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1.  A New Strategy to Stabilize Oxytocin in Aqueous Solutions: I. The Effects of Divalent Metal Ions and Citrate Buffer 
The AAPS Journal  2011;13(2):284-290.
In the current study, the effect of metal ions in combination with buffers (citrate, acetate, pH 4.5) on the stability of aqueous solutions of oxytocin was investigated. Both monovalent metal ions (Na+ and K+) and divalent metal ions (Ca2+, Mg2+, and Zn2+) were tested all as chloride salts. The effect of combinations of buffers and metal ions on the stability of aqueous oxytocin solutions was determined by RP-HPLC and HP-SEC after 4 weeks of storage at either 4°C or 55°C. Addition of sodium or potassium ions to acetate- or citrate-buffered solutions did not increase stability, nor did the addition of divalent metal ions to acetate buffer. However, the stability of aqueous oxytocin in aqueous formulations was improved in the presence of 5 and 10 mM citrate buffer in combination with at least 2 mM CaCl2, MgCl2, or ZnCl2 and depended on the divalent metal ion concentration. Isothermal titration calorimetric measurements were predictive for the stabilization effects observed during the stability study. Formulations in citrate buffer that had an improved stability displayed a strong interaction between oxytocin and Ca2+, Mg2+, or Zn2+, while formulations in acetate buffer did not. In conclusion, our study shows that divalent metal ions in combination with citrate buffer strongly improved the stability of oxytocin in aqueous solutions.
doi:10.1208/s12248-011-9268-7
PMCID: PMC3085697  PMID: 21448747
citrate buffer; divalent metal ions; improved stability; oxytocin
2.  Aggregation Kinetics of Citrate and Polyvinylpyrrolidone Coated Silver Nanoparticles in Monovalent and Divalent Electrolyte Solutions 
Environmental science & technology  2011;45(13):5564-5571.
The aggregation kinetics of silver nanoparticles (AgNPs) that were coated with two commonly used capping agents—citrate and polyvinylpyrrolidone (PVP)—were investigated. Time-resolved dynamic light scattering (DLS) was employed to measure the aggregation kinetics of the AgNPs over a range of monovalent and divalent electrolyte concentrations. The aggregation behavior of citrate-coated AgNPs in NaCl was in excellent agreement with the predictions based on Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, and the Hamaker constant of citrate-coated AgNPs in aqueous solutions was derived to be 3.7 × 10-20 J. Divalent electrolytes were more efficient in destabilizing the citrate-coated AgNPs, as indicated by the considerably lower critical coagulation concentrations (2.1 mM CaCl2 and 2.7 mM MgCl2 vs. 47.6 mM NaCl). The PVP-coated AgNPs were significantly more stable than citrate-coated AgNPs in both NaCl and CaCl2, which is likely due to steric repulsion imparted by the large, non-charged polymers. The addition of humic acid resulted in the adsorption of the macromolecules on both citrate- and PVP-coated AgNPs. The adsorption of humic acid induced additional electrosteric repulsion that elevated the stability of both nanoparticles in suspensions containing NaCl or low concentrations of CaCl2. Conversely, enhanced aggregation occurred for both nanoparticles at high CaCl2 concentrations due to interparticle bridging by humic acid clusters.
doi:10.1021/es200157h
PMCID: PMC3137917  PMID: 21630686
3.  Effect on Postpartum Hemorrhage of Prophylactic Oxytocin (10 IU) by Injection by Community Health Officers in Ghana: A Community-Based, Cluster-Randomized Trial 
PLoS Medicine  2013;10(10):e1001524.
Cynthia Stanton and colleagues conducted a cluster-randomized controlled trial in rural Ghana to assess whether oxytocin given by injection by community health officers at home births was a feasible and safe option in preventing postpartum hemorrhage.
Please see later in the article for the Editors' Summary
Background
Oxytocin (10 IU) is the drug of choice for prevention of postpartum hemorrhage (PPH). Its use has generally been restricted to medically trained staff in health facilities. We assessed the effectiveness, safety, and feasibility of PPH prevention using oxytocin injected by peripheral health care providers without midwifery skills at home births.
Methods and Findings
This community-based, cluster-randomized trial was conducted in four rural districts in Ghana. We randomly allocated 54 community health officers (stratified on district and catchment area distance to a health facility: ≥10 km versus <10 km) to intervention (one injection of oxytocin [10 IU] one minute after birth) and control (no provision of prophylactic oxytocin) arms. Births attended by a community health officer constituted a cluster. Our primary outcome was PPH, using multiple definitions; (PPH-1) blood loss ≥500 mL; (PPH-2) PPH-1 plus women who received early treatment for PPH; and (PPH-3) PPH-2 plus any other women referred to hospital for postpartum bleeding. Unsafe practice is defined as oxytocin use before delivery of the baby. We enrolled 689 and 897 women, respectively, into oxytocin and control arms of the trial from April 2011 to November 2012. In oxytocin and control arms, respectively, PPH-1 rates were 2.6% versus 5.5% (RR: 0.49; 95% CI: 0.27–0.88); PPH-2 rates were 3.8% versus 10.8% (RR: 0.35; 95% CI: 0.18–0.63), and PPH-3 rates were similar to those of PPH-2. Compared to women in control clusters, those in the intervention clusters lost 45.1 mL (17.7–72.6) less blood. There were no cases of oxytocin use before delivery of the baby and no major adverse events requiring notification of the institutional review boards. Limitations include an unblinded trial and imbalanced numbers of participants, favoring controls.
Conclusion
Maternal health care planners can consider adapting this model to extend the use of oxytocin into peripheral settings including, in some contexts, home births.
Trial registration
ClinicalTrials.gov NCT01108289
Please see later in the article for the Editors' Summary
Editors' Summary
Background
Many women in low-and middle-income countries die unnecessarily during childbirth, even though the solutions to prevent or manage complications are well known. Maternal death rates are highest amongst poor women living in remote areas, as they are least likely to have access to adequate health care. One of the United Nation's Millennium Development Goals is to reduce maternal death rates by three-quarters by 2015. Between 1990 and 2010, these rates were nearly halved. So there is still some way to go to meet the target.
One of the major causes of maternal death is excessive bleeding after birth, known as postpartum hemorrhage (PPH). The highest rates of PPH are found in Africa (28% of births), with an overall global rate of 11%. PPH can be caused by the uterus not contracting after the baby is born, damage to tissues and blood vessels, retention of the placenta, and problems with blood-clotting.
PPH can be prevented by an injection of oxytocin (a hormone) or with tablets of the drug misoprostol immediately after birth. Other drugs exist but are used much less frequently in low-income countries. If the mother does bleed excessively, then these interventions can also be used to treat PPH in the hours following birth. These drugs cause the uterus to contract. Continued severe bleeding requires emergency treatment in hospital. The World Health Organization (WHO) recommends that in situations where women give birth without the assistance of a trained midwife, priority should be given to preventing PPH because access to emergency services may be limited.
Why Was This Study Done?
Of the two most common options for preventing PPH, oxytocin is generally the preferred choice. It has the advantage of having no side effects, whereas misoprostol can cause fever and shivering. A repeat injection of oxytocin can also be given if the mother continues to bleed excessively, whereas a dose of misoprostol after birth should only be given once. A major concern about both drugs is that the timing of administration must be precise. Giving a drug that causes the uterus to contract before birth can be harmful to both mother and baby. A disadvantage of oxytocin is that it requires someone trained and authorized to give an injection. For this reason, oxytocin has so far been generally limited to hospitals and clinics, where it can be administered by medically trained professionals. Another disadvantage is that oxytocin is weakened by heat, which means its storage and use may be impractical in hot countries.
The main aim of this study was therefore to find out whether health workers without midwifery skills are able to administer oxytocin safely when attending home births in poor, rural communities.
What Did the Researchers Do and Find?
The researchers carried out a cluster-randomized controlled trial in four rural districts in Ghana, working with community health officers (CHOs). CHOs are trained for two years in giving childhood immunizations and antenatal and postnatal care, but are not trained midwives. 54 CHOs were randomized to one of two groups. The CHOs in the first group gave a preventative oxytocin injection to the mother at every birth they attended. The oxytocin was administered using a pre-filled, disposable device called Uniject that is easier and quicker to use than a syringe and needle. The packaging also included a heat-sensitive label that indicated whether the oxytocin still met the manufacturer's criteria for an acceptably potent drug. CHOs in the second group acted as controls, and did not give any oxytocin injections to prevent PPH. The women seen by each CHO formed a cluster. Comparisons were made across the clusters of women that either received or did not receive the preventative intervention.
The researchers found that the women who were given a preventative oxytocin injection lost less blood after birth than the women who did not receive a preventative injection. There were also fewer cases of PPH amongst the women who received oxytocin for PPH prevention. 2.6% of the women who received a preventative oxytocin injection experienced PPH, compared to 5.5% of the controls. Therefore the risk of PPH was approximately halved. There were no cases of oxytocin use before delivery of the baby and no difference in the frequency of other birth complications between women in the intervention and control groups.
What Do These Findings Mean?
These findings show that under the trial conditions, CHOs can safely administer oxytocin injections when attending home births in poor, rural settings. This intervention also proved practical to use in the Uniject format.
The study therefore suggests that oxytocin should be considered for use in regions where maternal deaths from PPH are still unacceptably high. There are also several noteworthy limitations, such as unblinding and the imbalance between participant groups. The researchers emphasized that their findings do not mean that oxytocin is always a better choice than misoprostol for home births. Many factors will influence which intervention is the most feasible, such as the local availability of sufficiently skilled health professionals, the relative cost and availability of the two drugs, as well as ease of access to emergency health services.
Additional Information
Please access these websites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1001524.
This study is further discussed in a PLOS Medicine Perspective by João Paulo Souza
WHO Factsheet on Maternal Mortality
United Nations Population Fund's Goals to improve sexual and reproductive health
doi:10.1371/journal.pmed.1001524
PMCID: PMC3794862  PMID: 24130463
4.  Synergistic effects of metal ion and the pre-senile cataract-causing G98R αA-crystallin: self-aggregation propensities and chaperone activity 
Molecular Vision  2009;15:2050-2060.
Purpose
αA- and αB-crystallins are abundantly present in the eye lens, belong to the small heat shock protein family, and exhibit molecular chaperone activity. They are also known to interact with metal ions such as Cu2+, and their metal-binding modulates the structure and chaperone function. Unlike other point mutations in αA-crystallin that cause congenital cataracts, the G98R mutation causes pre-senile cataract. We have investigated the effect of Cu2+ on the structure and function of G98R αA-crystallin.
Methods
Fluorescence spectroscopy and isothermal titration calorimetry were used to study Cu2+ binding to αA- and G98R αA-crystallin. Circular dichroism spectroscopy was used to study secondary and tertiary structures, and dynamic light scattering was used to determine the hydrodynamic radii of the proteins. Chaperone activity and self-aggregation of the wild type and the mutant protein in the absence and the presence of the metal ions was monitored using light scattering.
Results
Our fluorescence quenching and isothermal titration calorimetric studies show that like αA-crystallin, G98R αA-crystallin binds Cu2+ with picomolar range affinity. Further, both wild type and mutant αA-crystallin inhibit Cu2+-induced generation of reactive oxygen species with similar efficiency. However, G98R αA-crystallin undergoes pronounced self-aggregation above a certain concentration of Cu2+ (above subunit to Cu2+ molar ratio of 1:3 in HEPES-NaOH buffer, pH 7.4). At concentrations of Cu2+ below this ratio, G98R αA-crystallin is more susceptible to Cu2+-induced tertiary and quaternary structural changes than αA-crystallin. Interestingly, Cu2+ binding increases the chaperone-like activity of αA-crystallin toward the aggregation of citrate synthase at 43 °C while it decreases the chaperone-like activity of G98R αA-crystallin. Mixed oligomer formation between the wild type and the mutant subunits modulates the Cu2+-induced effect on the self-aggregation propensity. Other heavy metal ions, namely Cd2+ and Zn2+ but not Ca2+, also promote the self-aggregation of G98R αA-crystallin and decrease its chaperone-like activity.
Conclusions
Our study demonstrates that unlike wild type αA-crystallin, G98R αA-crystallin and its mixed oligomers with wild type protein are vulnerable to heavy metal ions. Our study provides insight into aspects of how environmental factors could augment phenotype(s) in certain genetically predisposed conditions.
PMCID: PMC2768467  PMID: 19862354
5.  Sorption of Heavy Metals to the Filamentous Bacterium Thiothrix Strain A1 
A study was undertaken to determine the ability of the filamentous bacterium Thiothrix strain A1 to sorb heavy metals from solution. Cells of Thiothrix strain A1 were harvested, washed, and suspended in solutions of metals. After an equilibration period, biomass was separated from solution and the metal content in acid-digested cells and/or filtrates was determined by atomic absorption spectrophotometry. Sorption of nickel and zinc was very rapid; most of the sorbed metal was bound in less than 10 min. The sorption data for copper fit the Freundlich isotherm, and nickel and zinc data fit biphasic Freundlich isotherms. Sorption of both nickel and zinc was dependent on cell age. Cells harvested 24 h after inoculation sorbed approximately one-half of the amount of metal per gram cell protein than did cells harvested after 48, 72, or 96 h. Calcium and magnesium effectively competed with zinc for binding sites, whereas potassium had only a slight effect on the capacity of cells to sorb zinc. The primary mechanism of metal sorption apparently was ion exchange, because 66 to 75% of nickel or zinc could be desorbed by placing metal-laden cells in a solution of 5 mM CaCl2. A competition experiment with nickel and zinc indicated that both metals occupied the same sorption sites. The strong chelating agents EDTA and NTA effectively prevented metal uptake, but lactate enhanced the uptake of nickel. Thiothrix strain A1 grown in nickel-containing medium had a relatively low uptake of nickel compared with uptake by resting cells suspended in a simple buffer solution.
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PMCID: PMC182077  PMID: 16348924
6.  A Study of the Hydration of the Alkali Metal Ions in Aqueous Solution 
Inorganic Chemistry  2011;51(1):425-438.
The hydration of the alkali metal ions in aqueous solution has been studied by large angle X-ray scattering (LAXS) and double difference infrared spectroscopy (DDIR). The structures of the dimethyl sulfoxide solvated alkali metal ions in solution have been determined to support the studies in aqueous solution. The results of the LAXS and DDIR measurements show that the sodium, potassium, rubidium and cesium ions all are weakly hydrated with only a single shell of water molecules. The smaller lithium ion is more strongly hydrated, most probably with a second hydration shell present. The influence of the rubidium and cesium ions on the water structure was found to be very weak, and it was not possible to quantify this effect in a reliable way due to insufficient separation of the O–D stretching bands of partially deuterated water bound to these metal ions and the O–D stretching bands of the bulk water. Aqueous solutions of sodium, potassium and cesium iodide and cesium and lithium hydroxide have been studied by LAXS and M–O bond distances have been determined fairly accurately except for lithium. However, the number of water molecules binding to the alkali metal ions is very difficult to determine from the LAXS measurements as the number of distances and the temperature factor are strongly correlated. A thorough analysis of M–O bond distances in solid alkali metal compounds with ligands binding through oxygen has been made from available structure databases. There is relatively strong correlation between M–O bond distances and coordination numbers also for the alkali metal ions even though the M–O interactions are weak and the number of complexes of potassium, rubidium and cesium with well-defined coordination geometry is very small. The mean M–O bond distance in the hydrated sodium, potassium, rubidium and cesium ions in aqueous solution have been determined to be 2.43(2), 2.81(1), 2.98(1) and 3.07(1) Å, which corresponds to six-, seven-, eight- and eight-coordination. These coordination numbers are supported by the linear relationship of the hydration enthalpies and the M–O bond distances. This correlation indicates that the hydrated lithium ion is four-coordinate in aqueous solution. New ionic radii are proposed for four- and six-coordinate lithium(I), 0.60 and 0.79 Å, respectively, as well as for five- and six-coordinate sodium(I), 1.02 and 1.07 Å, respectively. The ionic radii for six- and seven-coordinate K+, 1.38 and 1.46 Å, respectively, and eight-coordinate Rb+ and Cs+, 1.64 and 1.73 Å, respectively, are confirmed from previous studies. The M–O bond distances in dimethyl sulfoxide solvated sodium, potassium, rubidium and cesium ions in solution are very similar to those observed in aqueous solution.
The hydration of alkali metal ions has been studied by large angle X-ray scattering, LAXS, and double difference infrared spectroscopy. The obtained M−O bond distances from LAXS have been compared to relevant crystal structures, conclusions about hydration numbers in aqueous solution have been made, and new ionic radii have been proposed. Hydration numbers of six, seven, eight and eight are proposed for the sodium, potassium, rubidium and cesium ions in aqueous solution.
doi:10.1021/ic2018693
PMCID: PMC3250073  PMID: 22168370
7.  Divalent Metal Ion Transport across Large Biological Ion Channels and Their Effect on Conductance and Selectivity 
Electrophysiological characterization of large protein channels, usually displaying multi-ionic transport and weak ion selectivity, is commonly performed at physiological conditions (moderate gradients of KCl solutions at decimolar concentrations buffered at neutral pH). We extend here the characterization of the OmpF porin, a wide channel of the outer membrane of E. coli, by studying the effect of salts of divalent cations on the transport properties of the channel. The regulation of divalent cations concentration is essential in cell metabolism and understanding their effects is of key importance, not only in the channels specifically designed to control their passage but also in other multiionic channels. In particular, in porin channels like OmpF, divalent cations modulate the efficiency of molecules having antimicrobial activity. Taking advantage of the fact that the OmpF channel atomic structure has been resolved both in water and in MgCl2 aqueous solutions, we analyze the single channel conductance and the channel selectivity inversion aiming to separate the role of the electrolyte itself, and the counterion accumulation induced by the protein channel charges and other factors (binding, steric effects, etc.) that being of minor importance in salts of monovalent cations become crucial in the case of divalent cations.
doi:10.1155/2012/245786
PMCID: PMC3449104  PMID: 23008773
8.  The permeability of endplate channels to monovalent and divalent metal cations 
The Journal of General Physiology  1980;75(5):493-510.
The relative permeability of endplate channels to monovalent and divalent metal ions was determined from reversal potentials. Thallium is the most permeant ion with a permeability ratio relative to Na+ of 2.5. The selectivity among alkali metals is weak with a sequence, Cs+ greater than Rb+ greater than K+ greater than Na+ greater than Li+, and permeability ratios of 1.4, 1.3, 1.1, 1.0, and 0.9. The selectivity among divalent ions is also weak, with a sequence for alkaline earths of Mg++ greater than Ca++ greater than Ba++ greater than Sr++. The transition metal ions Mn++, Co++, Ni++, Zn++, and Cd++ are also permeant. Permeability ratios for divalent ions decreased as the concentration of divalent ion was increased in a manner consistent with the negative surface potential theory of Lewis (1979 J. Physiol. (Lond.). 286: 417--445). With 20 mM XCl2 and 85.5 mM glucosamine.HCl in the external solution, the apparent permeability ratios for the alkaline earth cations (X++) are in the range 0.18--0.25. Alkali metal ions see the endplate channel as a water-filled, neutral pore without high-field-strength sites inside. Their permeability sequence is the same as their aqueous mobility sequence. Divalent ions, however, have a permeability sequence almost opposite from their mobility sequence and must experience some interaction with groups in the channel. In addition, the concentrations of monovalent and divalent ions are increased near the channel mouth by a weak negative surface potential.
PMCID: PMC2215258  PMID: 6247423
9.  Complementary Metal Ion Specificity of the Metal-Citrate Transporters CitM and CitH of Bacillus subtilis 
Journal of Bacteriology  2000;182(22):6374-6381.
Citrate uptake in Bacillus subtilis is stimulated by a wide range of divalent metal ions. The metal ions were separated into two groups based on the expression pattern of the uptake system. The two groups correlated with the metal ion specificity of two homologous B. subtilis secondary citrate transporters, CitM and CitH, upon expression in Escherichia coli. CitM transported citrate in complex with Mg2+, Ni2+, Mn2+, Co2+, and Zn2+ but not in complex with Ca2+, Ba2+, and Sr2+. CitH transported citrate in complex with Ca2+, Ba2+, and Sr2+ but not in complex with Mg2+, Ni2+, Mn2+, Co2+, and Zn2+. Both transporters did not transport free citrate. Nevertheless, free citrate uptake could be demonstrated in B. subtilis, indicating the expression of at least a third citrate transporter, whose identity is not known. For both the CitM and CitH transporters it was demonstrated that the metal ion promoted citrate uptake and, vice versa, that citrate promoted uptake of the metal ion, indicating that the complex is the transported species. The results indicate that CitM and CitH are secondary transporters that transport complexes of divalent metal ions and citrate but with a complementary metal ion specificity. The potential physiological function of the two transporters is discussed.
PMCID: PMC94783  PMID: 11053381
10.  Ca2+-Citrate Uptake and Metabolism in Lactobacillus casei ATCC 334 
Applied and Environmental Microbiology  2013;79(15):4603-4612.
The putative citrate metabolic pathway in Lactobacillus casei ATCC 334 consists of the transporter CitH, a proton symporter of the citrate-divalent metal ion family of transporters CitMHS, citrate lyase, and the membrane-bound oxaloacetate decarboxylase complex OAD-ABDH. Resting cells of Lactobacillus casei ATCC 334 metabolized citrate in complex with Ca2+ and not as free citrate or the Mg2+-citrate complex, thereby identifying Ca2+-citrate as the substrate of the transporter CitH. The pathway was induced in the presence of Ca2+ and citrate during growth and repressed by the presence of glucose and of galactose, most likely by a carbon catabolite repression mechanism. The end products of Ca2+-citrate metabolism by resting cells of Lb. casei were pyruvate, acetate, and acetoin, demonstrating the activity of the membrane-bound oxaloacetate decarboxylase complex OAD-ABDH. Following pyruvate, the pathway splits into two branches. One branch is the classical citrate fermentation pathway producing acetoin by α-acetolactate synthase and α-acetolactate decarboxylase. The other branch yields acetate, for which the route is still obscure. Ca2+-citrate metabolism in a modified MRS medium lacking a carbohydrate did not significantly affect the growth characteristics, and generation of metabolic energy in the form of proton motive force (PMF) was not observed in resting cells. In contrast, carbohydrate/Ca2+-citrate cometabolism resulted in a higher biomass yield in batch culture. However, also with these cells, no generation of PMF was associated with Ca2+-citrate metabolism. It is concluded that citrate metabolism in Lb. casei is beneficial when it counteracts acidification by carbohydrate metabolism in later growth stages.
doi:10.1128/AEM.00925-13
PMCID: PMC3719530  PMID: 23709502
11.  Increasing Mechanical Strength of Gelatin Hydrogels by Divalent Metal Ion Removal 
Scientific Reports  2014;4:4706.
The usage of gelatin hydrogel is limited due to its instability and poor mechanical properties, especially under physiological conditions. Divalent metal ions present in gelatin such as Ca2+ and Fe2+ play important roles in the gelatin molecule interactions. The objective of this study was to determine the impact of divalent ion removal on the stability and mechanical properties of gelatin gels with and without chemical crosslinking. The gelatin solution was purified by Chelex resin to replace divalent metal ions with sodium ions. The gel was then chemically crosslinked by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC). Results showed that the removal of divalent metal ions significantly impacted the formation of the gelatin network. The purified gelatin hydrogels had less interactions between gelatin molecules and form larger-pore network which enabled EDC to penetrate and crosslink the gel more efficiently. The crosslinked purified gels showed small swelling ratio, higher crosslinking density and dramatically increased storage and loss moduli. The removal of divalent ions is a simple yet effective method that can significantly improve the stability and strength of gelatin hydrogels. The in vitro cell culture demonstrated that the purified gelatin maintained its ability to support cell attachment and spreading.
doi:10.1038/srep04706
PMCID: PMC3988488  PMID: 24736500
12.  STUDIES ON ISOLATED CELL COMPONENTS  
The Journal of General Physiology  1952;35(5):781-796.
1. The effects of morganic ions, electrolyte concentration, and pH on the appearance and volume of the isolated rat liver nucleus have been studied. Nuclei were isolated by differential centrifugation in a buffered salt-sucrose mixture at pH 7.1. Nuclear volumes were determined photographically. 2. In solutions of NaCl, of KCl, and in potassium phosphate buffers the nuclear volume decreased markedly with an increase in concentration from 0.001 M to 0.05 M but remained essentially constant with further increase in concentration to 1.0 M. The effects of CaCl2 and MgCl2 differed from those of NaCl and KCl in that a smaller volume was obtained in concentrations less than 0.15 M, and in the case of CaCl2 an increase in volume was obtained in more concentrated solutions. The volume changes are considered to be due primarily to ionic effects on the nuclear colloids rather than to osmotic behavior. 3. Treatment of nuclei with DNAase prevented the characteristic volume changes resulting from ion effects, suggesting the importance of DNA in nuclear volume changes. 4. The optical changes in isolated nuclei in various concentrations of KCl, NaCl, CaCl2, MgCl2, and in potassium phosphate buffers as observed under phase contrast illumination are described. CaCl2 gave the most marked nuclear changes from the conditions in the uninjured cell and caused shrinkage and granulation in 0.001 M concentration. The effects of CaCl2 were also manifested in 0.88 M sucrose, in mixtures with monovalent salts, and in serum. Changes in nuclear volume and optical appearance which occurred in salt solutions and in 0.1 N HCl were readily reversible. 5. Nuclear volume remained constant between pH 8.91 and 5.12 and decreased in more acid solutions. 6. Sucrose had no appreciable osmotic effect, and in hyperosmotic solution. (0.88 M) nuclei showed swelling and rupture comparable to that in distilled water. 7. The results are considered in relation to the requirements of nuclear isolation media. 8. Rat liver nuclei isolated in a buffered salt-sucrose medium by differential centrifugation exhibited a pattern of size distribution similar to that of fixed nuclei but were of considerably larger volume. The ratio of the volumes of the peak frequencies of the two chief size groups was 1:1.9.
PMCID: PMC2147317  PMID: 14955619
13.  Use of Divalent Metal Ions in the DNA Cleavage Reaction of Human Type II Topoisomerases† 
Biochemistry  2009;48(9):1862-1869.
All type II topoisomerases require divalent metal ions in order to cleave and ligate DNA. In order to further elucidate the mechanistic basis for these critical enzyme-mediated events, the role of the metal ion in the DNA cleavage reaction of human topoisomerase IIβ was characterized and compared to that of topoisomerase IIα. The present study utilized divalent metal ions with varying thiophilicities in conjunction with DNA cleavage substrates that substituted a sulfur atom for the 3′-bridging oxygen or the non-bridging oxygens of the scissile phosphate. Based on time courses of DNA cleavage, cation titrations, and metal ion mixing experiments, we propose the following model for the use of divalent metal ions by human type II topoisomerases. First, both enzymes employ a two-metal-ion mechanism to support DNA cleavage. Second, an interaction between one divalent metal ion and the 3′-bridging atom of the scissile phosphate greatly enhances enzyme-mediated DNA cleavage, most likely by stabilizing the leaving 3′-oxygen. Third, there is an important interaction between a divalent second metal ion and a non-bridging atom of the scissile phosphate that stimulates DNA cleavage mediated by topoisomerase IIβ. If this interaction exists in topoisomerase IIα, its effects on DNA cleavage are equivocal. This last aspect of the model highlights a difference in metal ion utilization during DNA cleavage mediated by human topoisomerase IIα and IIβ.
doi:10.1021/bi8023256
PMCID: PMC2693261  PMID: 19222228
14.  Divalent Cu, Cd, and Pb Biosorption in Mixed Solvents 
Dead dried Chlorella vulgaris was studied in terms of its performance in binding divalent copper, cadmium, and lead ions from their aqueous or 50% v/v methanol, ethanol, and acetone solutions. The percentage uptake of cadmium ions exhibited a general decrease with decrease in dielectric constant values, while that of copper and lead ions showed a general decrease with increase in donor numbers. Uptake percentage becomes less sensitive to solvent properties the larger the atomic radius of the biosorbed ion, and uptake of copper was the most affected. FT-IR analyses revealed stability of the biomass in mixed solvents and a shift in vibrations of amide(I) and (II), carboxylate, glucose ring, and metal oxygen upon metal binding in all media. ΔνCOO values (59–69 cm−1) confirmed bidentate metal coordination to carboxylate ligands. The value of νasCOO increased slightly upon Cu, Cd, and Pb biosorption from aqueous solutions indicating lowering of symmetry, while a general decrease was noticed in mixed solvents pointing to the opposite. M–O stretching frequencies increased unexpectedly with increase in atomic mass as a result of solvent effect on the nature of binding sites. Lowering polarity of the solvent permits variations in metal-alga bonds strengths; the smaller the metal ion, the more affected.
doi:10.1155/2009/561091
PMCID: PMC2726428  PMID: 19688108
15.  Quantitative and Comprehensive Decomposition of the Ion Atmosphere around Nucleic Acids 
Journal of the American Chemical Society  2007;129(48):14981-14988.
The ion atmosphere around nucleic acids critically affects biological and physical processes such as chromosome packing, RNA folding, and molecular recognition. However, the dynamic nature of the ion atmosphere renders it difficult to characterize. The basic thermodynamic description of this atmosphere, a full accounting of the type and number of associated ions, has remained elusive. Here we provide the first complete accounting of the ion atmosphere, using buffer equilibration and atomic emission spectroscopy (BE-AES) to accurately quantitate the cation association and anion depletion. We have examined the influence of ion size and charge on ion occupancy around simple, well-defined DNA molecules. The relative affinity of monovalent and divalent cations correlates inversely with their size. Divalent cations associate preferentially over monovalent cations; e.g., with Na+ in four-fold excess of Mg2+ (20 vs. 5 mM), the ion atmosphere nevertheless has three-fold more Mg2+ than Na+. Further, the dicationic polyamine putrescine2+ does not compete effectively for association relative to divalent metal ions, presumably because of its lower charge density. These and other BE-AES results can be used to evaluate and guide the improvement of electrostatic treatments. As a first step, we compare the BE-AES results to predictions from the widely-used nonlinear Poisson Boltzmann (NLPB) theory and assess the applicability and precision of this theory. In the future, BE-AES in conjunction with improved theoretical models, can be applied to complex binding and folding equilibria of nucleic acids and their complexes, to parse the electrostatic contribution from the overall thermodynamics of important biological processes.
doi:10.1021/ja075020g
PMCID: PMC3167487  PMID: 17990882
16.  Regulation of the L-lactase dehydrogenase from Lactobacillus casei by fructose-1,6-diphosphate and metal ions. 
Journal of Bacteriology  1975;121(3):777-784.
The lactate dehydrogenase of Lactobacillus casei, like that of streptococci, requires fructose-1,6-diphosphate (FDP) for activity. The L. casei enzyme has a much more acidic pH optimum (pH 5.5) than the streptococcal lactate dehydrogenases. This is apparently due to a marked decrease in the affinity of the enzyme for the activator with increasing pH above 5.5; the concentration of FDP required for half-maximal velocity increase nearly 1,000-fold from 0.002 mM at pH 5.5 to 1.65 mM at 6.6. Manganous ions increase the pH range of activity particularly on the alkaline side of the optimum by increasing the affinity for FDP. This pH dependent metal ion activation is not specific for Mn2+. Other divalent metals, Co2+, Cu2+, Cd2+, Ni2+, Fe2+, Fe2+, and Zn2+ but not Mg2+, will effectively substitute for Mn2+, but the pH dependence of the activation differs with the metal ion used. The enzyme is inhibited by a number of commonly used buffering ions, particularly phosphate, citrate, and tris (hydroxymethyl) aminomethane-maleate buffers, even at low buffer concentrations (0.02 M). These buffers inhibit by affecting the binding of FDP.
PMCID: PMC246003  PMID: 234946
17.  Effect of Pressure-Induced Changes in the Ionization Equilibria of Buffers on Inactivation of Escherichia coli and Staphylococcus aureus by High Hydrostatic Pressure 
Applied and Environmental Microbiology  2013;79(13):4041-4047.
Survival rates of Escherichia coli and Staphylococcus aureus after high-pressure treatment in buffers that had large or small reaction volumes (ΔV°), and which therefore underwent large or small changes in pH under pressure, were compared. At a low buffer concentration of 0.005 M, survival was, as expected, better in MOPS (morpholinepropanesulfonic acid), HEPES, and Tris, whose ΔV° values are approximately 5.0 to 7.0 cm3 mol−1, than in phosphate or dimethyl glutarate (DMG), whose ΔV° values are about −25 cm3 mol−1. However, at a concentration of 0.1 M, survival was unexpectedly better in phosphate and DMG than in MOPS, HEPES, or Tris. This was because the baroprotective effect of phosphate and DMG increased much more rapidly with increasing concentration than it did with MOPS, HEPES, or Tris. Further comparisons of survival in solutions of salts expected to cause large electrostriction effects (Na2SO4 and CaCl2) and those causing lower electrostriction (NaCl and KCl) were made. The salts with divalent ions were protective at much lower concentrations than salts with monovalent ions. Buffers and salts both protected against transient membrane disruption in E. coli, but the molar concentrations necessary for membrane protection were much lower for phosphate and Na2SO4 than for HEPES and NaCl. Possible protective mechanisms discussed include effects of electrolytes on water compressibility and kosmotropic and specific ion effects. The results of this systematic study will be of considerable practical significance in studies of pressure inactivation of microbes under defined conditions but also raise important fundamental questions regarding the mechanisms of baroprotection by ionic solutes.
doi:10.1128/AEM.00469-13
PMCID: PMC3697583  PMID: 23624471
18.  Cation Charge and Size Selectivity of the C2 Domain of Cytosolic Phospholipase A2† 
Biochemistry  2002;41(4):1109-1122.
C2 domains regulate numerous eukaryotic signaling proteins by docking to target membranes upon binding Ca2+. Effective activation of the C2 domain by intracellular Ca2+ signals requires high Ca2+ selectivity to exclude the prevalent physiological metal ions K+, Na+, and Mg2+. The cooperative binding of two Ca2+ ions to the C2 domain of cytosolic phospholipase A2 (cPLA2-α) induces docking to phosphatidylcholine (PC) membranes. The ionic charge and size selectivities of this C2 domain were probed with representative mono-, di-, and trivalent spherical metal cations. Physiological concentrations of monovalent cations and Mg2+ failed to bind to the domain and to induce docking to PC membranes. Superphysiological concentrations of Mg2+ did bind but still failed to induce membrane docking. In contrast, Ca2+, Sr2+, and Ba2+ bound to the domain in the low micromolar range, induced electrophoretic mobility shifts in native polyacrylamide gels, stabilized the domain against thermal denaturation, and induced docking to PC membranes. In the absence of membranes, the degree of apparent positive cooperativity in binding of Ca2+, Sr2+, and Ba2+ decreased with increasing cation size, suggesting that the C2 domain binds two Ca2+ or Sr2+ ions, but only one Ba2+ ion. These stoichiometries were correlated with the abilities of the ions to drive membrane docking, such that micromolar concentrations of Ca2+ and Sr2+ triggered docking while even millimolar concentrations of Ba2+ yielded poor docking efficiency. The simplest explanation is that two bound divalent cations are required for stable membrane association. The physiological Ca2+ ion triggered membrane docking at 20-fold lower concentrations than Sr2+, due to both the higher Ca2+ affinity of the free domain and the higher affinity of the Ca2+-loaded domain for membranes. Kinetic studies indicated that Ca2+ ions bound to the free domain are retained at least 5-fold longer than Sr2+ ions. Moreover, the Ca2+-loaded domain remained bound to membranes 2-fold longer than the Sr2+- loaded domain. For both Ca2+ and Sr2+, the two bound metal ions dissociate from the protein–membrane complex in two kinetically resolvable steps. Finally, representative trivalent lanthanide ions bound to the domain with high affinity and positive cooperativity, and induced docking to PC membranes. Overall, the results demonstrate that both cation charge and size constraints contribute to the high Ca2+ selectivity of the C2 domain and suggest that formation of a cPLA2-α C2 domain–membrane complex requires two bound multivalent metal ions. These features are proposed to stem from the unique structural features of the metal ion-binding site in the C2 domain.
PMCID: PMC3676915  PMID: 11802709
19.  ION SERIES AND THE PHYSICAL PROPERTIES OF PROTEINS. I 
1. This paper contains experiments on the influence of acids and alkalies on the osmotic pressure of solutions of crystalline egg albumin and of gelatin, and on the viscosity of solutions of gelatin. 2. It was found in all cases that there is no difference in the effects of HCl, HBr, HNO3, acetic, mono-, di-, and trichloracetic, succinic, tartaric, citric, and phosphoric acids upon these physical properties when the solutions of the protein with these different acids have the same pH and the same concentration of originally isoelectric protein. 3. It was possible to show that in all the protein-acid salts named the anion in combination with the protein is monovalent. 4. The strong dibasic acid H2SO4 forms protein-acid salts with a divalent anion SO4 and the solutions of protein sulfate have an osmotic pressure and a viscosity of only half or less than that of a protein chloride solution of the same pH and the same concentration of originally isoelectric protein. Oxalic acid behaves essentially like a weak dibasic acid though it seems that a small part of the acid combines with the protein in the form of divalent anions. 5. It was found that the osmotic pressure and viscosity of solutions of Li, Na, K, and NH4 salts of a protein are the same at the same pH and the same concentration of originally isoelectric protein. 6. Ca(OH)2 and Ba(OH)2 form salts with proteins in which the cation is divalent and the osmotic pressure and viscosity of solutions of these two metal proteinates are only one-half or less than half of that of Na proteinate of the same pH and the same concentration of originally isoelectric gelatin. 7. These results exclude the possibility of expressing the effect of different acids and alkalies on the osmotic pressure of solutions of gelatin and egg albumin and on the viscosity of solutions of gelatin in the form of ion series. The different results of former workers were probably chiefly due to the fact that the effects of acids and alkalies on these proteins were compared for the same quantity of acid and alkali instead of for the same pH.
PMCID: PMC2140403  PMID: 19871850
20.  Chelating Agent Shock of Bacteriophage T5 
Journal of Virology  1968;2(9):944-950.
When two strains of phage T5 (heat-susceptible form T5st+ and its heat-resistant mutant T5st) were placed in solutions containing various high concentrations of chelating agents (sodium citrate and ethylenediaminetetraacetic acid) at room temperature, they could be effectively inactivated by rapid dilution in distilled water of relatively low temperatures (2 to 37 C). This phenomenon has been termed “chelating agent shock” (CAS). The susceptibility of phage T5 to CAS increased with an increase in the concentration of chelating agents and with an increase in temperature of the water used for rapid dilution. Under any given condition, T5st+ was much more sensitive to CAS than was T5st. Phage T5 was protected against inactivation by the addition of monovalent or divalent metal salts, but not by the addition of nonionic solutes, to the shocking water prior to CAS treatment. This finding is compatible with the view that cations combined with the phage protein are removed by the chelating agent, although no metal ion has been identified in the phage protein. Alternatively, since the chelating agents used are polyanions, they may bind relatively tightly to the protein subunits in the head of T5, thereby distorting the structure of the phage head. Rapid dilution of these distorted particles could lead to loss of phage DNA. No evidence for recovery of phage activity could be obtained by the addition of metal salts to the inactivated phage after CAS. The morphological properties of phage inactivated by CAS are similar to those of heat-inactivated T5 phage. Electron micrographs showed that most of the phage particles consisted of empty head membranes; some of the particles had lost their tails. Both heritable and nonheritable resistance to heat was accompanied by resistance to CAS in phage T5. The sensitive element detected by each test seemed to be the same.
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PMCID: PMC375716  PMID: 4972945
21.  Proton-Nuclear Magnetic Resonance Analyses of the Substrate Specificity of a β-Ketolase from Pseudomonas putida, Acetopyruvate Hydrolase† 
Journal of Bacteriology  1999;181(16):5051-5059.
A revised purification of acetopyruvate hydrolase from orcinol-grown Pseudomonas putida ORC is described. This carbon-carbon bond hydrolase, which is the last inducible enzyme of the orcinol catabolic pathway, is monomeric with a molecular size of ∼38 kDa; it hydrolyzes acetopyruvate to equimolar quantities of acetate and pyruvate. We have previously described the aqueous-solution structures of acetopyruvate at pH 7.5 and several synthesized analogues by 1H-nuclear magnetic resonance (NMR)-Fourier transform (FT) experiments. Three 1H signals (2.2 to 2.4 ppm) of the methyl group are assigned unambiguously to the carboxylate anions of 2,4-diketo, 2-enol-4-keto, and 2-hydrate-4-keto forms (40:50:10). A 1H-NMR assay for acetopyruvate hydrolase was used to study the kinetics and stoichiometries of reactions within a single reaction mixture (0.7 ml) by monitoring the three methyl-group signals of acetopyruvate and of the products acetate and pyruvate. Examination of 4-tert-butyl-2,4-diketobutanoate hydrolysis by the same method allowed the conclusion that it is the carboxylate 2-enol form(s) or carbanion(s) that is the actual substrate(s) of hydrolysis. Substrate analogues of 2,4-diketobutanoate with 4-phenyl or 4-benzyl groups are very poor substrates for the enzyme, whereas the 4-cyclohexyl analogue is readily hydrolyzed. In aqueous solution, the arene analogues do not form a stable 2-enol structure but exist principally as a delocalized π-electron system in conjugation with the aromatic ring. The effects of several divalent metal ions on solution structures were studied, and a tentative conclusion that the enol forms are coordinated to Mg2+ bound to the enzyme was made. 1H–2H exchange reactions showed the complete, fast equilibration of 2H into the C-3 of acetopyruvate chemically; this accounts for the appearance of 2H in the product pyruvate. The C-3 of the product pyruvate was similarly labelled, but this exchange was only enzyme catalyzed; the methyl group of acetate did not undergo an exchange reaction. The unexpected preference for bulky 4-alkyl-group analogues is discussed in an evolutionary context for carbon-carbon bond hydrolases. Routine one-dimensional 1H-NMR in normal 1H2O is a new method for rapid, noninvasive assays of enzymic activities to obtain the kinetics and stoichiometries of reactions in single reaction mixtures. Assessments of the solution structures of both substrates and products are also shown.
PMCID: PMC93995  PMID: 10438778
22.  Release of Alkaline Phosphatase from Cells of Pseudomonas aeruginosa by Manipulation of Cation Concentration and of pH1 
Journal of Bacteriology  1970;104(2):748-753.
Pseudomonas aeruginosa ATCC 9027 contains an inducible alkaline phosphatase. The enzyme is readily removed from 14-hr cells by washes in 0.2 m MgCl2, pH 8.4. Similar washes in tris(hydroxymethyl)aminomethane buffer, 20% sucrose, monovalent ions, or water partially release enzyme from the cells. The release of alkaline phosphatase is correlated with an increased release of protein and retention of internal enzymes. The effect of 0.2 m MgCl2 washing upon the cells is minimal since both viability and growth rates remain unchanged as compared to water washing. Although cells are plasmolyzed in both 0.2 m MgCl2 and 20% sucrose, it is evident that plasmolysis alone is unable to account for total enzyme release and that a divalent metal, i.e. Mg2+, augments the release pattern. Growing cells in the presence of increasing concentrations of MgCl2 or at increased pH values results in an almost total secretion of the enzyme to the culture filtrate. The findings suggest that P. aeruginosa alkaline phosphatase is linked to the exocytoplasmic region through divalent metal ion, presumably Mg2+, bridges.
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PMCID: PMC285053  PMID: 4992367
23.  ION SERIES AND THE PHYSICAL PROPERTIES OF PROTEINS. II 
1. Our results show clearly that the Hofmeister series is not the correct expression of the relative effect of ions on the swelling of gelatin, and that it is not true that chlorides, bromides, and nitrates have "hydrating," and acetates, tartrates, citrates, and phosphates "dehydrating," effects. If the pH of the gelatin is taken into considertion, it is found that for the same pH the effect on swelling is the same for gelatin chloride, nitrate, trichloracetate, tartrate, succinate, oxalate, citrate, and phosphate, while the swelling is considerably less for gelatin sulfate. This is exactly what we should expect on the basis of the combining ratios of the corresponding acids with gelatin since the weak dibasic and tribasic acids combine with gelatin in molecular proportions while the strong dibasic acid H2SO4 combines with gelatin in equivalent proportions. In the case of the weak dibasic acids he anion in combination with gelatin is therefore monovalent and in the case of the strong H2SO4 it is bivalent. Hence it is only the valency and not the nature of the ion in combination with gelatin which affects the degree of swelling. 2. This is corroborated in the experiments with alkalies which show that LiOH, NaOH, KOH, and NH4OH cause the same degree of swelling at the same pH of the gelatin solution and that this swelling is considerably higher than that caused by Ca(OH)2 and Ba(OH)2 for the same pH. This agrees with the results of the titration experiments which prove that Ca(OH)2 and Ba(OH)2 combine with gelatin in equivalent proportions and that hence the cation in combination with the gelatin salt with these two latter bases is bivalent. 3. The fact that proteins combine with acids and alkalies on the basis of the forces of primary valency is therefore not only in full agreement with the influence of ions on the physical properties of proteins but allows us to predict this influence qualitatively and quantitatively. 4. What has been stated in regard to the influence of ions on the swelling of the different gelatin salts is also true in regard to the influence of ions on the relative solubility of gelatin in alcohol-water mixtures. 5. Conductivity measurements of solutions of gelatin salts do not support the theory that the drop in the curves for swelling, osmotic pressure, or viscosity, which occurs at a pH 3.3 or a little less, is due to a drop in the concentration of ionized protein in the solution; nor do they suggest that the difference between the physical properties of gelatin sulfate and gelatin chloride is due to differences in the degree of ionization of these two salts.
PMCID: PMC2140423  PMID: 19871862
24.  Activity, folding and Z-DNA formation of the 8-17 DNAzyme in the presence of monovalent ions 
The effect of monovalent ions on both the reactivity and global folding of the 8–17 DNAzyme is investigated and the results are compared with the hammerhead ribozyme, which has similar size and secondary structure. In contrast to the hammerhead ribozyme, the 8–17 DNAzyme activity is not detectable in the presence of 4 M K+, Rb+, and Cs+ and the complex, [Co(NH3)6]3+. Only Li+, NH4+ and to a lesser extent Na+ showed detectable activity. The observed rate constants (kobs ~10−3 min−1 for Li+ and NH4+) are ~1000-fold lower than that in the presence of 10 mM Mg2+, and ~200,000-fold slower than the estimated rate in the presence of 100 µM Pb2+. Since the hammerhead ribozyme displays monovalent ion-dependent activity that is often within ~10-fold of divalent metal ion-dependent activity, these results suggest that the 8–17 DNAzyme, obtained by in vitro selections has evolved to have a more stringent divalent metal ion requirement for high activity as compared to the naturally occurring ribozymes, making the 8–17 DNAzyme an excellent choice as a Pb2+ sensor with high selectivity. In contrast to the activity data, folding was observed in the presence of all the monovalent ions investigated, although those monovalent ions that do not support DNAzyme activity have weaker binding affinity (Kd ~0.35 M for Rb+ and Cs+), while those that confer DNAzyme activity possess stronger affinity (Kd ~0.22 M for Li+, Na+ and NH4 +). In addition, a correlation between metal ion charge density, binding affinity and enzyme activity was found among mono- and divalent metal ions except Pb2+; higher charge density resulted in stronger affinity and higher activity, suggesting that the observed folding and activity is at least partially due to electrostatic interactions between ions and the DNAzyme. Finally, circular dichroism (CD) study has revealed Z-DNA formation with the monovalent metal ions, Zn2+ and Mg2+; the Kd values obtained using CD were in the same range as those obtained from folding studies using FRET. However, Z-DNA formation was not observed with Pb2+. These results indicate that Pb2+-dependent function follows a different mechanism from the monovalent metal ions and other divalent metal ions; in the presence of latter metal ions, metal-ion dependent folding and structural changes, including formation of Z-DNA, play an important role in the catalytic function of the 8–17 DNAzyme.
doi:10.1021/ja8082939
PMCID: PMC2765493  PMID: 19326878
8–17 DNAzyme; Monovalent ions; Activity; Fluorescence Resonance Energy Transfer (FRET); Circular Dichroism (CD)
25.  Structure of Cationized Glycine, Gly·M2+ (M = Be, Mg, Ca, Sr, Ba), in the Gas Phase: Intrinsic Effect of Cation Size on Zwitterion Stability 
The journal of physical chemistry. A  2000;104(43):9793-9796.
Interactions between divalent metal ions and biomolecules are common both in solution and in the gas phase. Here, the intrinsic effect of divalent alkaline earth metal ions (Be, Mg, Ca, Sr, Ba) on the structure of glycine in the absence of solvent is examined. Results from both density functional and Moller–Plesset theories indicate that for all metal ions except beryllium, the salt-bridge form of the ion, in which glycine is a zwitterion, is between 5 and 12 kcal/mol more stable than the charge-solvated structure in which glycine is in its neutral form. For beryllium, the charge-solvated structure is 5–8 kcal/mol more stable than the salt-bridge structure. Thus, there is a dramatic change in the structure of glycine with increased metal cation size. Using a Hartree–Fock-based partitioning method, the interaction between the metal ion and glycine is separated into electrostatic, charge transfer and deformation components. The charge transfer interactions are more important for stabilizing the charge-solvated structure of glycine with beryllium relative to magnesium. In contrast, the difference in stability between the charge-solvated and salt-bridge structure for magnesium is mostly due to electrostatic interactions that favor formation of the salt-bridge structure. These results indicate that divalent metal ions dramatically influence the structure of this simplest amino acid in the gas phase.
doi:10.1021/jp002970e
PMCID: PMC1343513  PMID: 16467899

Results 1-25 (803222)