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1.  Inverse Temperature Transition of Elastin Like Motifs in Major Ampullate Dragline Silk: MD Simulations of Short Peptides and NMR Studies of Water Dynamics 
Soft matter  2014;10(5):773-785.
Using deuterium 2D T1-T2 Inverse Laplace Transform (ILT) NMR we have investigated the distribution, population, and dynamics of waters of hydration in major ampullate N. clavipes and A. aurantia silk as a function of temperature. In both samples studied, correlation times much larger than that of free water are measured and in some cases appear to increase with increasing temperature over the range of 5 to 60 °C(corresponding to reduced tumbling). In addition, the experimental data point to a reduction in the population of water localized in the silk with increasing temperature in the range of 20 to 50°C. Molecular dynamics simulations were performed to probe the thermal characteristics of a variety of repeating motifs found in the two silk samples. The repeating motifs GLGSQ, GAAAAAAG, GPGGY, GPGQQ, GPSG, and GPSGPGS found in N. clavipes, GLGSQ, GYGSG, GPGSG, and GPGSQ found in A. aurantia silk were found to exhibit a thermal property observed in short elastin peptides known as the “inverse temperature transition”. This is a well known characteristic exhibited by short peptides consisting of (VPGXG)n motifs (where X is any amino acid other than proline) found in elastin, a protein responsible for the elasticity of vertebrate tissues. In qualitative agreement with experimental measurements of water in the silks, all the peptides studied in simulation show evidence of an increase in sidechain contacts and peptide hydrogen bonds, concomitant with a decrease in radius of gyration and localized water as the temperature is raised from approximately 5 to 60° C.
PMCID: PMC3914981  PMID: 24511323
2.  NMR Studies of Thermo-responsive Behavior of an Amphiphilic Poly(asparagine) Derivative in Water 
Polymer  2014;55(1):278-286.
The thermo-responsive behavior of a unique biocompatible polymer, poly(N-substituted α/β-asparagine) derivative (PAD), has been studied with several NMR methods. The 1H and 13C solution NMR measurements of the PAD in DMSO-d6 were used to investigate the isolated polymer and perform spectral assignments. By systematic addition of D2O we have tracked structural changes due to aggregation and observed contraction of hydrophilic side chains. Solution and cross polarization / magic angle spinning (CP/MAS) 13C NMR approaches were implemented to investigate the aggregates of the PAD aqueous solution during the liquid to gel transition as the temperature was increased. At temperatures near 20 °C, all of the peaks from the PAD were observed in the 13C CP/MAS and 13C solution NMR spectra, indicating the presence of polymer chain nodes. Increasing the temperature to 40 °C resulted in a partial disentanglement of the nodes due to thermal agitation and further heating resulted in little to no additional structural changes. Deuterium T1–T2 and T2–T2 two-dimensional relaxation spectroscopies using an inverse Laplace transform, were also implemented to monitor the water–PAD interaction during the phase transition. At temperatures near 20 °C the dynamical characteristics of water were manifested into one peak in the deuterium T1–T2 map. Increasing the temperature to 40 °C resulted in several distinguishable reservoirs of water with different dynamical characteristics. The observation of several reservoirs of water at the temperature of gel formation at 40 °C is consistent with a physical picture of a gel involving a network of interconnected polymer chains trapping a fluid. Further increase in temperature to 70 °C resulted in two non-exchanging water reservoirs probed by deuterium T2–T2 measurements.
PMCID: PMC4299659  PMID: 25614708
3.  The role of mode of delivery on elastic fiber architecture and vaginal vault elasticity: a rodent model study 
Journal of the mechanical behavior of biomedical materials  2013;29:10.1016/j.jmbbm.2013.08.025.
We report on an experimental study of the role of mode of delivery and pregnancy on the architecture of vaginal elastic fibers and vaginal vault elasticity in female Sprague-Dawley rats. In primiparous rats submitted to spontaneous or Cesarean delivery and virgin rats submitted to simulated delivery, the tortuosity of elastic fibers (defined as the ratio of length to end-to-end distance) was observed to decrease when measured two days to two weeks postpartum. In addition, the measured tortuosity of elastic fibers in multiparous rats was greater than that of virgin rats. The tortuosity of elastic fibers of all rats measured at two days postpartum were found to be similar to that of multiparous rats. At two weeks postpartum the measured tortuosity of vaginal elastic fibers was indistinguishable from virgin rats, regardless of the delivery method. Borrowing from the field of polymer physics, a model is suggested that connects elastic fiber tortuosity to the resulting tension under an applied stress; fibers having high tortuosity are expected to provide less structural support than more linear, low tortuosity fibers. To probe the macroscopic effects in elasticity due to architectural changes observed in elastic fibers, we have measured the stiffness of the vaginal vault in each cohort using a pressure-infusion system. The vaginal vault stiffness of all primiparous rats measured two weeks postpartum was greater than that measured two days postpartum. In addition, the vaginal vault of virgin rats was stiffer than that of multiparous rats. These observations confirmed that vaginal vault elastic fibers undergo significant remodeling due to pregnancy and parturition, and that the complex remodeling may be a significant contributor to tissue elasticity. Remarkably, regardless of the mode of delivery or simulated tissue trauma, elastic fiber tortuosity is observed to decrease from two days to two weeks postpartum indicating the onset of repair and recovery of tissue stiffness.
PMCID: PMC3857332  PMID: 24099948
Elastic Fiber; Pelvic Organ Prolapse
4.  Effects of experimental imperfections on a spin counting experiment 
Spin counting NMR is an experimental technique that allows a determination of the size and time evolution of networks of dipolar coupled nuclear spins. This work reports on an average Hamiltonian treatment of two spin counting sequences and compares the efficiency of the two cycles in the presence of flip errors, RF inhomogeneity, phase transients, phase errors, and offset interactions commonly present in NMR experiments. Simulations on small quantum systems performed using the two cycles reveal the effects of pulse imperfections on the resulting multiple quantum spectra, in qualitative agreement with the average Hamiltonian calculations. Experimental results on adamantane are presented, demonstrating differences in the two sequences in the presence of pulse errors.
PMCID: PMC3667418  PMID: 23648319
5.  Multispin correlations and pseudo-thermalization of the transient density matrix in solid-state NMR: free induction decay and magic echo 
Quantum unitary evolution typically leads to thermalization of generic interacting many-body systems. There are very few known general methods for reversing this process, and we focus on the magic echo, a radio-frequency pulse sequence known to approximately “rewind” the time evolution of dipolar coupled homonuclear spin systems in a large magnetic field. By combining analytic, numerical, and experimental results we systematically investigate factors leading to the degradation of magic echoes, as observed in reduced revival of mean transverse magnetization. Going beyond the conventional analysis based on mean magnetization we use a phase encoding technique to measure the growth of spin correlations in the density matrix at different points in time following magic echoes of varied durations and compare the results to those obtained during a free induction decay (FID). While considerable differences are documented at short times, the long-time behavior of the density matrix appears to be remarkably universal among the types of initial states considered – simple low order multispin correlations are observed to decay exponentially at the same rate, seeding the onset of increasingly complex high order correlations. This manifestly athermal process is constrained by conservation of the second moment of the spectrum of the density matrix and proceeds indefinitely, assuming unitary dynamics.
PMCID: PMC3661221  PMID: 23710125
6.  NMR Studies of Localized Water and Protein Backbone Dynamics in Mechanically Strained Elastin 
The journal of physical chemistry. B  2011;115(47):13935-13942.
We report on measurements of the dynamics of localized waters of hydration and the protein backbone of elastin, a remarkable resilient protein found in vertebrate tissues, as a function of the applied external strain. Using deuterium 2D T1–T2 NMR, we separate four reservoirs in the elastin–water system characterized by water with distinguishable mobilities. The measured correlation times corresponding to random tumbling of water localized to the protein is observed to decrease with increasing strain and is interpreted as an increase in its orientational entropy. The NMR T1 and T1ρ relaxation times of the carbonyl and aliphatic carbons of the protein backbone are measured and indicate a reduction in the correlation time as the elastomer strain is increased. It is argued, and supported by MD simulation of a short model elastin peptide [VPGVG]3, that the observed changes in the backbone dynamics give rise to the development of an entropic elastomeric force that is responsible for elastins’ remarkable elasticity.
PMCID: PMC3622950  PMID: 22017547
7.  Thermal hysteresis in the backbone and side chain dynamics of the elastin mimetic peptide [VPGVG]3 revealed by 2H NMR 
We report on experimental measurements of the backbone and side chain dynamics of the elastin mimetic peptide [VPGVG]3 by 2H NMR echo spectroscopy and 2D T1-T2 correlation relaxometry. The T1 and T2 relaxation times of the Gly α-deuterons and Val α-, β- and γ- deuterons of a hydrated sample reveal a thermal hysteresis when the temperature is raised from −10°C to 45°C and then subsequently cooled back to −10°C. In addition, near 30°C we observe a reduction in the slope of the T1(T) and T2(T) heating curves, indicating a structural change that appears to be correlated well to the known inverse temperature transition of this peptide. The thermal dependence of the correlation times of the Gly α-deuterons are well fit by an Arrhenius Law, from which we have measured Eact =(20.0 ± 3.1) kJ/mol when the sample is heated, and Eact =(10.9 ± 2.8) kJ/mol when cooled. Molecular dynamics simulations support the notion that the measured activation energy is determined largely by the extent of localized water, which is observed to decrease with increasing temperature from approximately 25°C to 42°C.
PMCID: PMC3257400  PMID: 22142235
8.  Investigation of the effect of a variety of pulse errors on spin I=1 quadrupolar alignment echo spectroscopy 
We report on an analysis of a well known three-pulse sequence for generating and detecting spin I=1 quadrupolar order when various pulse errors are taken into account. In the situation of a single quadrupolar frequency, such as the case found in a single crystal, we studied the potential leakage of single and/or double quantum coherence when a pulse flip error, finite pulse width effect, RF transient or a resonance offset is present. Our analysis demonstrates that the four-step phase cycling scheme studied is robust in suppressing unwanted double and single quantum coherence as well as Zeeman order that arise from the experimental artifacts, allowing for an unbiased measurement of the quadrupolar alignment relaxation time, T1Q. This work also reports on distortions in quadrupolar alignment echo spectra in the presence of experimental artifacts in the situation of a powdered sample, by simulation. Using our simulation tool, it is demonstrated that the spectral distortions associated with the pulse artifacts may be minimized, to some extent, by optimally choosing the time between the first two pulses. We highlight experimental results acquired on perdeuterated hexamethylbenzene and polyethelene that demonstrate the efficacy of the phase cycling scheme for suppressing unwanted quantum coherence when measuring T1Q. It is suggested that one employ two separate pulse sequences when measuring T1Q to properly analyze the short time behavior of quadrupolar alignment relaxation data.
PMCID: PMC3148855  PMID: 21664160
Quadrupolar Relaxation; Quadrupolar Interaction; Multiple Quantum Filtering; T1Q
9.  Measurement of the Exchange Rate of Waters of Hydration in Elastin by 2D T2-T2 Correlation Nuclear Magnetic Resonance Spectroscopy 
New journal of physics  2011;13:2-16.
We report on the direct measurement of the exchange rate of waters of hydration in elastin by T2-T2 exchange spectroscopy. The exchange rates in bovine nuchal ligament elastin and aortic elastin at temperatures near, below and at the physiological temperature are reported. Using an Inverse Laplace Transform (ILT) algorithm, we are able to identify four components in the relaxation times. While three of the components are in good agreement with previous measurements that used multi-exponential fitting, the ILT algorithm distinguishes a fourth component having relaxation times close to that of free water and is identified as water between fibers. With the aid of scanning electron microscopy, a model is proposed allowing for the application of a two-site exchange analysis between any two components for the determination of exchange rates between reservoirs. The results of the measurements support a model (described elsewhere [1]) wherein the net entropy of bulk waters of hydration should increase upon increasing temperature in the inverse temperature transition.
PMCID: PMC3144479  PMID: 21804764
10.  Investigation of the dynamical properties of water in elastin by deuterium Double Quantum Filtered NMR 
The anisotropic motion of tightly bound waters of hydration in bovine nuchal ligament elastin has been studied by deuterium Double Quantum Filtered (DQF) NMR. The experiments have allowed for a direct measurement of the degree of anisotropy within pores of elastin over a time scale ranging from 100 μs to 30 ms, corresponding to a tortuous spatial displacement ranging from 0.2 to 7 μm. We studied the anisotropic motion of deuterium nuclei in D2O hydrated elastin over a temperature of −15 °C to 37 °C and in solvents with varying dielectric constants. Our experimental measurements of the residual quadrupolar interaction as a function of temperature are correlated to the existing notion of hydrophobic collapse near 20 °C.
PMCID: PMC2925226  PMID: 20452263
Double Quantum Filter; Quadrupolar interaction; Elastin; Nuchal Ligament; fibers; Deuterium NMR
11.  Simulation studies of instrumental artifacts on spin I=1 double quantum filtered NMR spectroscopy 
We report on the results of a simulation based study of the effect of various experimental artifacts for spin I=1 double quantum filtered NMR. The simulation captures the effects of static field inhomogeneity, finite pulse widths, phase errors, transients and radio frequency inhomogeneity. We simulated the spectral distortions introduced under these errors for four, eight and sixteen step phase cycles that are well known in the NMR community. The dominating pulse errors are radio frequency field inhomogeneity and antisymmetric pulse transients. These errors result in the reduction of signal intensity as well as an introduction of distortions in the detected double quantum filtered spectrum. Using the simulation tool we studied the improvement one obtains when implementing a sixteen step phase cycle over a four step phase cycle. The results indicate that implementing a sixteen step phase cycle over an eight or four step phase cycle does not result in a significant reduction in the DQF intensity loss, or reduction in spectral distortions for antisymmetric transients.
PMCID: PMC2976484  PMID: 20451432

Results 1-11 (11)