Two-point hydrogen bonding between acid and base functionalities provides a convenient method for the modular assembly of proton-coupled electron transfer (PCET) networks, especially when that interface comprises an amidinium and two-point anionic partner; a system is presented that permits the proton configuration within the interface to be determined when pKa values of the conjugate acids are known.
observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates
from a Co(II) impurity. This impurity is oxidized to produce the well-known
Co-OEC heterogeneous cobaltate catalyst, which is an active water
oxidation catalyst. We present results from electron paramagnetic
resonance spectroscopy, nuclear magnetic resonance line broadening
analysis, and electrochemical titrations to establish the existence
of the Co(II) impurity as the major source of water oxidation activity
that has been reported for Co4O4 molecular cubanes.
Differential electrochemical mass spectrometry is used to characterize
the fate of glassy carbon at water oxidizing potentials and demonstrate
that such electrode materials should be used with caution for the
study of water oxidation catalysis.
Substrate turnover in class Ia ribonucleotide reductase (RNR) requires reversible radical transport across two subunits over 35 A, which occurs by a multi-step proton-coupled electron transfer mechanism. Using a photooxidant-labeled β2 subunit of Escherichia coli class Ia RNR, we demonstrate photoinitiated oxidation of a tyrosine in an α2:β2 complex, which results in substrate turnover. Using site-directed mutations of the redox-active tyrosines at the subunit interface—Y356F(β) and Y731F(α)—this oxidation is identified to be localized on Y356. The rate of Y356 oxidation depends on the presence of Y731 across the interface. This observation supports the proposal that unidirectional PCET across the Y356(β)–Y731(α)–Y730(α) triad is crucial to radical transport in RNR.
reductases (RNRs) catalyze the conversion of nucleotides
to deoxynucleotides in all organisms. Active E. coli class Ia RNR is an α2β2 complex
that undergoes reversible, long-range proton-coupled electron transfer
(PCET) over a pathway of redox active amino acids (β-Y122 → [β-W48] → β-Y356 → α-Y731 → α-Y730 → α-C439) that spans ∼35 Å.
To unmask PCET kinetics from rate-limiting conformational changes,
we prepared a photochemical RNR containing a [ReI] photooxidant
site-specifically incorporated at position 355 ([Re]-β2), adjacent to PCET pathway residue Y356 in β. [Re]-β2 was further modified by replacing Y356 with 2,3,5-trifluorotyrosine
to enable photochemical generation and spectroscopic observation of
chemically competent tyrosyl radical(s). Using transient absorption
spectroscopy, we compare the kinetics of Y· decay in the presence
of substrate and wt-α2, Y731F-α2 ,or C439S-α2, as well as with
3′-[2H]-substrate and wt-α2. We
find that only in the presence of wt-α2 and the unlabeled
substrate do we observe an enhanced rate of radical decay indicative
of forward radical propagation. This observation reveals that cleavage
of the 3′-C–H bond of substrate by the transiently formed
C439· thiyl radical is rate-limiting in forward PCET
through α and has allowed calculation of a lower bound for the
rate constant associated with this step of (1.4 ± 0.4) ×
104 s–1. Prompting radical propagation
with light has enabled observation of PCET events heretofore inaccessible,
revealing active site chemistry at the heart of RNR catalysis.
Proton-coupled electron transfer (PCET) underpins energy conversion in biology. PCET may occur with the unidirectional or bidirectional transfer of a proton and electron and may proceed synchronously or asynchronously. To illustrate the role of PCET in biology, this review presents complementary biological and model systems that explore PCET in electron transfer (ET) through hydrogen bonds [azurin as compared to donor-acceptor (D–A) hydrogen-bonded networks], the activation of C–H bonds [alcohol dehydrogenase and soybean lipoxygenase (SLO) as compared to Fe(III) metal complexes], and the generation and transport of amino acid radicals [photosystem II (PSII) and ribonucleotide reductase (RNR)as compared to tyrosine-modified photoactive Re(I) and Ru(II) complexes]. In providing these comparisons, the fundamental principles of PCET in biology are illustrated in a tangible way.
amino acid radical; lipoxygenase; photosystem II; ribonucleotide reductase; tyrosine
Proton-coupled electron transfer (PCET) is a fundamental mechanism important in a wide range of biological processes including the universal reaction catalysed by ribonucleotide reductases (RNRs) in making de novo, the building blocks required for DNA replication and repair. These enzymes catalyse the conversion of nucleoside diphosphates (NDPs) to deoxynucleoside diphosphates (dNDPs). In the class Ia RNRs, NDP reduction involves a tyrosyl radical mediated oxidation occurring over 35 Å across the interface of the two required subunits (β2 and α2) involving multiple PCET steps and the conserved tyrosine triad [Y356(β2)–Y731(α2)–Y730(α2)]. We report the synthesis of an active photochemical RNR (photoRNR) complex in which a Re(I)-tricarbonyl phenanthroline ([Re]) photooxidant is attached site-specifically to the Cys in the Y356C-(β2) subunit and an ionizable, 2,3,5-trifluorotyrosine (2,3,5-F3Y) is incorporated in place of Y731 in α2. This intersubunit PCET pathway is investigated by ns laser spectroscopy on [Re356]-β2:2,3,5-F3Y731-α2 in the presence of substrate, CDP, and effector, ATP. This experiment has allowed analysis of the photoinjection of a radical into α2 from β2 in the absence of the interfacial Y356 residue. The system is competent for light-dependent substrate turnover. Time-resolved emission experiments reveal an intimate dependence of the rate of radical injection on the protonation state at position Y731(α2), which in turn highlights the importance of a well-coordinated proton exit channel involving the key residues, Y356 and Y731, at the subunit interface.
Hydrogen-bonded interfaces are essential structural elements in biology. Furthermore they can mediate electron transport by coupling the electron to proton transfer within the interface. The specific hydrogen-bonding configuration and strength have a large impact on the proton transfer, which exchanges the hydrogen bonded donor and acceptor species (i.e. NH---O → N---HO. Modulations of the hydrogen-bonding environment, such as the hydrogen-bond stretch and twist modes, affect the proton transfer dynamics. Here we present transient grating and echo peak shift measurements of the NH stretch vibrations of four doubly hydrogen-bonded cyclic dimers in their electronic ground state. The equilibrium vibrational dynamics exhibit strong coherent modulations that we attribute to coupling of the high-frequency NH vibration to the low frequency inter-dimer stretch and twist modes and not to interference between multiple Fermi-resonances that dominate the substructure of the linear spectra.
The presentation of two phenols on a xanthene backbone is akin to the tyrosine dyad (Y730 and Y731) of ribonucleotide reductase. X-ray crystallography reveals that the two phenol moieties are cofacially disposed at 4.35 Å. Cyclic voltammetry (CV) reveals that phenol oxidation is modulated within the dyad, which exhibits a splitting of one-electron waves with the second oxidation of the phenol dyad occurring at larger positive potential than that of a typical phenol. In contrast, a single phenol appended to a xanthene exhibits a two-electron (ECE) process, consistent with reported oxidation pathways of phenols in acetonitrile. The perturbation of the phenol potential by stacking is reminiscent of a similar effect for guanines stacked within DNA base pairs.
Polynuclear transition metal complexes,
which frequently constitute
the active sites of both biological and chemical catalysts, provide
access to unique chemical transformations that are derived from metal–metal
cooperation. Reductive elimination via ligand-bridged binuclear intermediates
from bimetallic cores is one mechanism by which metals may cooperate
during catalysis. We have established families of Rh2 complexes
that participate in HX-splitting photocatalysis in which metal–metal
cooperation is credited with the ability to achieve multielectron
photochemical reactions in preference to single-electron transformations.
Nanosecond-resolved transient absorption spectroscopy, steady-state
photocrystallography, and computational modeling have allowed direct
observation and characterization of Cl-bridged intermediates (intramolecular
analogues of classical ligand-bridged intermediates in binuclear eliminations)
in halogen elimination reactions. On the basis of these observations,
a new class of Rh2 complexes, supported by CO ligands,
has been prepared, allowing for the isolation and independent characterization
of the proposed halide-bridged intermediates. Direct observation of
halide-bridged structures establishes binuclear reductive elimination
as a viable mechanism for photogenerating energetic bonds.
A ratiometric fluorescent pH sensor based on CdSe/CdZnS nanocrystal quantum dots (NCs) has been designed for biological pH ranges. The construct is formed from the conjugation of a pH dye (SNARF) to NCs coated with a poly(amido amine) (PAMAM) dendrimer. The sensor exhibits a well–resolved ratio response at pH values between 6 and 8 under linear or two–photon excitation, and in the presence of a 4% bovine serum albumin (BSA) solution.
Electrochemical energy-storage devices have the potential to be clean and efficient, but their current cost and performance limit their use in numerous transportation and stationary applications. Many organic molecules are abundant, economical and electrochemically active; if selected correctly and rationally designed, these organic molecules offer a promising route to expand the applications of these energy-storage devices. In this study, polycyclic aromatic hydrocarbons are introduced within a functionalized few-walled carbon nanotube matrix to develop high-energy, high-power positive electrodes for pseudocapacitor applications. The reduction potential and capacity of various polycyclic aromatic hydrocarbons are correlated with their interaction with the functionalized few-walled carbon nanotube matrix, chemical configuration and electronic structure. These findings provide rational design criteria for nanostructured organic electrodes. When combined with lithium negative electrodes, these nanostructured organic electrodes exhibit energy densities of ∼350 Wh kg−1electrode at power densities of ∼10 kW kg−1electrode for over 10,000 cycles.
Electrochemically active organic molecules are an important class of electrode materials for energy storage. Here, the authors report organic electrodes made of polycyclic aromatic hydrocarbons and functionalized few-walled carbon nanotubes, which show promising electrochemical performance.
Incorporation of 2,3,6-trifluorotyrosine (F3Y) and a rhenium bipyridine ([Re]) photooxidant into a peptide corresponding to the C-terminus of the β protein (βC19) of Escherichia coli ribonucleotide reductase (RNR) allows for the temporal monitoring of radical transport into the α2 subunit of RNR. Injection of the photogenerated F3Y radical from the [Re]–F3Y–βC19 peptide into the surface accessible Y731 of the α2 subunit is only possible when the second Y730 is present. With the Y–Y established, radical transport occurs with a rate constant of 3 × 105 s−1. Point mutations that disrupt the Y–Y dyad shut down radical transport. The ability to obviate radical transport by disrupting the hydrogen bonding network of the amino acids composing the co-linear proton-coupled electron transfer pathway in α2 suggests a finely tuned evolutionary adaptation of RNR to control the transport of radicals in this enzyme.
Ribonucleotide Reductase; Proton-Coupled Electron Transfer; Radical Transport
Acidity, hypoxia and glucose levels characterize the tumor microenvironment rendering pH, pO2 and pGlucose, respectively, important indicators of tumor health. To this end, understanding how these parameters change can be a powerful tool for the development of novel and effective therapeutics. We have designed optical chemosensors that feature a quantum dot and an analyte-responsive dye. These non-invasive chemosensors permit pH, oxygen, and glucose to be monitored dynamically within the tumor microenvironment by using multiphoton imaging.
Supramolecular assemblies of a quantum dot (QD) associated to palladium(II) porphyrins have been developed to detect oxygen (pO2) in organic solvents. Palladium porphyrins are sensitive in the 0–160 torr range, making them ideal phosphors for in vivo biological oxygen quantification. Porphyrins with meso pyridyl substituents bind to the surface of the QD to produce self–assembled nanosensors. Appreciable overlap between QD emission and porphyrin absorption features results in efficient Förster resonance energy transfer (FRET) for signal transduction in these sensors. The QD serves as a photon antenna, enhancing porphyrin emission under both one– and two–photon excitation, demonstrating that QD–palladium porphyrin conjugates may be used for oxygen sensing over physiological oxygen ranges.
We report single-particle photoluminescence (PL) intermittency (blinking) with high on-time fractions in colloidal CdSe quantum dots (QD) with conformal CdS shells of 1.4 nm thickness, equivalent to approximately 4 CdS monolayers. All QDs observed displayed on-time fractions > 60% with the majority > 80%. The high-on-time-fraction blinking is accompanied by fluorescence quantum yields (QY) close to unity (up to 98% in an absolute QY measurement) when dispersed in organic solvents and a monoexponential ensemble photoluminescence (PL) decay lifetime. The CdS shell is formed in high synthetic yield using a modified selective ion layer adsorption and reaction (SILAR) technique that employs a silylated sulfur precursor. The CdS shell provides sufficient chemical and electronic passivation of the QD excited state to permit water solubilization with greater than 60% QY via ligand exchange with an imidazole-bearing hydrophilic polymer.
Energy transfer between a CdSe/ZnS nanocrystal (NC) donor and a rhodamine isothiocyanate (RITC) acceptor has been achieved via a functionalized poly(acrylic acid) (PAA) encapsulating layer over the surface of the NC. The modification of PAA with both N-octylamine (OA) and 5-amino-1-pentanol (AP), [PAA-OA-AP], allows for the simultaneous water-solubilization and functionalization of the NCs, underscoring the ease of synthesizing NC-acceptor conjugates with this strategy. Photophysical studies of the NC-RITC constructs showed that energy transfer is efficient, with kFRET approaching 108 s−1. The ease of the covalent conjugation of molecules to NCs with PAA-OA-AP coating, together with efficient energy transfer, makes the NCs encapsulated with PAA-OA-AP attractive candidates for sensing applications.
Fold-up fluorophore: A new paradigm for designing self-referencing fluorescent nanosensors is demonstrated by interfacing a pH-triggered molecular conformational switch with quantum dots. Analytedependent, large-amplitude conformational motion controls the distance between the nanocrystal energy donor and an organic FRET acceptor. The result is a fluorescence signal capable of reporting pH values from individual endosomes in living cells.
biosensors; fluorescent probes; quantum dots; molecular devices; molecular machines
Amidine-appended ferrocene derivatives form a supramolecular assembly with Ru(ii)(bpy-COOH) (L)22+ complexes (bpy-COOH is 4-CO2H-4′-CH3-bpy and L = bpy, 2,2′-bipyridine or btfmbpy, 4,4′-bis (trifluoromethyl)-2,2′-bipyridine). Steady-state, time-resolved spectroscopy and kinetic isotope effects establish that the metal-to-ligand charge transfer excited states of the Ru(ii) complexes are quenched by proton-coupled energy transfer (PCEnT). These results show that proton motion can be effective in mediating not only electron transfer (ET) but energy transfer (EnT) as well.
Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.
Amino acid radical generation and transport are fundamentally important to numerous essential biological processes to which small molecule models lend valuable mechanistic insights. Pyridyl-amino acid-methyl esters are appended to a rhenium(I) tricarbonyl 1,10-phenanthroline core to yield rhenium–amino acid complexes with tyrosine ([Re]–Y–OH) and phenylalanine ([Re]–F). The emission from the [Re] center is more significantly quenched for [Re]–Y–OH upon addition of base. Time-resolved studies establish that excited-state quenching occurs by a combination of static and dynamic mechanisms. The degree of quenching depends on the strength of the base, consistent with a proton-coupled electron transfer (PCET) quenching mechanism. Comparative studies of [Re]–Y–OH and [Re]–F enable a detailed mechanistic analysis of a bidirectional PCET process.
Photochemical ribonucleotide reductases (photoRNRs) have been developed to study the proton-coupled electron transfer (PCET) mechanism of radical transport in class I E. coli ribonucleotide reductase (RNR). The transport of the effective radical occurs along several conserved aromatic residues across two subunits: β2(•Y122→W48→Y356)→ α2(Y731→Y730→C439). The current model for RNR activity suggests that radical transport is strongly controlled by conformational gating. The C-terminal tail peptide (Y- βC19) of β2 is the binding determinant of β2 to α2 and contains the redox active Y356 residue. A photoRNR has been generated synthetically by appending a Re(bpy)(CO)3CN ([Re]) photo-oxidant next to Y356 of the 20-mer peptide. Emission from the [Re] center dramatically increases upon peptide binding, serving as a probe for conformational dynamics and Y356 protonation state. The diffusion coefficient of [Re]-Y-βC19 has been measured (kd1 = 6.1 × 10−7 cm−1 s–1), along with the dissociation rate constant for the [Re]-Y-βC19:α2 complex (7000 s−1 > koff > 400 s−1). Results from detailed time-resolved emission and absorption spectroscopies reveal biexponential kinetics, suggesting a large degree of conformational flexibility in the α2:[Re]-Y-βC19 complex that partitions the N-terminus of the peptide into both bound and solvent-exposed fractions.
Two nanocrystal-osmium(II) polypyridyl (NC-Os(II)PP) conjugates have been designed to detect oxygen in biological environments. Polypyridines appended with a single free amine were linked with facility to a carboxylic acid functionality of a semiconductor NC overlayer to afford a biologically stable amide bond. The Os(II)PP complexes possess broad absorptions that extend into the red spectral region; this absorption feature makes them desirable acceptors of energy from NC donors. Fluorescence resonance energy transfer (FRET) from the NC to the Os(II)PP causes an enhanced Os(II)PP emission with a concomitant quenching of the NC emission. Owing to the large two-photon absorption cross-section of the NCs, FRET from NC to the Os(II)PP can be established under two-photon excitation conditions. In this way, two-photon processes of metal polypyridyl complexes can be exploited for sensing. The emission of the NC is insensitive to oxygen, even at 1 atm, whereas excited states of both osmium complexes are quenched in the presence of oxygen. The NC emission may thus be used as an internal reference to correct for fluctuations in the photoluminescence intensity signal. These properties taken together establish NC-Os(II)PP conjugates as competent ratiometric, two-photon oxygen sensors for application in biological microenvironments.
Personalized energy (PE) is a transformative idea that provides a new modality for the planet’s energy future. By providing solar energy to the individual, an energy supply becomes secure and available to people of both legacy and non-legacy worlds, and minimally contributes to increasing the anthropogenic level of carbon dioxide. Because PE will be possible only if solar energy is available 24 hours a day, 7 day a week, the key enabler for solar PE is an inexpensive storage mechanism. HX (X = halide or OH−) splitting is a fuel-forming reaction of sufficient energy density for large scale solar storage but the reaction relies on chemical transformations that are not understood at the most basic science level. Critical among these are multielectron transfers that are proton-coupled and involve the activation of bonds in energy poor substrates. The chemistry of these three italicized areas is developed, and from this platform, discovery paths leading to new HX and H2O splitting catalysts are delineated. For the case of the water splitting catalyst, it captures many of the functional elements of photosynthesis. In doing so, a highly manufacturable and inexpensive method has been discovered for solar PE storage.
A Zn(II) porphyrin-amidinium is the excited state electron donor (D) to a naphthalene diimide acceptor (A) appended with either a carboxylate or sulfonate functionality. The two-point hydrogen bond (---[H+]---) formed between the amidinium and carboxylate or sulfonate establishes a proton-coupled electron transfer (PCET) pathway for charge transfer. The two D---[H+]---A assemblies differ only by the proton configuration within the hydrogen bonding interface. Specifically, the amidinium transfers a proton to the carboxylate to form a non-ionized amidine-carboxylic acid two-point hydrogen network whereas the amidinium maintains both protons when bound to the sulfonate functionality forming an ionized amidinium-sulfonate two-point hydrogen network. These two interface configurations within the dyads thus allow for a direct comparison of PCET kinetics for the same donor and acceptor juxtaposed by an ionized and non-ionized hydrogen-bonded interface. Analysis of PCET kinetics ascertained from transient absorption and transient emission spectroscopy reveal that the ionized interface is more strongly impacted by the local solvent environment, thus establishing that the initial static configuration of the proton interface is a critical determinant to the kinetics of PCET.