Showing papers on "Proton-coupled electron transfer published in 2001"
TL;DR: This theoretical framework allows predictions of rates, mechanisms, and kinetic isotope effects for proton-coupled electron transfer reactions.
Abstract: This Account presents a theoretical formulation for proton-coupled electron transfer reactions. The active electrons and transferring protons are treated quantum mechanically, and the free energy surfaces are obtained as functions of collective solvent coordinates corresponding to the proton and electron transfer reactions. Rate expressions have been derived in the relevant limits, and methodology for including the dynamical effects of the solvent and protein has been developed. This theoretical framework allows predictions of rates, mechanisms, and kinetic isotope effects for proton-coupled electron transfer reactions.
405 citations
TL;DR: In this article, the same authors performed ab initio (RHF, MP2, CASSCF, and CASPT2) calculations for the electronic ground and lowest excited singlet states of phenol.
Abstract: Ab initio (RHF, MP2, CASSCF, and CASPT2) calculations have been performed for the electronic ground and lowest excited singlet states of phenol, the complexes of phenol with water and ammonia, and the corresponding cations. In agreement with recent experiments it is found that proton transfer is a barrierless process in the phenol−(H2O)3 and phenol−NH3 cations, whereas no proton transfer occurs in the phenol−H2O cation. Novel aspects of the reaction dynamics in the excited-state manifold of the neutral clusters are revealed by the calculations. Predissociation of the S1(ππ*) state by a low-lying 1πσ* state leads to a concerted electron and proton-transfer reaction from the chromophore to the solvent. The excited-state reaction is endothermic in phenol−H2O and phenol−(H2O)3 clusters but exothermic (though activated) in the phenol−NH3 complex. These results substantiate recent reinterpretations of spectroscopic and kinetic data on hydrogen-transfer reactions in phenol−ammonia clusters. The close relationshi...
267 citations
TL;DR: It is concluded that at the Ag/SAM interface the energy barrier for the PT processes of the adsorbed Cyt-c is raised by the electric field, which may represent a possible mechanism for controlling biological redox reactions via changes of the transmembrane potential.
Abstract: Cytochrome c (Cyt-c) was electrostatically bound to self-assembled monolayers (SAM) on an Ag electrode, which are formed by ω-carboxyl alkanethiols of different chain lengths (Cx). The dynamics of the electron-transfer (ET) reaction of the adsorbed heme protein, initiated by a rapid potential jump to the redox potential, was monitored by time-resolved surface enhanced resonance Raman (SERR) spectroscopy. Under conditions of the present experiments, only the reduced and oxidized forms of the native protein state contribute to the SERR spectra. Thus, the data obtained from the spectra were described by a one-step relaxation process yielding the rate constants of the ET between the adsorbed Cyt-c and the electrode for a driving force of zero electronvolts. For C16- and C11-SAMs, the respective rate constants of 0.073 and 43 s-1 correspond to an exponential distance dependence of the ET (β = 1.28 A-1), very similar to that observed for long-range intramolecular ET of redox proteins. Upon further decreasing th...
178 citations
TL;DR: The Marcus theory formalism based on ground-state energetics and self-exchange rates, originally developed for electron transfer processes, is also valuable for hydrogen atom transfer.
Abstract: The transfer of a hydrogen atom—a proton and an electron—is a fundamental process in chemistry and biology. A variety of hydrogen atom transfer reactions, involving iron complexes, phenols, hydroxylamines, tBuOOH, toluene, and related radicals, are shown to follow the Marcus cross relation. Thus, the Marcus theory formalism based on ground-state energetics and self-exchange rates, originally developed for electron transfer processes, is also valuable for hydrogen atom transfer. Compounds that undergo slow proton transfer (C–H bonds) or slow electron transfer (cobalt complexes) also undergo slow hydrogen atom transfer. Limitations of this approach are also discussed.
153 citations
147 citations
TL;DR: H-bonding between Y(Z) and His, together with the Coulombic interaction between negative tyrosinate and positive imidazolium, are suggested to play a decisive role in alleviating constraints in the water oxidase.
122 citations
TL;DR: In this article, the results of experimental studies on photo-induced electron transfer in DNA were presented and analyzed within the framework of semi-classical electron transfer theory, and the π-stacked bases of DNA provided a better medium for electron transfer than the sigma-bonded pathways of proteins and saturated hydrocarbons.
Abstract: Photoinduced electron transfer (PET) in DNA can occur via one of two mechanisms, single-step superexchange and multi-step hole hopping. The dynamics of superexchange charge separation and charge recombination has recently been investigated in several structurally well-defined systems. In each of these systems, an electron acceptor is separated from a guanine or deazaguanine nucleobase donor by a variable number of A:T base pairs. The results of experimental studies on these and related systems are presented and analyzed within the framework of semi-classical electron transfer theory. Comparison of the results with those for other bridge-mediated electron transfer systems indicates that the π-stacked bases of DNA provide a better medium for electron transfer than the sigma bonded pathways of proteins and saturated hydrocarbons but do not function as a molecular wire.
88 citations
TL;DR: In this paper, the photophysical and photochemical properties of excited-state double proton transfer (ESDPT) in 7-azaindole and its corresponding analogues are reviewed.
Abstract: Contemporary progress regarding guest/host types of excited-state double proton transfer has been reviewed, among which are the biprotonic transfer within doubly H-bonded host/guest complexes, the transfer through a solvent bridge relay, the intramolecular double proton transfer and solvation dynamics coupled proton transfer. Of particular emphases are the photophysical and photochemical properties of excited-state double proton transfer (ESDPT) in 7-azaindole and its corresponding analogues. From the chemical aspect, two types of ESDPT reaction, namely the catalytic and non-catalytic types of ESDPT, have been classified and reviewed separately. For the case of static host/guest hydrogen-bonded complexes both hydrogen-bonding strength and configuration (i.e. geometry) play key roles in accounting for the reaction dynamics. In addition to the dynamical concern, excited-state thermodynamics are of importance to fine-tune the proton transfer reaction in the non-catalytic host/guest type of ESDPT. The mechanisms of protic solvent assisted ESDPT, depending on host molecules and proton-transfer models, have been reviewed where the plausible resolution is deduced. Particular attention has been given to the excited-state proton transfer dynamics in pure water, aiming at its future perspective in biological applications. Finally, the differentiation in mechanism between solvent diffusive reorganization and solvent relaxation to affect the host/guest ESPT dynamics is made and discussed in de tail.
79 citations
TL;DR: A comparative theoretical investigation of single electron transfer, single proton transfer (PT), and proton-coupled electron transfer (PCET) reactions in iron bi-imidazoline complexes is presented, indicating that the similarity of the rates for ET and PCET is due mainly to the compensation of the smaller outer-sphere solvent reorganization energy for PCET by the larger coupling for ET.
Abstract: A comparative theoretical investigation of single electron transfer (ET), single proton transfer (PT), and proton-coupled electron transfer (PCET) reactions in iron bi-imidazoline complexes is presented. These calculations are motivated by experimental studies showing that the rates of ET and PCET are similar and are both slower than the rate of PT for these systems (Roth, J. P.; Lovel, S.; Mayer, J. M. J. Am. Chem. Soc. 2000, 122, 5486). The theoretical calculations are based on a multistate continuum theory, in which the solute is described by a multistate valence bond model, the transferring hydrogen nucleus is treated quantum mechanically, and the solvent is represented as a dielectric continuum. For electronically nonadiabatic electron transfer, the rate expressions for ET and PCET depend on the inner-sphere (solute) and outer-sphere (solvent) reorganization energies and on the electronic coupling, which is averaged over the reactant and product proton vibrational wave functions for PCET. The small o...
71 citations
TL;DR: The kinetic model shows that the radical pair P700(+)A(0)(-) forms with 1.3 ps rise kinetics after creation of electronically excited P700*, which is appreciably faster than previously estimated by other groups.
Abstract: The charge separation P700*A(0) --> P700(+)A(0)(-) and the subsequent electron transfer from the primary to secondary electron acceptor have been studied by subtracting absorption difference profiles for cyanobacterial photosystem I (PS I) complexes with open and closed reaction centers. Samples were excited at 660 nm, which lies toward the blue edge of the core antenna absorption spectrum. The resulting PS I kinetics were analyzed in terms of the relevant P700, P700(+), A(0), and A(0)(-) absorption spectra. In our kinetic model, the radical pair P700(+)A(0)(-) forms with 1.3 ps rise kinetics after creation of electronically excited P700*. The formation of A(1)(-) via electron transfer from A(0)(-) requires approximately 13 ps. The kinetics of the latter step are appreciably faster than previously estimated by other groups (20--50 ps).
68 citations
TL;DR: Kinetic solvent isotope effects on the electron-transfer kinetics associated with the oxidation of guanine by a 2-aminopurine (2AP) neutral radical separated by 2 or 3 thymine, or 6 adenine residues on the same strand in 15-mer double-stranded oligonucleotides in H2O or D2O were measured.
Abstract: Kinetic solvent isotope effects on the electron-transfer kinetics associated with the oxidation of guanine by a 2-aminopurine (2AP) neutral radical separated by 2 or 3 thymine, or 6 adenine residues on the same strand in 15-mer double-stranded oligonucleotides in H2O or D2O were measured. The evolution in time of the oxidized form of guanine, the neutral radical G(−H)•, and the electron acceptor, the neutral 2AP(−H)• radical, were followed directly by a spectroscopic laser pulse-induced transient absorption technique on a 1−200 μs time scale. About 70−100% of this one-electron-transfer reaction occurs on this relatively slow time scale in the three oligonucleotide duplexes studied. The rate constants of formation of G(−H)• in the oligonucleotides are larger in H2O than in D2O by a factor of 1.3−1.7. This kinetic isotope effect suggests that the electron-transfer reaction from G to 2AP(−H)• is coupled to a deprotonation of G•+, and a protonation of 2AP-, the primary products of the electron-transfer reacti...
TL;DR: The free energy dependence of electron transfer in a few small-molecule donor--acceptor systems having hydrogen-bonding appendages was studied and it was observed that the rate depends very feebly on distance, but distance dependence studies did not confirm the prediction of enhanced rates at larger distances in the inverted region.
Abstract: The free energy dependence of electron transfer in a few small-molecule donor−acceptor systems having hydrogen-bonding appendages was studied to evaluate the role of diffusion in masking the inverted region in bimolecular PET reactions. A small fraction of the probe molecules associate and this led to the simultaneous observation of unimolecular and diffusion-mediated quenching of the probe fluorescence. Free energy dependence studies showed that the unimolecular electron transfer obeys Marcus behavior and the diffusion-mediated electron transfer obeys Rehm−Weller behavior. The absence of an inverted region in bimolecular PET reactions is thus attributed to diffusion. The results of the free energy dependence studies suggest that distance dependence of electron transfer plays a role in masking the inverted region. To ascertain this aspect we have carried out a study of the distance dependence of electron transfer in the hydrogen-bonded donor−acceptor systems. For a system in the normal region an exponenti...
TL;DR: In this paper, the possibility of spontaneous proton transfer between a proton donor AH and an acceptor B has been analyzed using the hydrogen bond complexes, as suitable model systems.
Abstract: The possibility of observing a spontaneous proton transfer (PT) between a proton donor AH and a proton acceptor B has been analyzed using, as suitable model systems, the hydrogen bond complexes bet...
TL;DR: It is shown by optical spectroscopy that about one half of the fastest rate of electron transfer between the haem groups actually occurs at least three orders of magnitude faster, after photolysis of carbon monoxide from the half-reduced bovine heart enzyme.
TL;DR: The enthalpy and entropy of electron transfer in the purified photosystem I trimer complexes from Synechocystis sp.
Abstract: The thermodynamic properties of electron transfer in biological systems are far less known in comparison with that of their kinetics. In this paper the enthalpy and entropy of electron transfer in the purified photosystem I trimer complexes from Synechocystis sp. PCC 6803 have been studied, using pulsed time-resolved photoacoustics on the 1 Is time scale. The volume contraction of reaction centers of photosystem I, which results directly from the light-induced charge separation forming P 700 + FA/FB - from the excited-state P700*, is determined to be -26 ( 2A 3 . The enthalpy of the above electron-transfer reaction is found to be -0.39 ( 0.1 eV. Photoacoustic estimation of the quantum yield of photochemistry in the purified photosystem I trimer complex showed it to be close to unity. Taking the free energy of the above reaction as the difference of their redox potentials in situ allows us to calculate an apparent entropy change (T¢S )o f+0.35 ( 0.1 eV. These values of ¢V and T¢S are similar to those of bacterial reaction centers. The unexpected sign of entropy of electron transfer is tentatively assigned, as in the bacterial case, to the escape of counterions from the surface of the particles. The apparent entropy change of electron transfer in biological system is significant and cannot be neglected.
TL;DR: In this paper, the free energy transformations that occur in nitrogenase are described based on theoretical analysis of structural and electrochemical data, made possible by recent X-ray structural data and motivated by closely related electrochemical studies.
Abstract: Nitrogenase consists of two proteins that work in concert to reduce atmospheric dinitrogen to a biologically useful form, ammonia (Curr. Opin. Chem. Biol. 2000, 4, 559−566; Chem. Rev. 1996, 96, 2965−2982). The smaller of the proteins (the so-called Fe protein) shuttles high-energy electrons to the larger subunit (the so-called MoFe protein) where the reduction of dinitrogen molecules takes place. The Fe protein catalyzes the hydrolysis of two MgATP molecules per electron transferred to the MoFe protein. The physical mechanism that couples the ATP hydrolysis and electron-transfer reactions in nitrogenase is one of the “great mysteries” of nitrogen fixation. Our goal is to describe the free-energy transformations that occur in nitrogenase based upon theoretical analysis of structural and electrochemical data. The electrostatic and thermodynamic analysis described here, made possible by recent X-ray structural data (and motivated by closely related electrochemical studies: Biochemistry 1997, 36, 12976−12983...
TL;DR: The authors studied photosystem II core particles from photoautotrophic mutants where the residue D1-E189 was replaced by glutamine, arginine and lysine which were expected to electrostatically differ from the glutamate in the wild-type (WT), and found the rates of electron transfer were the same as in the WT.
TL;DR: A solvent kinetic isotope effect study of electron transfer in two complex flavoproteins, xanthine oxidase and trimethylamine dehydrogenase, has been undertaken, and a rationale is presented as to why the physical layout of redox-active sites not be uniformly increasing in reduction potential in the direction of physiological electron transfer.
TL;DR: It was found that this conformational change of the Rieske protein on linear electron transport does not limit the electron transport rate, if it occurs with a time constant of at least 1000 s(-1).
TL;DR: In this paper, a simple and efficient pH-induced molecular switching has been devised based on a symmetric bridged ruthenium dimer, where the electronic coupling and delocalization can be modulated to induce class III (deprotonated bridging ligand), class II (protonated), or even an intermediate borderline class II/class III system features.
TL;DR: In this article, the intersecting-state model is applied to calculate the energy barriers and rates of atom transfers, S N 2 reactions, proton transfers and electron transfers, including electron transfer at the metal-electrolyte interface.
TL;DR: In this paper, a new approach for the molecular dynamics simulation of proton-coupled electron transfer reactions in solution is presented, where the solute is represented by a four-state valence bond model, and the solvent is described by explicit solvent molecules.
Abstract: A new approach for the molecular dynamics simulation of proton-coupled electron transfer reactions in solution is presented. The solute is represented by a four-state valence bond model, and the solvent is described by explicit solvent molecules. The nuclear quantum effects of the transferring hydrogen are incorporated with a procedure based on a series of purely classical molecular dynamics simulations. The resulting mixed electronic/vibrational free energy surfaces depend on two solvent reaction coordinates corresponding to electron and proton transfer. This approach is shown to be equivalent to adiabatic mixed quantum/classical molecular dynamics, in which the nuclear quantum effects are included during the simulation, under well-defined, physically reasonable conditions. The results of the application of this approach to a model system are compared to those from a previous study based on a dielectric continuum treatment of the solvent. In addition, specific molecular motions of the solvent associated ...
TL;DR: The vinylene-bridged ferrocene−hydroquinone complex, 1, underwent 2-electron oxidation and 2-proton elimination by a redox reaction with 1,1‘-dichloroferrocenium hexafluorophosphate as a 1-electrons oxidizing agent in methanol to form 3, which includes a novel allene and a quinonoid structure.
Abstract: The vinylene-bridged ferrocene−hydroquinone complex, 2-(2-ferrocenylvinyl)hydroquinone, 1, underwent 2-electron oxidation and 2-proton elimination by a redox reaction with 1,1‘-dichloroferrocenium hexafluorophosphate as a 1-electron oxidizing agent in methanol to form 3, which includes a novel allene and a quinonoid structure. The quinonoid compound 3 shows interesting proton response to generate a semiquinone−ferrocenium compound 2-1 through proton-coupled intramolecular electron transfer in acetonitrile.
TL;DR: The application of photochemical methods to the study of intramolecular electron transfer in hemoproteins is reviewed, thus selecting a small, well-defined sector of this otherwise enormous field.
08 Oct 2001
TL;DR: Effects of environmental changes on the reaction pattern of excitation energy trapping and transformation into the "stable" radical pair P+Q(A)-, have been analyzed in isolated reaction centers of the anoxygenic purple bacterium Rhodobacter sphaeroides.
TL;DR: In this paper, a new method for the generation of transient neutral biradicals in liquid solution is reported, where photoinduced electron transfer in aqueous solution of the structures shown above leads to neutral Biradicals with peptide spacers.