Dissociative electron transfer reactions

We consider recent data on dissociative electron transfer reactions in which the electron transfer causes practically concerted dissociation of the chemical bond in the reagent. We discuss considerable experimental data on reactions in the gas phase and in solutions, and also existing theoretical models for describing the kinetics of these complex processes. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 2, pp. 67–78, March–April, 1998.     

Light-induced electron transfer reactions

The reactions of the luminescent excited states of the polypyridine-ruthenium(II) complexes (^*RuL₃²⁺) with electron acceptors and donors are discussed. These electron transfer reactions convert the excited state into RuL₃³⁺ and RuL₃⁺, respectively. The former ruthenium complex is a more powerful oxidant and the latter is a more powerful reductant than the excited state itself. Some applications of these complexes in the conversion and storage of solar energy are presented. Theoretical models for electron transfer reactions are described and the implications of these models for the quenching and back electron transfer reactions are discussed. It is pointed out that the exploitation of the inverted region may provide a useful means of slowing down back electron transfer reactions.     

Photochemical electron transfer reactions in alkylcobaloximes

It is shown that photolysis with visible light (λ > 420 nm) of any alkylcobaloxime procedes via a mechanism involving an initial electron transfer reaction from an equatorial ligand to the central metal to produce a cobalt(II) species which retains both original axial ligands. In a subsequent rearrangement of the equatorial ligand a hydrogen atom is ejected.     

Photochemical electron transfer reactions of tirapazamine

The absorption and fluorescence spectra of 3-aminobenzo-1,2,4-triazine di-N-oxide (tirapazamine) have been recorded and exhibit a dependence on solvent that correlates with the Dimroth ET30 parameter. Time-dependent density functional theory calculations reveal that the transition of tirapazamine in the visible region is pi-->pi* in nature. The fluorescence lifetime is 98+/-2 ps in water. The fluorescence quantum yield is approximately 0.002 in water. The fluorescence of tirapazamine is efficiently quenched by electron donors via an electron-transfer process. Linear Stern-Volmer fluorescence quenching plots are observed with sodium azide, potassium thiocyanate, guanosine monophosphate and tryptophan (Trp) methyl ester hydrochloride. Guanosine monophosphate, tyrosine (Tyr) methyl ester hydrochloride and Trp methyl ester hydrochloride appear to quench the fluorescence at a rate greater than diffusion control implying that these substrates complex with tirapazamine in its ground state. This complexation was detected by absorption spectroscopy.     

Activation entropy of electron transfer reactions

We report microscopic calculations of free energies and entropies for intramolecular electron transfer reactions. The calculation algorithm combines the atomistic geometry and charge distribution of a molecular solute obtained from quantum calculations with the microscopic polarization response of a polar solvent expressed in terms of its polarization structure factors. The procedure is tested on a donor–acceptor complex in which ruthenium donor and cobalt acceptor sites are linked by a four-proline polypeptide. The reorganization energies and reaction energy gaps are calculated as a function of temperature by using structure factors obtained from our analytical procedure and from computer simulations. Good agreement between two procedures and with direct computer simulations of the reorganization energy is achieved. The microscopic algorithm is compared to the dielectric continuum calculations. We found that the strong dependence of the reorganization energy on the solvent refractive index predicted by continuum models is not supported by the microscopic theory. Also, the reorganization and overall solvation entropies are substantially larger in the microscopic theory compared to continuum models.     

Salt effects in photoinduced electron transfer reactions

Metal salts and oxygen react synergistically to inhibit back-electron-transfer in photoinduced reactions.     

Barrierless electron transfer bond fragmentation reactions

The ultrafast N-O bond fragmentation in a series of N-methoxypyridyl radicals, formed by one-electron reduction of the corresponding N-methoxypyridiniums, has been investigated as potentially barrierless electron-transfer-initiated chemical reactions. A model for the reaction involving the electronic and geometric factors that control the shape of the potential energy surface for the reaction is described. On the basis of this model, molecular structural features appropriate for ultrafast reactivity are proposed. Femtosecond kinetic measurements on these reactions are consistent with a kinetic definition of an essentially barrierless reaction, i.e., that the lifetime of the radical is a few vibrational periods of the fragmenting bond, for the p-methoxy-N-methoxypyridyl radical.     

Energetics of electron transfer reactions in polar solvents

The free energy change of an electron transfer reaction in a polar solvent is rigorously analyzed within the framework of the dielectric continuum model. An appropriate expression for the electrostatic energy between the two product ions separated by R is derived. The present result does not support a recent claim by Suppan that, if R is close to the contact distance, the electrostatic energy should be much larger in magnitude than estimated from the usual expression −e²/ε_sR.     

Mechanisms of electron transfer through proteins

The analysis of modern physical mechanisms of electron transfer in proteins is given. The tunnel electron transfer and donor–acceptor electron transfer through conducting states of a protein chain are discussed in detail. The expressions for the values of the electron resonance interaction and the formulas for probabilities of electron transfer between vibronic levels of donor and acceptor states in the presence of “transverse” and “longitudinal” relaxation are given.     

1,3-二碘六氟丙烷的电子转移反应

在四醋酸铅的催化下,二氟二碘甲烷(CF2I2,1)与四氟乙烯加成生成1,3-二碘六氟丙烷(ICF2CF2CF2I,3).3与烯烃、炔烃和丙二酸二乙酯阴离子发生电子转移反应。     

Photochemical initiation of electron transfer reactions.

It is quite apparent that the use of photoinitiated electron transfer has become a powerful, if not dominating, technique in the study of biological electron transfer. It provides a means to measure directly very fast processes and, through the choice of approach (flavin semiquinones or related, metal substitution in hemes or modification with ruthenium) and experimental conditions, provides the ability to probe different features of the electron transfer mechanism. Nevertheless, much remains to be done to fully understand biological electron transfer. The use of photoinitiated electron transfer has clearly established a role for a number of factors involved in controlling the kinetics of electron transfer, including driving force, distance, intervening media, dynamics (conformational gating) and orientation of redox centers. However, we have only scratched the surface in regard to understanding in molecular terms the details of electron transfer in physiologically relevant systems. Thus, even relatively simple and well characterized systems like cytochrome c-cytochrome c peroxidase remain obscure in terms of the through-protein electron paths (intervening media) and the role of protein dynamics in controlling electron transfer kinetics. Indeed, it is the through-protein paths and conformational gating that are unique to biological systems and provide nature with the capability of modulating electron transfer kinetics to optimize biological function. Of the techniques described here, the use of flavin semiquinones is clearly the least invasive in that there is no evidence that flavin semiquinones bind to or perturb physiologically relevant systems. However, this approach is constrained in that precise distances and orientations are not always known, and the range of driving forces available is limited. In contrast, metal substitution and ruthenation allow the positions of interacting redox centers to be reasonably well defined and can provide a very large range of driving force. This latter point is particularly important since it provides a means to discriminate between rate limiting electron transfer and conformational gating. Nevertheless, chemically modifying redox proteins runs the risk of structural and electrostatic alterations which can be difficult to detect but have profound effects on the redox kinetics. Moreover, the intrinsic protein dynamics can be affected, resulting, in the worst case, in changes in conformational gating which cannot be resolved from rate limiting electron transfer. Given the early stage of development of photo-initiated electron transfer, substantial progress can be expected in the next few years. No doubt new approaches will be developed and existing approaches further refined. Especially important, the theoretical basis for interpreting and understanding electron transfer will continue to evolve.(ABSTRACT TRUNCATED AT 400 WORDS)     

Protein dynamics and electronic fluctuation effects in electron transfer reactions of membrane-bound proteins and metalloprotein complexes

We have analyzed specific long-range features of the electron transfer reactions in the cytochrome c/cytochrome b₅ complex, and the bacteriopheophytin/quinone and quinone/bacteriochlorophyll special pair cation radical long-range electron transfers. The analysis rests on an electron transfer theory which incorporates vibrational dispersion in the protein and solvent environments, and environmental fluctuation effects on the electronic transmission coefficient.The dispersion represents broadly the “inverse” temperature dependence of the photosynthetic reactions, but an explicit temperature dependence of the rate parameters is needed for quantitative data agreement. The data suggest that the transmission coefficient may exhibit some reaction free energy and temperature dependence caused by fluctuations in the environmental nuclear motion. The data are presently insufficiently diagnostic in this respect, but transmembrane potential induced rate variations over wide temperature ranges could lead to its clarification.     

Theory of photoinduced heterogeneous electron transfer

We consider electron injection into the conduction band of a semiconductor, from an electronically excited state of a dye molecule, adsorbed on its surface. For arbitrary width of the conduction band, the survival probability of the excited state can be calculated using a Green's-function approach. We show that the existence of a split-off state can play an important role in the total injection probability. In the wide band limit, the survival probability decays exponentially, but for finite band widths it does not. We further investigate the effect of vibrations on the process. A Green's operator technique may be used to solve this too exactly. We show that the problem may be reduced to a non-Hermitian eigenvalue problem for the vibrational states alone. Exact results can be obtained for arbitrary bandwidth and for a few vibrational degrees of freedom. In the wide band limit, the dynamics is particularly simple and we find that (1) the survival probability of the excited state is unchanged by the inclusion of vibrational motion, but (2) each vibrational state now has a finite lifetime. Numerical results are presented for the effects of reorganization energy, energy of the injecting level, and the variation of the matrix element for the electron injection, on the survival probability of the electron in the excited state. As an illustration of the approach, we also present results of numerical calculation of the absorption spectrum of perylene adsorbed on TiO(2) and compare it with experimental results.     

Kinetics of outer-sphere electron transfer reactions involving synthetic models of copper “blue"” proteins

A model is constructed for a two-dimensional cellular automaton describing an autooscillatory chemical reaction in a spatially distributed medium on the basis of experimental data on the reentry map of the Belousovo-Zhabotinskii (BZ) reaction. It is shown that the behavior of such a cellular automaton is qualitatively similar to the temporal evolution of spatial structures in the BZ reaction.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 1, pp. 31–36, January, 1993.     

Low-field CIDNP study of photoinduced electron transfer reactions

Chemically induced dynamic nuclear polarization in low magnetic field (low-field CIDNP) has been detected and studied in photoinduced electron transfer reactions in the polar solvent acetonitrile. For the radical-ion reactions two different approaches to interpret the low-field CIDNP are demonstrated: interpretation of the low-field CIDNP sign on the basis of quality relationships, and numerical calculations of the CIDNP field dependence. Analysis shows that low-field CIDNP in these reactions is sensitive to the value of the electron exchange interactions in radical-ion pairs.     

Photoinduced electron transfer reactions of rose bengal and selected electron donors

The photoinduced electron transfer reactions of the triplet state of rose bengal (RB) and several electron donors were investigated by the complementary techniques of steady state and time-resolved electron paramagnetic resonance (EPR) and laser flash photolysis (LFP). The yield of radicals varied with the light fluence rate, RB concentration and, in particular, the electron donor used. Thus for L-dopa (dopa, dihydroxyphenylalanine) only 10% of RB anion radical (RB√−) was produced, with double the yield observed with NADH (NAD, nicotinamide adenine dinucleotide) as quencher and more than three times the yield observed with ascorbate as quencher. Quenching of the RB triplet was both reactive and physical with total quenching rate constants of 4 × 10⁸ mol⁻¹ dm³ s⁻¹ and 8.5 × 10⁸ mol⁻¹ dm³ s⁻¹ for ascorbate and NADH respectively. The rate constant for the photoinduced electron transfer from ascorbate to RB triplet was 1.4 × 10⁸ mol⁻¹ dm³ s⁻¹ as determined by Fourier transform EPR (FT EPR). FT EPR spectra were spin polarized in emission at early times indicating a radical pair mechanism for the chemically induced dynamic electron polarization. Subsequent to the initial electron transfer production of radicals, a complex series of reactions was observed, which were dominated by processes such as recombination, disproportionation and secondary (bleaching) reactions.

It was observed that back electron transfer reactions could be prevented by mild oxidants such as ferric compounds and duroquinone, which were efficiently reduced by RB√−.