Reversible Electron Transfer Reactions Research Articles

Electron transfer in the quenching of triplet methylene blue by complexes of iron(II)

A Q-switched pulsed ruby laser emitting 1.0-J flashes at 693.4 nm was used in an investigation by means of flash photolysis-kinetic spectrophotometry of the mechanism of quenching of 2 to 10 ..mu..M triplet methylene blue by substitution-inert complexes of Fe(II). Complexes included Fe(H/sub 2/O)/sub 6//sup 2 +/, H/sub 2/Fe/sup II/(CN)/sub 6//sup 2 -/, HFe/sup II/(CN)/sub 6//sup 3 -/, Fe/sup II/(CN)/sub 6//sup 4 -/, Fe/sup II/(CN)/sub 4/bpy/sup 2 -/, Fe/sup II/(CN)/sub 2/(bpy)/sub 2//sup 0/, ferrocene/sup 0/, and Fe/sup II/(bpy)/sub 3//sup 2 +/. System variables included solvent (H/sub 2/O, aqueous EtOH, aqueous CH/sub 3/CN, and aqueous DMF), pH (2.1 to 8.2) and ionic strength (0.01 to 1.6 M). Energy transfer from triplet methylene blue to quencher is not possible in any of the cases investigated, and quenching proceeds via partial or complete electron transfer. Rate constants for quenching, k/sub q/, were diffusion-controlled with all quenchers except Fe/sup II/(H/sub 2/O)/sub 6//sup 2 +/ and, possibly, Fe/sup II/(bpy)/sub 3//sup 2 +/. Efficiency of net electron transfer in the quenching process was measured directly and found to vary with quencher, solvent, and state of protonation of triplet methylene blue and quencher. The kinetics of the reverse electron-transfer reaction between complexes of Fe(III) and semimethylene blue,more » MBH/sup +/, was studied independently by generating MBH/sup +/ by quenching triplet methylene blue with diphenylamine in the presence of a complex of Fe(III). Values of the second-order specific rate of reserve electron transfer were measured directly. Mechanistic features for the diffusion-controlled quenching of /sup 3/MBH/sup 2 +/ by complexes of Fe(II), Fe/sup II/L/sub x//sup m/ consistent with the data are proposed.« less

MODELS FOR BACTERIAL PHOTOSYNTHESIS: ELECTRON TRANSFER FROM PHOTOEXCITED SINGLET BACTERIOPHEOPHYTIN TO METHYL VIOLOGEN AND m‐DINITROBENZENE

Abstract. As a model for the primary reactions of photosynthesis, we studied photochemical electron transfer from bacteriopheophytin (BPh) to methyl viologen (MVC12) and to m‐dinitrobenzene (m‐DNB) in solution. Both MVC12 and m‐DNB cause reductions in the lifetime of the first excited singlet state of BPh (BPh*), in the fluorescence quantum yield, and in the quantum yield of the triplet state, BPh +. The quenching of BPh* probably results from electron transfer, which generates short‐lived radical pairs involving the BPh radical cation (BPh+) and the reduced form of the quencher. Electron transfer from BPh* is thermodynamically favorable, but that from BPhT is not. From the magnitude of the quenching, we calculate rate constants for electron transfer in collision complexes formed between BPh* and MVC12 or m‐DNB. Measurements of the quantum yield of the free BPh+ radical indicate that about 3/4 of the [BPh+ MV+] radical pairs decay by reverse electron transfer, rather than dissociating to give the free radicals. Essentially all of the [BPh+m‐DNB +] radical pairs must decay by reverse electron transfer, because free BPh+ cannot be detected in this case. From these data, we estimate the rate constants for the reverse electron transfer reactions. The higher probability of dissociation in the [BPh+ MV+] radical pair can be explained by coulombic repulsion.The rate of the primary electron transfer reaction in photosynthetic bacteria is comparable to that of forward electron transfer in the BPh* collision complexes. Reverse electron transfer, however, is at least 103‐times slower in the radical pair formed in the bacterial reaction center than it is in [BPh+m‐DNB−], and more than 104‐times slower than in [BPh+ MV+]. The explanation for this dramatic and crucially important difference remains unclear, but several possibilities are discussed.

265 NM LASER FLASH PHOTOLYSIS OF SOME ORTHO‐SUBSTITUTED ANILIDES AND RELATED N‐FORMYLKYNURENINE DERIVATIVES

Abstract— The physical and chemical properties of the triplet state of eight ortho‐substituted anilides including N‐formylkynurenine (FK), the major trp UV‐photooxidation product and a remarkable photodynamic agent, have been investigated using both pulse radiolysis and 265 nm laser flash photolysis techniques. The molar extinction coefficient, the intersystem‐crossing quantum yield and the oscillator strength of the T1→Tn absorption band (Λmax˜ 450 nm) have been determined. It is shown that anilides having n,π* triplets readily react with most solvents whereas those having π,π* triplets slowly react with alcohols. In both cases, the semi‐reduced species are formed. In water, the formation of the semi‐reduced. species most probably involves the first excited singlet state. The triplet state properties of the FK derivatives (i.e. ortho‐substituted anilides having a side chain bearing charged groups such as carboxylic or amino groups) are strongly modified by the ionization state of the charged side chain. In the case of the FK derivatives possessing an uncharged amino group, quenching of the triplet state occurs via a fast reversible electron transfer reaction from the NH2 to the triplet anilide.

Mechanisms involved in the chemical inhibition of the eosin-sensitized photooxidation of trypsin.

A large series of compounds was screened for ability to protect trypsin from eosin-sensitized photodynamic inactivation. Eosin-sensitized photooxidation reactions of this type typically proceed via the triplet state of the dye and often involve singlet state oxygen as the oxidizing entity. In order to determine the mechanisms by which trypsin is protected from photoinactivation, a number of good protective agents (inhibitors) and some non-protective agents were selected for more detailed flash photolysis studies. Good inhibitors such as p-phenylenediamine, n-propyl gallate, serotonin creatinine sulfate and p-toluenediamine competed efficiently with oxygen and with trypsin for reaction with the triplet state of eosin. The inhibitors were shown to quench triplet eosin to the ground state and/or reduce triplet eosin to form the semireduced eosin radical and an oxidized form of the inhibitor. In the latter case, oxidized inhibitor could react by a reverse electron transfer reaction with the semi-reduced eosin radical to regenerate ground state eosin and the inhibitor. The good inhibitors also competed effectively with trypsin for oxidation by semioxidized eosin, thus giving another possible protective mechanism. Non-inhibitors such as halogen ions and the paramagnetic ions Co++, Cu++ and Mn++ reacted only slowly with triplet and with seimioxidized eosin. The primary pathway for the eosin-sensitized photooxidation of trypsin at pH 8.0 involved singlet oxygen, although semioxidized eosin may also participate.