Electron transfer reaction, conformational

D.R. McMillin, Purdue University In addition to the charge effects discussed by Professor Sykes, I would like to add that structural effects may help determine electron transfer reactions between biological partners. A case in point is the reaction between cytochrome C551 and azurin where, in order to explain the observed kinetics, reactive and unreactive forms of azurin have been proposed to exist in solution (JL). The two forms differ with respect to the state of protonation of histidine-35 and, it is supposed, with respect to conformation as well. In fact, the lH nmr spectra shown in the Figure provide direct evidence that the nickel(II) derivative of azurin does exist in two different conformations, which interconvert slowly on the nmr time-scale, depending on the state of protonation of the His35 residue (.2) As pointed out by Silvestrini et al., such effects could play a role in coordinating the flow of electrons and protons to the terminal acceptor in vivo. 

The electron-transfer reactions between the /3-cyclodextrin (/3-CD) N-substituted phenothiazine derivatives and /3-CD.ATPO (4-acetoxy-2,2,6,6-tetramethyl-1-oxopiperidinium hexachloroantimonate) were found to be influenced by the conformations of the phenothiazine derivatives restricted by the /3-CD cavity. N-Phenylphenothiazine (PPT) and A-phenylethylphenothiazine (PEPT), included by /3-CD, can transfer an electron to the /S-CD.ATP complex. No electron transfer was observed between the /3-CD.A-benzylphenothiazine (/3-CD.BPT) complex under the same conditions. The conformation of the /3-CD.BPT complex is such that the oxidation centre was shielded by the /3-CD wall and the substituent. However, electron-transfer reactions between y-CD.BPT and /3-CD.ATP and nitric acid occurred. ... 

Perhaps the most Important effect of conformational variations In electron transfer reactions would be to alter the distances and the relative orientations of donors and acceptors. In photosynthetic RC s, where the primary donors and acceptors lie within 4-5A of each other ( ), small structural displacements (, 5A) may significantly affect rates of back reactions. If they occur rapidly (24), (Conformational movements on a picosecond time scale are not Inconsistent with resonance Raman data on photo-dlssoclated heme-CO complexes (25)), On a longer time scale, protein rearrangements triggered by and propagating from the chromophores may also help subsequent reactions such as the transport of protons that Is Initiated by the primary photochemical event In the R,C, (26),... 

It is thus expected that the conformation of the polymer-ligand chain will influence the reactivity of a metal complex. The influence of the conformational change of a poly(N-vinyl-2-methylimidazole)(PVMI 3) ligand has been studied in the electron-transfer reaction of a Co(III) complex in aqueous-alcoholic solvents87. ... 

The mechanism of proton translocation in complexes I and IV is not yet understood. Here, the electron-transfer reactions may cause protein conformational changes that open gates for proton movement first on one side of the membrane and then on the other. 

The mechanism of the regulation of electron transfer in metalloproteins has been investigated 61) and two relevant examples have been discussed in the first one the molecular mechanism controlling the electron transfer reactions is restricted to the immediate chemical environment of the metal center (azurin), while in the second one it involves a conformational transition of the whole quaternary structure of the enzyme. The power of the kinetic approach in detecting significant intermediates was emphasized 6t>. The Cu metal complex site of azurin has a distorted tetrahedral... 

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

As indicated previously, NMR may be used simply as an analytical technique for monitoring the decomposition of a reactant or formation of a product. In addition, NMR and ESR merit a special mention due to their importance in studying the dynamics of systems at equilibrium these so-called equilibrium methods do not alter the dynamic equilibrium of the chemical process under study. They have been used to study, for example, -transfer reactions, valence isomerisations, conformational interconversions, heteronuclear isotopic exchange processes (NMR) and electron-transfer reactions (ESR). These techniques can be applied to the study of fast or very fast reactions by analysis of spectral line broadening [16,39],... 

Jinnouchi and Okazaki performed AIMD studies of the first-electron transfer reaction with 1 hydronium ion, 9 water molecules, and 12 Pt atoms at 350 K as shown in Fig. 13.108 The adsorbed water molecules and the hydronium ion hydrated the adsorbed oxygen atoms, and proton transfer through the constructed hydrogen bonds frequently occurred. When the conformation of these species satisfied certain conditions, the oxygen dissociation with the proton transfer reaction was induced and three OH were generated on the platinum surface (Fig. 14). The authors concluded that the oxygen dissociation tendency is one of the dominant factors for the reactivity of the cathode catalyst. This work demonstrates the power of AIMD that does not require specific assumption in order to describe charge transfer. 

Graige, M. S., Feher, G., and Okamura, M. Y., 1998, Conformational gating of the electron transfer reaction Qa Qb QaQb bacterial reaction centers of Rhodobacter sphaeroides determined by a driving force assay. Proc. Natl. Acad. Sci. USA, 95 11679911684. 

Hammes-Schiffer S. Theoretical perspectives on proton-coupled electron transfer reactions. Acc. Chem. Res. 2001 34 273-281. Khoshtariya DE, Wei J, Liu H, Yue H, Waldeck DH. Charge-transfer mechanism for cytochrome c adsorbed on nanometer thick films, distinguishing frictional control from conformational gating. J. Am. Chem. Soc. 2003 125 7704-7714. 

Numerous reviews have appeared which are relevant to aromatic photochemistry and are also of general interest and application these include accounts of the chemistry of excited-state complexes by Davidson, the photochemical electron-transfer reactions of ethylenes and related compounds by Mattes and Farid, photosolvolyses and photoreactions involving carbo-cations by Cristol and Bindel, conformational flexibility and photochemistry by Wagner, asymmetric photochemistry in solution by Rau, and the photochemistry of iminium salts and related heteroaromatic systems by Mariano." The photochemistry of fragrance materials has been considered by Shibamoto and Mihara, and in their second article they deal with aromatic compounds and phototoxicity. ... 

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