Showing papers on "Proton-coupled electron transfer published in 1994"

Mechanism for Proton-Coupled Electron-Transfer Reactions

Abstract: We introduce a model whereby the rate of an electron-transfer reaction can be modulated by an intervening proton-transfer reaction. The mechanism for this modulation is the assumed dependence of the electronic matrix element, which enables the electron to transfer from donor to acceptor, on the configuration of the proton(s) that are potentially undergoing a proton-transfer reaction. As the proton configuration changes, so does the electronic matrix element and, consequently, the electron-transfer rate depends on the proton configuration and dynamics, for this proton-coupled electron-transfer reaction, we find that the observed rate constant may have a significant isotope effect upon replacing the protons by deuterons and may also exhibit a temperature dependence above that expected for a conventional electron-transfer reaction

Passage from Concerted to Stepwise Dissociative Electron Transfer as a Function of the Molecular Structure and of the Energy of the Incoming Electron. Electrochemical Reduction of Aryldialkyl Sulfonium Cations

Abstract: The electrochemical reductive cleavage of the carbon-sulfur bond in the title compounds offers the example of a reaction where the concerted or stepwise character of the electron-transfer-bond-breaking process is a function of molecular structure. As with the reductive cleavage of the carbon-halogen bond in benzyl halides and of the nitrogen-halogen bond in aromatic N-halosultams, the two main factors governing the nature of the mechanism are the LUMO energy and the bond strength in the starting molecule: the higher the former and the weaker the latter, the greater the tendency for the concerted mechanism to prevail over the stepwise mechanism and vice versa. Consistently with the effect of these two mechanism-governing factors, two borderline cases were identified where the reaction passes from the concerted pathway to the stepwise pathway upon increasing the driving force by raising the scan rate and thus shifting the reduction potential toward negative values. The reasons for possible variations of the concerted or stepwise character of the mechanism of reductive cleavages upon changing the mode of electron injection are discussed. The electrochemical reduction of sulfonium salts has been the object of several studies in the past from which the reaction mechanism in Scheme 1 has emerged.283 The radical resulting from the initial reductive cleavage reaction may give rise tovarious products not involving further consumption of electrons or, in most cases, be further reduced to the carbanion, which is eventually converted into the corresponding hydrocarbon. The starting sulfonium cation plays the role of the acid in the latter reaction when the medium does not contain additional acids, as when the reaction is carried out in unbuffered aprotic solvents and when at least one carbon bonded to the sulfur bears at least one hydrogen. Half of the starting sulfonium cation is then converted into the corresponding ylid, and the overall reaction follows consequently a one electron per molecule stoichiometry (Scheme 2).4 Previous electrochemical studies indicate that the most stable radical is formed upon reductive cleavage as, for example, phenacyl, detected under the form of acetophenone, in phenacyldiethyl and phenylmethyl sulfonium s a l t ~ ; ~ M cyanomethyl in the dimethylcyanomethyl sulfonium cation;S* benzyl in the benzyldimethyl sulfonium cation, as detected by means of a radical t r a ~ ; ~ b and methyl, isopropyl, benzyl, p-cyanobenzyl, phenacyl, and .CH2(Ph)C=C(CN)2 in the corresponding phenylmethyl and 1 -naphthylmethyl sulfonium salts, as attested by the formation (1) (a) Universite Denis Diderot. (b) Eastman Kodak Co. (2) (a) For a review, see: ref 2b. (b) Chambers, J. Q. Organic Sulfur Compounds. In Encyclopedia ofEIectrochemistry ofrhe Elements, Organic Section; Bard, A. J., Lund, H., Eds.; Dekker: New York, 1978; Vol. XII, pp (3) (a) Saeva, F. D.; Morgan, B. P. J. Am. Chem. Soc. 1984, 106,4124. (b) Saeva, F. D. Tetrahedron 1986,42, 6123. (c) Saeva, F. D. Top. Curr. Chem. 1990, 156,61. (4) (a) The roleof ylid formationhasbeen pointedout in earlypolarographic studies of phenacyldiethyl and phenylmethyl sulfonium Ylid formation during the reduction in aprotic solvents has been taken advantage of for synthetic purposes, making them react with carbonyl compounds.”.’ (b) Savbnt, J.-M. C. R. Hebd. Seances Acad. Sci. 1963, 257, 448. (c) Savht, J.-M. C. R. Hebd. SeancesAcad.Sci. 1964,258,585. (d) Savht, J.-M. Bull. SOC. Chim. 1967, 481. (e) Shono, T.; Mitani, M. Tetrahedron Lett. 1969, 687. ( f ) Shono, T.; Akazawa, T.; Mitani, M. Tetrahedron 1969, 687. (5) (a) Wagenknecht, J. H.; Baizer, M. M. J . Electrochem. SOC. 1967, 114, 1095. (b) Baizer, M. M. J . Org. Chem. 1966, 31, 3847. 329-502. 0002-7863/94/ 1516-7864$04.50/0 Scheme 1 \ \ + R e + S 7-” + e ’ 1 ( in one or two steps ? )

Path of electron transfer in photosystem 1: direct evidence of forward electron transfer from A1 to Fe-Sx.

Abstract: Pulsed EPR spectroscopy and selective removal of the iron-sulfur centers in photosystem 1 have been used to study forward electron transfer from the secondary electron acceptor A1. At cryogenic temperatures where forward electron transfer is inhibited, we have observed a g = 2.003 electron spin-echo signal presenting a characteristic phase shift. This out-of-phase signal is attributed to the electron spin-polarized pair P700+/A1-, it decays with t1/e = 23 microseconds, reflecting the recombination reaction. At room temperature the out-of-phase signal is also observed, but it decays with t1/c = 200 ns in untreated photosystem 1, due to forward electron transfer from A1- to one of the iron-sulfur centers. This rate is unchanged in Fe-SA/B-depleted PS1 but is lost when the iron-sulfur center Fe-Sx is removed. In the preparations depleted of all iron-sulfur centers the out-of-phase signal decays with t1/c = 1.3 microseconds, reflecting either the back reaction or the decay of polarization. These results demonstrate that the electron transfer pathway in photosystem 1 is P700-->A1-->Fe-SX-->Fe-SA/B.

Long-range electron transfer in peptides. Tyrosine reduction of the indolyl radical: reaction mechanism, modulation of reaction rate, and physiological considerations

Abstract: We have observed long-range electron transfer (LRET) across the oligoproline spacer from the tyrosine side chain to the indolyl cation radical derived from the 1-electron oxidation of 1-N-methyltryptophan (Metrp .+ ). On the basis of these results and measured bimolecular electron transfers in the model system p-cresol/N-methylindole, phenol O-H bond breaking can clearly accompany the 1-electron transfer under conditions in which H-atom transfer to the indole nitrogen is impossible. With Metrp .+ as the electron acceptor, the rate of the LRET process across the oligoproline spacer is an order of magnitude higher than with the tryptophanyl radical (Trp . ) as electron acceptor

Redox mechanisms in inorganic chemistry

Abstract: The outer-sphere mechanism the inner-sphere mechanism intramolecular electron transfer multiple electron transfer.

Electron-transfer reactions with buckminsterfullerene, C60, in the gas phase

Abstract: The fundamental process of electron transfer is viewed from the new perspective provided by recent gas-phase measurements of reactions that remove electrons from the C60 molecule or donate electrons to singly and multiply charged C60 cations. Experimental results are reviewed for the chemical ionization of C60 by Penning ionization with metastable rare-gas atoms, single-electron transfer to atomic or molecular ions, double ionization by electron transfer, electron detachment reactions with He·+ or Ne·+, and multiple ionization by multiple-electron transfer to multiply charged rare-gas cations. Also, experimental results are reviewed for single-electron transfer to C·+ 60 and C2+ 60; a model previously proposed for the potential energy profile of single-electron transfer to C2+ 60 is included which argues in favour of a reaction barrier arising from the Coulombic repulsion between the single charged product ions of such a process. Results of measurements previously reported for single and double-e...

Efficient Photoinduced Orthogonal Energy and Electron Transfer Reactions via Phospholipid Membrane-Bound Donors and Acceptors

Abstract: A three component, liposome-bound photochemical molecular device (PMD) consisting of energy and electron transfer reactions is described. Bilayer membrane surface-associated dyes, 5,10,15,20-tetrakis[4-(trimethylammonio)-phenyl]-21H,2 3H-porphine tetra-p-tosylate salt and N,N[prime]-bis[(3-trimethylammonio)propyl]thiadicarbocya nine tribromide, are the energy donor and acceptor, respectively, in a blue light stimulated energy transfer reaction along the vesicle surface. The electronically excited cyanine is quenched by electron transfer from the phospholipid membrane bound triphenylbenzyl borate anion, which is located in the lipid bilayer interior. The PMD exhibits sequential reactions following electronic excitation with the novel feature that the steps proceed with orthogonal orientation: energy transfer occurs parallel to the membrane surface, and electron transfer occurs perpendicular to the surface. Photobleaching and fluorescence quenching experiments verify the transfer reactions, and Stern-Volmer analysis was used to estimate the reaction rate constants. At the highest concentrations examined of energy and electron acceptor ca. 60% of the photoexcited porphyrins were quenched by energy transfer to the cyanine. 56 refs., 6 figs., 3 tabs.

Dynamics of low-barrier proton transfer in polar solvents and protein media

Abstract: Proton transfer in chemical, physical, and biological systems frequently involves low and shallow barriers. This is caused by strong donor-acceptor interaction which also imposes fully adiabatic character on the process. Low or shallow barriers are not covered straightforwardly by tunnel views otherwise central in proton transfer theory. We introduce a formalism focused on a shallow enough proton transfer mode that the proton dynamics is indistinguishable from classical behaviour, strongly coupled to stochastic environmental nuclear motion. We derive rate constants for three cases, namely fast solvent relaxation, fast proton equilibration, and proton transfer followed by fast decay of the donor-acceptor complex. These cases may constitute useful descriptions of several kinds of processes including fast intramolecular proton transfer, hydrolytic and redox enzyme catalysis, and physical or biological proton conductivity.

Intramolecular electron transfer in yeast flavocytochrome b2 upon one-electron photooxidation of the fully reduced enzyme: evidence for redox state control of heme-flavin communication.

Abstract: Flavocytochrome b2, which has been fully reduced using L-lactate, can be rapidly oxidized by 1 equiv using the laser-generated triplet state of 5-deazariboflavin. Parallel photoinduced oxidation occurs at the reduced heme and at the fully reduced FMN (FMNH2) prosthetic groups of different enzyme monomers, producing the anion semiquinone of FMN and a ferric heme. Following the initial oxidation reaction, rapid intramolecular reduction of the ferric heme occurs with concomitant oxidation of FMNH2, generating the neutral FMN semiquinone. The observed rate constant for this intramolecular electron transfer is 2200 s-1, which is 1 order of magnitude larger than the turnover number under these conditions. A slower reduction of the heme prosthetic group also occurs with an observed rate constant of approximately 10 s-1, perhaps due to intersubunit electron transfer from reduced FMN to heme. The rapid intramolecular electron transfer between the FMNH2 and ferric heme is eliminated upon addition of excess pyruvate (Ki = 3.8 mM). This latter result indicates that pyruvate inhibition of catalytic turnover apparently can occur at the FMNH2-->heme electron transfer step. These results markedly differ from those previously obtained (Walker, M. C., & Tollin, G. (1991) Biochemistry 30, 5546-5555) and confirmed here for electron transfer within the one-electron reduced enzyme and for the effect of pyruvate binding, suggesting that intramolecular communication between the heme and flavin prosthetic groups can be controlled by the redox state of the enzyme and by ligand binding to the active site.

Electron transfer reaction between fixed redox sites in ion/electron mixed conducting polymers

Abstract: Summary form only given. Electrochemical reaction of redox molecules introduced in ionically conducting polymers can occur by two mechanisms. One mechanism is the physical diffusion of redox molecules to eletrode, followed by electron transfer reaction, and the other is the propagation of oxidized (reduced) sites generated at the electrode by electron self exchange reaction between redox sites. Redox active ion-conducting polymers whose reaction occurs by the latter mechanism are called ion/electron mixed conductors or redox conductors. In this study, the electron transfer mechanism at redox reaction has been examined in ionically conducting polymers having fixed redox sites. The polymers were prepared by radical copolymerization of vinylferrocene(VFc) or 3-vinyl-N-methylphenothiazine(VPT) as a redox monomer with methoxy nona(ethylen oxide) metacrylate(ME0/sub 9/) as an ionically conducting monomer. Reversible redox reaction in the bulk polymeric phase was observed in these copolymers, although the redox sites are affixed to the polymer backbone. Apparent diffusion coefficient (D) of the redox sites in the VFc-MEO/sub 9/ copolymers linearly increaced with increasing VFc composition with an intercept close to zero, which shows that D in the copolymer corresponds to the electron diffusuvity. Temperature dependence of the electron diffusion coefficient was very large and fit WLFeq. This indicates that the electron exchange rate greatly changes with temperature and is influenced by the dynamics of the polymer backbone. The electron transfer behavior in the VPT-ME0/sub 9/ copolymers were also explored and compared in these two series of the copolymers.

Charge transfer dynamics of donor-acceptor systems in solutions and sol-gel matrices: Code: FP4

Abstract: Charge transfer reactions are photoinduced by exciting with UV and visible light strongly coupled donor acceptor pairs. The systems investigated are composed either of porphyrin and phthalocyanine mixed dimers. In this case, the proton transfer competes with the electron transfer process.

Electron transfer reactions in 5-nitroisoquinolines

Abstract: 1-Chloromethyl-5-nitroisoquinoline, a new reductive alkylating agent, has been prepared and shown for the first time in isoquinoline series to react with 2-nitropropane anion to give C-alkylation by an SR^1 mechanism. This has been confirmed by the inhibitory effects of dioxygen, p-dinitrobenzene, cupric chloride and TEMPO. This reaction was followed by a basemediated nitrous acid elimination leading to new isopropylidene derivative. Introduction The inhibitory potency of various isoquinoline thiosemicarbazone derivatives (1) for the enzyme ribonucleotide reductase illustrates interest of the isoquinoline ring for medicinal chemistry. As part of our continuing studies on the Sp[*j1 reactions of heterocyclic alkylating agents (2) and in order to prepare new potentially antineoplastic agents involving electron transfer in their mode of action, we have investigated the synthesis of 1-chloromethyl-5-nitroisoquinoline and have studied its reactivity with the 2-nitropropane anion. Results 1-Chloromethyl-5-nitroisoquinoline f> has been prepared in five steps from the inexpensive and commercially available 2-phenylethylamine. The required starting material produced 2phenylethylacetamide 1 by acetylation. The synthesis of 1-methyl-3,4-dihydroisoquinoline was carried out utilizing the Bischler-Napieralski reaction (3), which was modified by use of polyphosphoric acid (PPA) instead of POCI3 as the dehydrating reagent. The employment of this modification resulted in an increase in the yield of £ from 30 to 80%. Derivative Ζ was then dehydrogenated with diphenyl disulfide as previously reported (4). Nitration of 1methylisoquinoline with HNO3-H2SO4 (5) resulted in only one compound, 1-methyl-5Vol. I, No. 1, 1994 Electron transfer reactions in 5-nitroisoquinolines nitroisoquinoline 4. Free radical chlorination (6) using N-chlorosuccinimide (NCS) of 4 gave 1chloromethyl-5-nitroisoquinoline 5 in 44% yield. Schemel AC20 PPA Ph2S2 NHCOCH3 NH2 ΟβΗβ 1 95% tetraline 2 80 % c h 3

Electron transfer and energy transfer reactions in photoexcited a-nonathiophene/C60 films and solutions

Abstract: Photoexcitation of a nonathiophene in film or solution across the π-π* energy gap produces a metastable triplet state. In the presence of C60, on the other hand, an ultra fast electron transfer from the photoexcited nonathiophene onto C60 is observed in films, whereas in solution C60 is involved in an efficient energy transfer reaction with the triplet-state nonathiophene.

Intramolecular proton transfer quenching in tryptophan and tryptamine: temperature dependence

Abstract: There are two dominant intramolecular quenching reactions for the deactivation of the excited singlet state of tryptophan and its related chromophores. These mechanisms are charge transfer to an electrophilic side chain group and proton transfer from a side chain to the indole ring. When occurring in D2O, the photoinduced proton transfer reaction results in an exchange of the hydrogen isotope at position 4 of the benzenoid ring of indole, which can be quantitated by 1H-NMR studies. We have determined the rate constant for this photoinduced H-D exchange for tryptophan and some of its analogs as a function of temperature. We show that the proton transfer quenching reaction makes a large contribution to the quenching of tryptophan, whereas for other chromophores, charge transfer quenching must dominate.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Nonadiabaticity in the photoinduced electron transfer reactions of metal complexes

Abstract: The rate of electron transfer (ET) in a variety of chemical and biological processes is influenced by factors like the free energy change (†G), the donor-acceptor electronic coupling and the medium. The effect of donor-acceptor electronic coupling on the rate of photoinduced intermolecular electron transfer is considered by taking Ru(II) and Cr(III) metal complexes in the excited state as electron acceptors and organic compounds as electron donors. The electronic coupling between the donor and acceptors depends strongly on donor-acceptor distance. The electron transfer distance is varied by introducing alkyl groups of different sizes either on the bipyridine ligand of the metal complex or on the quencher. The semiclassical theory of electron transfer expresses kET as the product of a nuclear and an electronic transmission coefficient (K n andK el respectively) and an effective nuclear-vibration frequency (v n),k ET =v nKel, Kn. The electron transfer reaction becomes nonadiabatic if the donor-acceptor distance is long. The change of electron transfer mechanism from adiabatic to nonadiabatic due to the introduction of bulky groups is explained in terms of semiclassical theory and from the temperature-dependence study of photoinduced electron transfer reactions of metal complexes.

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

Abstract: The kinetics of outer-sphere electron transfer reactions involving tetrathiamacrocyclic copper(II) complexes with K4[Fe(CN6)] and cyt c2+ was studied. The data obtained are discussed applying the Marcus approach to cross reactions. It was shown that the characteristics of electron transfer in CuL2+/+ redox pairs depend on the topology of the coordination polyhedron.