Sequence Of Electron Transfer Reactions Research Articles

Supersulfide biology and translational medicine for disease control.

For decades, the major focus of redox biology has been oxygen, the most abundant element on Earth. Molecular oxygen functions as the final electron acceptor in the mitochondrial respiratory chain, contributing to energy production in aerobic organisms. In addition, oxygen-derived reactive oxygen species including hydrogen peroxide and nitrogen free radicals, such as superoxide, hydroxyl radical and nitric oxide radical, undergo a complicated sequence of electron transfer reactions with other biomolecules, which lead to their modified physiological functions and diverse biological and pathophysiological consequences (e.g. oxidative stress). What is now evident is that oxygen accounts for only a small number of redox reactions in organisms and knowledge of biological redox reactions is still quite limited. This article reviews a new aspects of redox biology which is governed by redox-active sulfur-containing molecules-supersulfides. We define the term 'supersulfides' as sulfur species with catenated sulfur atoms. Supersulfides were determined to be abundant in all organisms, but their redox biological properties have remained largely unexplored. In fact, the unique chemical properties of supersulfides permit them to be readily ionized or radicalized, thereby allowing supersulfides to actively participate in redox reactions and antioxidant responses in cells. Accumulating evidence has demonstrated that supersulfides are indispensable for fundamental biological processes such as energy production, nucleic acid metabolism, protein translation and others. Moreover, manipulation of supersulfide levels was beneficial for pathogenesis of various diseases. Thus, supersulfide biology has opened a new era of disease control that includes potential applications to clinical diagnosis, prevention and therapeutics of diseases.

A Novel Pump–Pump–Probe Resonance Raman Approach Featuring Light-Induced Charge Accumulation on a Model Photosystem

Light-induced charge accumulation is at the heart of biomimetic systems aiming at solar fuel production in the realm of artificial photosynthesis. Understanding the mechanisms upon which these processes operate is a necessary condition to drive down the rational catalyst design road. We have built a nanosecond pump–pump–probe resonance Raman setup to witness the sequential charge accumulation process while probing vibrational features of different charge-separated states. By employing a reversible model system featuring methyl viologen (MV) as a dual electron acceptor, we have been able to watch the photosensitized production of its neutral form, MV0, resulting from two sequential electron transfer reactions. We have found that, upon double excitation, a fingerprint vibrational mode corresponding to the doubly reduced species appears at 992 cm–1 and peaks at 30 μs after the second excitation. This has been further confirmed by simulated resonance Raman spectra which fully support our experimental findings in this unprecedented buildup of charge seen by a resonance Raman probe.

Modulation of electrochemical property of carbon nanodot by post-chemical reductions

Abstract Influences of chemical reductions on the electrochemical properties of carbon nanodots (CNDs) were investigated. Sequential electron transfer reactions involving CNDs and several electrochromic molecules were analyzed by titration method to deduce electron accepting capabilities and upper-limit reduction potentials. CNDs prepared by alkali-assisted electrochemical oxidation of graphite were reduced by either sodium borohydride or hydrazine to obtain chemically reduced CNDs. Titration analysis of the electron transfer reactions involving methylviologen cation radicals and CNDs revealed that electron uptake per unit weight of CND increased nearly 30% after chemical reductions. Furthermore, detailed analyses of electron discharge processes involving the electron-filled CNDs and several leuco dyes provided fine estimation of upper-limit reduction potential and evidence of reduction potential shifts of CNDs to negative level by chemical reductions.

A general electron transfer reduction of lactones using SmI2–H2O

Herein we describe a strategy for the selective, electron transfer reduction of lactones of all ring sizes and topologies using SmI(2)-H(2)O and a Lewis base to tune the redox properties of the complex. The current protocol permits instantaneous reduction of lactones to the corresponding diols in excellent yields, under mild reaction conditions and with useful chemoselectivity. We demonstrate the broad utility of this transformation through the reduction of complex lactones and sensitive drug-like molecules. Sequential electron transfer reactions and syntheses of deuterated diols are also described.

Two redox-active beta-carotene molecules in photosystem II.

Photosystem II (PS II) contains secondary electron-transfer paths involving cytochrome b(559) (Cyt b(559)), chlorophyll (Chl), and beta-carotene (Car) that are active under conditions when oxygen evolution is blocked such as in inhibited samples or at low temperature. Intermediates of the secondary electron-transfer pathways of PS II core complexes from Synechocystis PCC 6803 and Synechococcus sp. and spinach PS II membranes have been investigated using low temperature near-IR spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. We present evidence that two spectroscopically distinct redox-active carotenoids are formed upon low-temperature illumination. The Car(+) near-IR absorption peak varies in wavelength and width as a function of illumination temperature. Also, the rate of decay during dark incubation of the Car(+) peak varies as a function of wavelength. Factor analysis indicates that there are two spectral forms of Car(+) (Car(A)(+) has an absorbance maximum of 982 nm, and Car(B)(+) has an absorbance maximum of 1027 nm) that decay at different rates. In Synechocystis PS II, we observe a shift of the Car(+) peak to shorter wavelength when oxidized tyrosine D (Y(D)*) is present in the sample that is explained by an electrostatic interaction between Y(D)* and a nearby beta-carotene that disfavors oxidation of Car(B). The sequence of electron-transfer reactions in the secondary electron-transfer pathways of PS II is discussed in terms of a hole-hopping mechanism to attain the equilibrated state of the charge separation at low temperatures.

Charge Hopping in Molecular Wires as a Sequence of Electron-Transfer Reactions

Charge transport in molecular wires is investigated theoretically within the framework of a simple hopping model. The model suggests that each elementary hopping step can be treated as an electron-transfer reaction between ionic and neutral states of π-conjugated structural units coupled through σ-bonded spacers. Within this mechanistic picture, the ability of wire to transport a charge depends crucially on the internal reorganization energy, λ. Using unrestricted Hartree−Fock and density functional theory methods, we evaluate λ for benzene, 3-methylbiphenyl, 2,6-dimethyl-1-phenyl-pyridinium (DMPP), and 4-(p-suflhydrylphenylpyridinium-1‘-yl)-2,6-dimethylpyridinium, selected as representative examples of structures used for chemical attachment to σ-bonded structural spacers in real molecular wires. The results are exploited to estimate the upper and lower limits of hole and electron mobility in wires that consist of aromatic ring units linked to the antipodal bridgeheads of σ-bonded molecular “cages”, bicy...

Investigation of the Competition between Electron and Energy Transfer in the Quenching of Aromatic Ketones in the Triplet State Using Picosecond Transient Grating Spectroscopy

The competition between electron transfer (ET) and triplet energy transfer (TT) in the quenching of benzophenone, xanthone, and anthraquinone in the triplet state by molecules with both a sufficiently small oxidation potential and low triplet state was investigated in the picosecond to microsecond time scales. In the longer time scale, the product distribution depends strongly on the relative exergonicity of ET and TT processes, the yield of the lower energy product being at least four times larger than that of the other product. Picosecond transient grating measurements reveal that if TT is more exergonic than ET, the TT product is predominantly formed by two sequential ET reactions, i.e., by spin-allowed back ET within the triplet geminate ion pair formed upon ET quenching. However, if ET is more exergonic than TT, no conversion from the TT product to the ET product could be detected. In this case, the product distribution in the microsecond time scale seems to reflect the competition between the two processes. When both processes are exergonic, ET appeared to be always faster than TT. This is in agreement with the severe orbital overlap requirement for TT via the Dexter exchange mechanism.

Formation of singlet oxygen in the system tris(bipyridine)ruthenium(II)–dimethyldioxirane

Decomposition of dimethyldioxirane in the presence of tris(bipyridine)ruthenium(II) complex (RuII) is accompanied by chemi-luminescence (CL) in the infrared spectrum region (1270 nm) due to the formation of singlet oxygen; reaction between RuIII and superoxide ion, which is formed in this process via a sequence of electron-transfer reactions, is assumed to be a CL step.

Modeling the sequence of electron transfer reactions in the single turnover of reduced, mammalian cytochrome c oxidase with oxygen.

Single-turnover studies of the reaction of reduced cytochrome oxidase with oxygen has led to a mechanistic model that specifies a linear sequence of electron transfer from cytochrome c to O2 via the four redox active metals of mitochondrial cytochrome oxidase. Fully reduced oxidase initially forms a dioxygen adduct within which two electrons are transferred at a rate of 6 x 10(4) s-1 to form a peroxy adduct. This two-electron step results in single-electron oxidation of cytochrome alpha 3 and cytochrome alpha. Cytochrome alpha is then re-reduced by CuA in an intramolecular reaction. Subsequent reoxidation of cytochrome alpha occurs at a rate nearly 100-fold slower than its initial oxidation. Oxidation of CuB in this sequence has been the most difficult to determine and here is specified as the third electron transfer step. This reaction sequence is unchanged in the presence of tightly bound cytochrome c, although cytochrome c is rapidly oxidized via CuA and cytochrome alpha. The linear model advanced here has CuA as the acceptor of electrons from cytochrome c, with cytochrome alpha serving as a bridge to deliver electrons from CuA (and cytochrome c) to the binuclear center, cytochrome alpha 3-CuB.

A molecular mechanism of the energetic coupling of a sequence of electron transfer reactions to endergonic reactions

A molecular mechanism of the energetic coupling of a sequence of electron transfer reactions to endergonic reactions is proposed and discussed from a physical point of view. The scheme represents a synthesis of concepts of electron transfer by tunneling and the conformational and chemiosmotic aspects of energy coupling processes. Its relation to existing experimental information and theoretical models is discussed, and further experimental tests are suggested.