Electron-proton coupling

Baptista, A.M. Martel, P.J. Soares, C.M., Simulation of electron-proton coupling with a Monte-Carlo method application to cytochrome C3 using continuum electrostatics, Biophys. J. 1999, 76, 2978-2998... 

Another case of a negative electron-proton coupling constant is the methyl radical CH3, where the unpaired electron is predominantly in a v MO perpendicular to the plane of the molecule. 

No generally applicable Q s can be given for methylene protons, for example of the propionates (Fig. 2), since the electron-proton coupling depends strongly on the sterical conformation of the molecule (16). 

Electron-proton coupling. Mechanism of ATPase reactions in energy-conversion... 

The nature of the electron/proton coupling in this reaction is a topic of great current interest. Unfortunately, much of the available experimental data comes from studies on Mn-depleted PSII preparations, which are incapable of water oxidation and which exhibit grossly altered kinetics of P680+ reduction. It is clear for such preparations that abstraction of the proton from Yz is rate-limiting for net electron transfer, although, even for this system, it is not known if the proton is removed before or after electron transfer (Hays et al., 1999). 

The continuum electrostatic approach has been considered also appropriate for studying biomolecular systems when electronic polarisation effects, typically neglected by the additive pair-wise potentials commonly use in MD and MC, are dominant in relation to the conformational flexibility, and when changes in protonation states of tritable sites [193-198] or electron-proton coupling phenomena [199-204] occur. 

The chapters in this volume offer overviews of electronic properties, electron transfer and electron-proton coupled charge transfer of biological molecules and macromolecules both in the natural aqueous solution environment and on metallic electrode surfaces, where the electrochemical potential controls biomolecular function. Redox metalloproteins and DNA-based molecules are primary targets, but amino acid and nucleobase building blocks are also addressed. Novel enviromnents where proteins and DNA-based molecules are inserted in metallic nanoparticle hybrids or in situ STM configurations are other focus areas. 

The structures of the RCs from Rb. sphaeroides and Rp. viridis show a striking identity, apart from the cytochrome-c subunit found only in the latter RC. The core regions around the bacteriochlorophylls and bacteriopheophytins, including the carotenoid, are particularly similar. New observations of water clusters close to the primary and secondary quinones are described and their impact on proton transfer processes is discussed. These findings help elucidate the intermeshed processes of electron-proton coupling in the RC. 

Still, the concept of hybridization is relevant even for electron-proton couplings ai because their values are practically determined by the spin density pi located on the next-neighbour atom i of the proton considered, for instance the unsaturated carbon C to which H is attached. These couplings are governed by relationships of the form... 

R450 I. Pecht, Electron/Proton Coupling in Biological Energy Transduction , FEES Lett., [online computer file], 2012, 586, 474. 

Interaction between the cluster and the aspartate carboxylate is an important part of the electron-proton coupling mechanism. Once the cluster is proto-nated, the pK of the D15 carboxylate decreases to a value approaching that observed when the cluster is oxidized (pKj pK2 pK Simultaneously, the pK of the cluster is sensitive to the protonation state of the aspartate. 

As discussed by Travers [102], this can be due to either (1) an increase in the intrachain diffusion rate or (2) a decrease in the electron-proton coupling constants. In case 1, hydration has an effect on the polaron mobility, i.e., the latter is enhanced in case 2, hydration modifies the polaron electronic wave function. In addition, T p shows that the low frequency contribution to the proton relaxation is not affected, which is not consistent with a change in the coupling constant. It can be concluded that the increase in the macroscopic conductivity observed on hydration is related to an increase of the on-chain polaron mobility. A possible explanation can be proposed in terms of a solvation effect of the counterions resulting in a depinning of the polarons. 

Since proteins have several polar or charged side chains, especially on their surface, electrostatics is a simple, yet quite reliable tool to study various processes involving proteins (Baker and McCammon 2009). Electrostatic interactions may play a role in protonation, ligand binding, enzymatic catalysis, redox processes, and electron-proton coupling (e.g., in photosynthesis). 

The NMR spectra of heterocyclic compounds with seven or more ring members are as diverse as the shape, size and degree of unsaturation of the compounds. NMR is perhaps the most important physical method to ascertain the structure, especially the conformational statics and dynamics, of large heterocycles. Proton-proton coupling constants provide a wealth of data on the shape of the molecules, while chemical shift data, heteroatom-proton coupling constants and heteronuclear spectra give information of the electronic structure. Details are found in Chapters 5.16-5.22. Some data on seven-membered rings are included in Table 10. 

The substrates of catabolism—proteins, carbohydrates, and lipids—are good sources of chemical energy because the carbon atoms in these molecules are in a relatively reduced state (Figure 18.9). In the oxidative reactions of catabolism, reducing equivalents are released from these substrates, often in the form of hydride ions (a proton coupled with two electrons, H ). These hydride ions are transferred in enzymatic dehydrogenase reactions from the substrates... 

Rosenthal J, Nocera DG. 2007. Role of proton-coupled electron transfer in 0-0 bond activation. Acc Chem Res 40 543. 

The reduction of O2 in W by hydroquinone derivatives (QH2) in O is a subject of interest, since the reaction might offer the fundamental information on the electron transport coupled with the proton transport at a biomembrane realized by the respiration [2,3,56]. 

Proton-coupled intramolecular electron transfer has been investigated for the quinonoid compounds linked to the ferrocene moiety by a 7r-conjugated spacer, 72 (171) and 75 (172). The complex 72 undergoes 2e oxidation in methanol to afford 74, which consists of an unusual allene and a quinonoid structure, with the loss of two hydrogen atoms from 72 (Scheme 2). The addition of CF3SO3H to an acetonitrile solution of 74 results in two intense bands around 450 nm, characteristic of a semi-quinone radical, and a weak broad band at lOOOnm in the electronic... 

Belevich I, Verkhovsky MI, Wikstrom M (2006) Proton-coupled electron transfer drives the proton pump of cytochrome c oxidase. Nature 440 829-832. 

Sobolewski AL, Domcke W (2003) Ab initio study of the excited-state coupled electron-proton-transfer process in the 2-aminopyridine dimer. Chem Phys 294 2763... 

Note that (1) in saturated systems proton-proton couplings are seldom observed beyond three bonds, but (2) in aromatic and heterocyclic systems, four- and even five-bond coupling is commonplace. This is because spin coupling is transferred by electrons. Where you have extended conjugation, you can expect to observe coupling over a greater number of bonds. 

Keywords Excited-state intramolecular proton transfer Fluorescence dye Photoinduced electron transfer Proton coupled electron transfer Relaxation dynamics... 

As we saw in the previous section, Strategy 1 plants utilize ferric reductases, with NADPH as electron donor, coupled to proton extrusion and a specific Fe(II) transport system localized in the root plasma membrane. Saccharomyces cerevisiae also uses cell surface reductases to reduce ferric iron, and in early studies (Lesuisse et ah, 1987 ... 

When ESR spectra were obtained for the benzene anion radical, [C6II6] and the methyl radical, CH3, the proton hyperfine coupling constants were found to be 3.75 and 23.0 G, respectively, i.e. they differ by about a factor of 6. Since the carbon atom of CH3 has a spin density corresponding to one unpaired electron and the benzene anion carries an electron spin density of 1/6, the two results suggest that the proton coupling to an electron in a n-orbital is proportional to the spin density on the adjacent carbon atom ... 

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