Proton transfer processes, coupled

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... 

These experiments demonstrate the importance of proton transfer processes during hole transfer through DNA. S. Steenken has already remarked that a proton shift between the G C bases stabilizes the positive charge [23]. If such a proton shift is coupled with the hole shift, a deuterium isotope effect should arise. Actually, H/D isotope effects are described by V. Shafiro-vich, M.D. Sevilla as well as H.H. Thorp in their articles of this volume. Experiments with our assay [22] also demonstrate (Fig. 16) that hole transfer in protonated DNA (H20 as solvent) is three times more efficient than in deuterated DNA (D20 as solvent). If this reflects a primary isotope effect, it shows that the charge transfer is coupled with a proton transfer. 

The values of 3/(NH,H) coupling constant observed for imine proton can be helpful in detection of the proton transfer processes and determination of mole fractions of tautomers in equilibrium. For NH-form, this value is close to 13 Hz, lower values usually indicate the presence of tautomeric equilibrium. It should be mentioned that the values below 2.4 Hz have not been reported. The chemical shift of C—OH (C-2 for imines, derivatives of aromatic ortho-hydroxyaldehydes or C-7 for gossypol derivatives) carbon to some extent can be informative, however, this value depends on type of substituents and should be interpreted with caution. 

These proton transfer processes increase the driving force of the electron transfer reactions, which can thus be considered in terms of a proton-coupled electron transfer process [57-60]. 

Other methods of calculating the O N separation dependent proton transfer rates, such as a Fermi Golden Rule approach (Siebrand et al. 1984), can also be employed. In this approach, two harmonic potential wells (e.g., O-H N and, O H-N) are considered to be coupled by an intermolecular term in the Hamiltonian. Inclusion of the van der Waals modes into this approximation involves integration of the coupling term over the proton and van der Waals mode wavefunctions for all initial and final states populated at a given temperature of the system. Such a procedure requires the reaction exothermicity and a functional form for the variation of the coupling as a function of well separation. In the present study, we employ the barrier penetration approach this approach is calculationally straightforward and leads to a clear qualitative physical picture of the proton transfer process. 

R160 S. H. Alarcon and A. C. Olivieri, Residual Dipolar Coupling Effects in Solid State MAS NMR. Investigation of Proton Transfer Processes in Solids p. Pl/263... 

Proton transfer is coupled to the dynamics of two nuclear mode sets with widely different timescales, viz. the proton modes themselves and the environmental protein conformational and external solvent modes. The proton is trapped at the donor fragment at the equilibrium environmental conformational/solvent configuration in the reactants state, but fluctuations in these nuclear coordinates induce a state of resonance between the proton vibrational levels in the reactants and products states. Environmental gating modes with their special status in proton transfer are parts of this process. 

Bacteriorhodopsin is the quintessential transmembrane ion pun ). It consists of a small, seven-helix protein where proton transport across the membrane is driven by photoisomerization of retinal from the all trans to the 13-cis,l5-anti configuration. A number of high-resolution crystal structures of the protein and its photointermediates have been used to propose several competing mechanisms describing proton translocation to fhe extracellular surface. Unresolved issues include understanding how conformational changes couple to proton transfer and the role played by water molecules in the proton transfer process. ... 

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. 

Proton-coupled electron transfer (PCET) is a ubiquitous process in biology and chemistry and plays an essential role in multi-electron, multi-proton transfer processes of biological relevance, such as photosynthesis and respira-tion. The coupling between proton and electron motions stabilizes reaction intermediates, by preventing charge build-up during the accumulation of redox equivalents. The key role of PCET natural photosynthesis is probably the major contributor to this effect. 

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