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[en] Highlights: • The dark-state recovery mechanism in BLUF photoreceptor was studied by QM/MM method. • Conserved His is a proton donor initiating tautomerization of the conserved Gln. • Conserved Tyr mediates forward and backward proton transfer steps. • The lifetime of the BLUF light state is controlled by a flavin-Gln hydrogen bond. Photoactivation of the bacterial photoreceptor BLUF is achieved by a unique tautomerization and rotation of the conserved Gln residue. To identify interactions controlling the lifetime of the high-energy ZZ-imide tautomer, we consider thermal dark-state recovery of the BLUF photoreceptor BlrB using MD and QM/MM calculations. We found that protonation/deprotonation of Gln51 by protonated His73 via Tyr9 was energetically favorable. The rate-limiting steps correspond to rearrangements of hydrogen bonds and rotation of the protonated Gln51 side chain. Notably, strengthening the hydrogen bond between protonated Gln51 and flavin O4 plays a central role in extending the lifetime of the photoactivated state.
[en] This book constitutes the proceedings of an international meeting held in September 19-21, 1990, at Zentrum fuer Interdisziplinaere Forschung, Universitaet Bielefeld, Germany. It describes various aspects of electron and proton transfer in chemistry and biology. The book starts with a survey of physiochemical principles of electron transfer in the gas and the solid phase, with thermodynamic and photochemical driving force. Inner and outer sphere mechanisms and the coupling of electron transfer to nuclear rearrangements are reviewed. These principles are applied to construct artificial photosynthesis. This leads to biological electron transfer involving proteins with transition metal and/or organic redox centres. The tuning of the free energy profile on the reaction trajectory through the protein by single amino acids or by the larger ensemble that determines the electrostatic properties of the reaction path is one major issue. Another one is the transformation of one-electron to paired-electron steps with protection against hazardous radical intermediates. The diversity of electron transport systems is represented in some chapters with emphasis on photosynthesis, respiration and nitrogenases. In photosynthesis of green plants light driven vectorial electron transfer is coupled to protolytic reactions, with about one quarter of the useful work derived from light quanta utilized for proton pumping across a coupling membrane. That is where the interchange of electrochemical (Dm) and chemical (ATP) forms of free energy storage and transfer in cellular energetics starts. The proton is distinguished from other reactants by an extremely small diameter and the ability of tunneling at reasonable rates. This is the basis for particular polarization, solvent and isotope effects as well as for hydrogen-bonded networks that are suited to long-range proton-transfer. (author). refs.; figs.; tabs
[en] the addition of benzylamine (BA) to benz-ylidenedimethylmalonate (BMM) take place in a single step in which the Cα-N bond formation and proton transfer to Cβ of BMM occur concurrently with a four-membered cyclic TS structure. The reaction center carbon, Cα , becomes more negative (ρY > 0) on going from the reactant to TS, but the negative charge development is stronger than that for the reactions of BID. The sign and magnitude of the cross-interaction constant, ρXY, is comparable to those for the normal bond formation processes in the SN2 and addition reactions. The normal kinetic isotope effects, kH/kD (>1), involving deuterated benzylamine nucleophiles (XC6H4CH2ND2), are somewhat smaller than those corresponding values for the reaction of CNS due to the smaller extent of bond formation in the TS. The relatively low ΔΗ≠ and large negative ΔS≠ values are also consistent with the mechanism proposed
[en] Adsorbed hydrogen in a transition metal may be to a first order approximation regarded as a system of protons interacting pairwise via long-range forces. By limiting momentum transfers in proton-proton scattering to small wavevectors, the relaxation function for a many-proton system is obtained analytically by a method of recurrence relations recently developed by Lee. Other dynamical quantities including diffusivity are also obtained analytically
[en] The authors review the transport capabilities of the MCNP and MCNPX Monte Carlo codes in the energy regimes in which tabular transport data are available. Giving special attention to neutron tables, they emphasize the measures taken to improve the treatment of a variety of difficult aspects of the transport problem, including unresolved resonances, thermal issues, and the availability of suitable cross sections sets. They also briefly touch on the current situation in regard to photon, electron, and proton transport tables.
[en] Details of the double-bond isomerization of 1-hexene over H-ZSM-5 were clarified using density functional theory. It is found that the reaction proceeds by a mechanism which involves the Brφnsted acid part of the zeolite solely. According to this mechanism, 1-hexene is first physically adsorbed on the acidic site, and then, the acidic proton transfers to one carbon atom of the double bond, while the other carbon atom of the double bond bonds with the Brφnsted host oxygen, yielding a stable alkoxy intermediate. Thereafter, the Brφnsted host oxygen abstracts a hydrogen atom from the C_6H_1_3 fragment and the C-O bond is broken, restoring the acidic site and yielding trans-2-hexene. The calculated activation barrier is 12.65 kcal/mol, which is in good agreement with the experimental value. These results well explain the energetic aspects during the course of double-bond isomerization and extend the understanding of the nature of the zeolite active sites