Tunneling, environmentally coupled

Maglia, G. and AUemann, R.K. (2003). Evidence for environmentally coupled hydrogen tunneling during dihydrofolate reductase catalysis. J. Am. Chem. Soc. 125, 13372-13373... 

Knapp, M.J. and Klinman, J.P. (2002). Environmentally coupled hydrogen tunneling. Linking catalysis to dynamics. Eur. J. Biochem. 269, 3113-3121... 

This treatment is confined to tunneling correction to TST and is not valid for any Marcus-like model (e.g., environmentally coupled tuimeling). Thus, an Arrhenius plot of KIEs can distinguish between data that might be fitted by a tunneling correction model (e.g., A]/Ah > 1 with A a 0) and data that can only be fitted by a Marcus-like model (e.g., Aj/Ah > 1 with large AEa)-... 

It must be emphasized that the semiclassical limits for the energy of activation (the slope of the Arrhenius plot) are not well defined. Consequently, in order to establish that nonclassical features are evident from temperature independent KIEs, the pre-exponential Arrhenius factor must be outside their semiclassical limits. For example, a recent paper misinterpreted nearly temperature independent KIEs with Ah/Ad close to unity as Evidence for environmentally coupled hydrogen tunneling during dihydrofolate reductase catalysis [84]. Actually, the temperature dependence of the KIEs in that study (AE = 3.0 + 0.7 kcal mol above 20 °C) was well within the semiclassical range. 

Figure 12.7. Illustration of Marcus-like models energy surface of environmentally coupled hydrogen tunneling. Two orthogonal coordinates are presented p, the environmental energy parabolas for the reactant state (R) and the product state (P) and q, the H-transfer potential surface at each p configuration. The gray shapes represent the populated states (e.g., the location of the...

Mixed labeling experiments with specifically isotopically substituted 4R- and 4S-NADPH cofactors established the primary and secondary kinetic isotope effects and their temperature dependence for the hydride transfer reaction. Indeed, resulting data could be rationalized only by a reaction model featuring an extensive tunneling contribution that is environmentally coupled. The difference in the observed and calculated intrinsic kinetic isotope effects requires a commitment factor arising from dissecting the pre-steady state hydride step into kinetic steps, one the actual hydride transfer step itself and the other a motion of the protein and/or nicotinamide associated with the hydride transfer step [17]. 

Fig. 1. The Marcus parabolic free energy surfaces corresponding to the reactant electronic state of the system (DA) and to the product electronic state of the system (D A ) cross (become resonant) at the transition state. The curves which cross are computed with zero electronic tunneling interaction and are known as the diabatic curves, and include the Born-Oppenheimer potential energy of the molecular system plus the environmental polarization free energy as a function of the reaction coordinate. Due to the finite electronic coupling between the reactant and charge separated states, a fraction k l of the molecular systems passing through the transition state region will cross over onto the product surface this electronically controlled fraction k l thus enters directly as a factor into the electron transfer rate constant...

In situ STM of metalloproteins with localized low-lying redox levels can be expected to follow ET patterns similar to metalloprotein ET in homogeneous solution and at electrochemical surfaces. The redox level is thus strongly coupled to the protein and solvent environment. A key notion is that the vacant local level (oxidized form) at equilibrium with the environmental nuclear motion is located well above the Fermi levels of both the substrate and tip, whereas, the occupied level (reduced form) at equilibrium is located well below the Fermi levels. Another central notion is that the local redox level at the transition metal centre is still much lower than environmental protein or solvent electronic levels. The redox level therefore constitutes a pronounced indentation in the tunnel barrier. This alone would strongly enhance tunnelling. Configurational fluctuations in the environment can, secondly take the redox level to such low values that temporary physical population occurs. This requires nuclear activation but can still be favourable due to the much shorter electron tunnel distances... 

Abstract This paper introduces the current state of research on the coupled Thermal-Hydraulic-Mechanical-Chemical (THMC) processes in geo-materials and applications for geotechnical and geo-environmental engineering in China. The paper puts equal emphasis on both the achievements of experimental results for fundamental studies, and the applications in rock mechanics and foundation engineering, tunnel engineering, geothermal mining, unsaturated soil and dynamic consolidation. At the end, it presents a model for chemical mining. 

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