Conformational processes energy requirements

The energy required to activate hydrogenase is in the order of 80kJ/mol. Several possible processes have been suggested so far, e.g. a rearrangement of the spatial conformation of the active site or of the surrounding protein in order to allow substrate accessibility to the active site. 

At the energies required for conformational conversions and other exchange processes which are amenable to study by NMR spectroscopy, the reacting molecules have state densities which are much lower than those of molecules undergoing isomerization and decomposition reactions which are generally found to obey RRKM kinetics. Whether these systems can be modeled with RRKM theory is a question of current interest. Table 8 lists molecules for which pressure-dependent gas-phase chemical exchange rate constants have been obtained. 

Discussion of the physicochemical sizes of substituents on the basis of conformational analysis of the torsional process of substituted cyclohexanes, cyclophanes, and diaryl derivatives have been summarized by Schlosser [5]. The energies required for the conformational exchange of an equatorial position to the axial position for each substituent in cyclohexane are listed in Table 1.47 [6]. For fluorine, the energy difference between the axial and equatorial environment is negligibly small as compared with methyl, while that of CF3 seems larger than that of z-Pr so far observed in the cyclohexane torsional system. From the... 

A mathematical method has been described for determining, from independent variables that include the contribution of valence-angle deformation, the potential barrier for conformational inversion in j8-D-glucopyranose. The energy required for the conversion of a chair form to a boat form was calculated to be 13.5 kcal.mole , and the energy required for the reverse process, 6.5 kcal.mole . Ex-... 

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