Hydride transfer between NAD+ analogues

Abstract

Rate (k) and equilibrium constants (K) have been measured for 17 non-degenerate hydride-transfer reactions, the donors and acceptors generally being variously substituted and elaborated pyridinium ions. Rate constants have also been measured for 6 degenerate reactions of the same type. These results have been used to test the Marcus theory of atom transfer. The Brønsted plot shows a good deal of scatter, but around K= 1.0 it has about the predicted slope (α) of 0.5. Using the measured values of k for degenerate reactions and assuming intrinsic reactivity to be constant within structural families, values of k for 12 non-degenerate reactions were calculated without introducing adjustable parameters. They reproduce 99% of the variation in the measured values, the measured values generally being larger than the calculated values. The data do not permit a test of the predicted variation of α with K.

A proposed mechanism involving electron transfer followed by proton transfer followed by another electron transfer, with metastable intermediates between the steps, can be excluded. Such a mechanism would lead to values of k below those predicted by the theory.