Marcus Theory of Hydride Transfer from an Anionic Reduced Deazafiavin to NAD+ Analogues

초록

Eighteen rate constants. k<inf>ij</inf>, for hydride transfer from the conjugate base of 1,5-dihvdro-3,10-dimethvl-5-diazaisoalloxazine to a variety of pyridinium, quinolinium, phenanthridinium, and acridinium ions have been determined. (All the oxidizing agents can be regarded as analogues of NAD+.) The k<inf>ij</inf> values span 7 powers of 10 and the corresponding equilibrium constants, k<inf>ij</inf>, span more than 13 powers of 10. For reactions with ΔG° near zero, the k<inf>ij</inf> values are close to those given by modified Marcus theory (ref 10). However, with more negative ΔG° values, the observed increase more strongly than the calculated values. Agreement can be produced by making the standard free energy of precursor complex formation, symbolized Wτ <inf>±</inf> here, to indicate that it applies to reactants of opposite charge, a linear function of ΔG°, and treating the slope and intercept of the linear relation as adjustable parameters. The best fit is obtained with Wτ± (in kJ mol-1) = -9.4 + 0.11 ΔG°. An average discrepancy between calculated and observed In k<inf>ij</inf> values of 0.5 is achieved, which is as good as the overall fit achieved for hydride transfer from neutral NADH analogues to NAD+ analogues (ref 10). The form and the parametrization of Wτ are shown to be a physically reasonable approximation for reactions with ΔG° < 0. These results strengthen the conclusion (ref 10) that a wide range of hydride transfer rates can be quantitatively understood without introducing high-energy metastable intermediates (radicals and radical ions). © 1988, American Chemical Society. All rights reserved.