Marcus Theory of Hydride Transfer from an Anionic reduced Deazaflavin to NAD+ Analogues

Article abstract of DOI:10.1021/ja00220a043

Eighteen rate constants, k_ij for hydride transfer from the conjugate base of 1,5-dihydro-3,10-dimethyl-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_ij values span 7 powers of 10 and the corresponding equilibrium constants, K_ij, span more than 13 powers of 10.For reactions with ΔG⁰ near zero, the k_ij values are close to those given by modified Marcus theory (ref 10).However, with more negative ΔG⁰ values, the observed k_ij increase more strogly than the calculated values.Agreement can be produced by making the standard free energy of precursor complex formation, symbolized W^T +- here, to indicate that it applies to reactants of opposite charge, a linear function of ΔG⁰, and treating the slope and interrcept of the linear relation as adjustable parameters.The best fit is obtained with W^T+-(in kJ*mol^-1)=-9.4+0.11ΔG⁰.An avarage discrepancy between calculated and observed ln k_ij values of 0.5 is achieved, which is a good as the overall fit achieved for hydride transfer from neutral NADH analogues to NAD⁺ analogues (ref 10).The form and the parameterization of W^f are shown to be a physically reasonable approximation for reactions with ΔG⁰<0.These results strengthen the conclusion (ref 10) that a wide range of hydride transfer rates can be quantitavely understood without introducing high-energy metastable intermediates (radicals and radical ions).