Effects of magnesium(II) ion on hydride-transfer reactions from an NADH model compound to p-Benzoquinone derivatives. the quantitative evaluation based on the reaction mechanism

The effects of Mg²⁺ ion on hydride-transfer reactions from an NADH model compound, 1 -benzyl-l,4- dihydronicotinamide (BNAH), to a series of p-benzoquinone derivatives (Q) as well as on the redox potentials of BNAH and Q in acetonitrile have been examined. The Mg²⁺ ion shows both accelerating and retarding effects on the hydride-transfer reactions depending on the p- benzoquinone derivative andthe Mg²⁺ concentration. Such dual effects of the Mg²⁺ ion have been well correlated with the change of the redox potentials of BNAH and Q in the presence of Mg²⁺ ion since it has been found that there is a simple correlation between the logarithm of the rate constant and the difference of the redox potentials between BNAH and Q in the absence and presence of Mg²⁺ ion. It is shown that a proposed reaction mechanism involving electron transfer from BNAH to Q followed by proton transfer from BNAH^+· to Q^-· in the rate-determining step of the hydride-equivalent transfer provides a quantitative evaluation of the single and unified correlation between the logarithm of the rate constant and the difference between the redox potentials of BNAH and Q in the absence and presence of Mg²⁺ ion. The effect of Mg²⁺ ion on the primary kinetic isotope effects on hydride transfer from BNAH to Q is also shown to be consistent with the proposed reaction mechanism. Moreover, a ternary complex involving BNAH, Mg²⁺ ion, and Q as a reaction intermediate prior to electron transfer from BNAH to Q has been detected for the first time.