Energetic Comparison between Photoinduced Electron-Transfer Reactions from NADH Model Compounds to Organic and Inorganic Oxidants and Hydride-Transfer Reactions from NADH Model Compounds to p-Benzoquinone Derivatives

Kinetic studies on photoinduced electron-transfer reactions from dihydropyridine compounds (PyH₂) as being NADH model compounds to organic and inorganic oxidants and hydride-transfer reactions from PyH₂to p-benzoquinone derivatives in the absence and presence of Mg²⁺ion are reported by determining over 150 rate constants. These results, combined with the values of Gibbs energy change of the photoinduced electron-transfer reactions as well as those of each step of the hydride-transfer reactions as being the e“-H⁺-e” sequence, which are determined independently, revealed that the rate constants of the photoinduced electron-transfer reactions obey the Rehm-Weller-Gibbs energy relationship and that the activation barrier of the hydride-transfer reactions from PyH₂to Q is dependent solely on the Gibbs energy changes of the initial electron transfer from PyH₂to Q and the following proton transfer from PyH₂'⁺to Q'~ and thus independent of the Gibbs energy change of the final electron transfer from PyHH to QH The retarding effect of Mg²⁺ion observed on the photoinduced electron transfer and hydride-transfer reactions of PyH₂is ascribed to the positive shifts of the redox potentials of the ground and excited states of PyH₂due to the complex formation with Mg²⁺ion.