Mechanisms of photo-oxidation of NADH model compounds by oxygen

When an oxygen-saturated acetonitrile (MeCN) solution containing the NADH model compound, 9,10-dihydro-10-methylacridine (AcrH₂), is irradiated with u.v. light firstly in the presence and secondly in the absence of perchloric acid (HClO₄), AcrH₂ is oxidized by oxygen to yield the 10-methylacridinium ion (AcrH⁺) and 10-methyl-9-acridone (Acr=O) respectively, and reduction of oxygen yields hydrogen peroxide. The u.v. irradiation of a neutral aqueous solution containing 1-benzyl-1,4- dihydronicotinamide (BNAH) also results in the oxidation of BNAH by oxygen to yield BNA⁺ and H₂O₂. Kinetic studies and detection of radical intermediates by e.s.r. spectroscopy have revealed that the photo-oxidation of NADH model compounds by oxygen proceeds via radical-chain reactions, which are initiated by electron transfer from the singlet excited state of NADH model compounds to oxygen. The chain carrier for the photo-oxidation of AcrH₂ in the presence of HClO₄ in MeCN as well as for the photo-oxidation of BNAH in a neutral aqueous solution is hydroperoxy radical (HO₂^.), while acridinylperoxy radical (AcrO₂^.) acts as a chain carrier for the photo-oxidation of AcrH₂ in the absence of HClO₄ in MeCN. Although no photo-oxidation of AcrH₂ by oxygen occurs under visible-light irradiation, AcrH₂ is efficiently oxidized to AcrH⁺ in the presence of a flavin analogue used as a sensitizer, in MeCN under otherwise identical conditions. The flavin-sensitized photo-oxidation of AcrH₂ by oxygen proceeds via one-electron reduction of a flavin analogue by AcrH ₂ followed by the efficient oxidation of the reduced flavin by oxygen without the appreciable contribution of radical-chain processes.