Semiquinone formation of d-amino-acid oxidase by irradiation

It has been found that oxidized d-amino-acid oxidase (d-amino-acid: oxygen oxidoreductase (deaminating), EC 1.4.3.3) is transformed to a semiquinone by irradiation with visible light. The semiquinone formation was detected by measurements of the optical absorption spectrum and electron spin resonance. A fairly stable semiquinone was obtained under anaerobic conditions but not under aerobic conditions since it easily reacted with molecular oxygen. After a cycle of semiquinone formation and reoxidation, the d-amino-acid oxidase activity was found to be completely restored. Although no evidence was obtained about an electron donor for the semiquinone formation, it did not appear likely, from the measurement of the activation energy of 5 kcal per mole, that water donated electrons. The electron spin resonance signal of the semiquinone exhibited an asymmetrical shape. Possible interpretations for the phenomena are discussed.