Structure and kinetics of the intermediate biradicals generated from intramolecular electron transfer reaction of FAD studied by an action spectrum of the magnetic field effect

The intramolecular electron transfer reaction induced by light irradiation of flavin–adenine dinucleotide (FAD) was studied by an action spectra of the magnetic field effect on the transient absorption. The action spectra depended on pH in the region of 2.0–4.0. They are reproduced by the sum of two components: T–T absorption and radical form of flavin. The contribution of T–T absorption is dominant at pH⩽2.3 whereas that of flavin radical is dominant at pH>3.3. The existence of the equilibrium between the radical pair and triplet excited state is concluded.