The mechanism of photoinduced electron transfer in the d-amino acid oxidase–benzoate complex from pig kidney: Electron transfer in the inverted region

d-Amino acid oxidase (DAAO) from pig kidney binds flavin adenine dinucleotide (FAD) as a cofactor. The DAAO–benzoate (Bz) complex (DAOB) is remarkably more stable than DAAO. It was previously reported that the fluorescence lifetime of FAD in DAAO is reduced by 66% upon Bz binding. The mechanism of photoinduced electron transfer (ET) in the DAOB monomer from Bz to the excited isoalloxazine (Iso*) was studied by molecular dynamics simulation (MDS) and by Kakitani and Mataga electron transfer theory (KM theory), and compared with that in DAAO. The ET parameters contained in KM theory were determined to obtain the fluorescence lifetime of DAOB (60 ps). The ET mechanism in DAOB is different from other flavoproteins, where neutral tryptophans and tyrosines are the ET donors, since the anionic Bz acts as the anionic ET donor in DAOB. Moreover, ET takes place in the inverted region. Charge transfer interaction (CT) was studied by semi-empirical molecular orbital (MO) methods. The dipole moment in the Iso*–Bz system was lower than that in the isoalloxazine (Iso)–Bz system, which was ascribed to CT from the negatively charged Bz to Iso*. The mean interaction energies between Iso and Bz over 100 snapshots were −16.8 and −8.1 kcal/mol in the excited and the ground states, respectively, with mean charge densities for the excited and the ground states of −0.354 and −0.013 at Iso and −0.646 and −0.987 at Bz, respectively. The ionization potential of Bz was evaluated as 7.3 eV by ET analysis, which was in excellent agreement with that obtained by MO calculations. The dielectric constant between Iso and the donors was 2.2 in DAOB, compared to 5.8 in DAAO. According to the MDS, several water molecules existed around Iso in DAAO that were all removed upon Bz binding resulting in a strongly non-polar environment near Iso in DAOB.