Ultrafast photoinduced electron transfer in o-aminobenzoate – d-Amino acid oxidase complex

Abstract Absorption spectrum of ortho-amino benzoate (oAB) – D-amino acid oxidase complex (ODC) displays a charge transfer (CT) band around 570 nm. The fluorescence decays of the second lowest electronic state of ODC were measured using a fluorescence up-conversion technique. The lifetimes were 100 fs and 130 fs at 500 nm and 514 nm fluorescence emission wavelengths, respectively. These ultrafast lifetimes are ascribed to the photoinduced electron transfer (PET) from the oAB to the excited isoalloxazine (Iso*) in the ODC. The PET mechanism in ODC was studied with Marcus-Kakitani-Mataga (MKM) theory. The protein structures were obtained by molecular dynamics simulation (MDS). Mean distances between Iso and oAB over 12,500 MDS snapshots were 0.60 in subunit A (Sub A) and 0.58 nm in Sub B. The calculated lifetimes at 514 nm-emission were 97 and 179 fs in Sub A and Sub B, while at 500 nm there were 97 and 101 fs, respectively. Solvent reorganization energy was quite different between Sub A and Sub B, which is main reason for difference in the calculated lifetimes. Electrostatic (ES) energies between the photoproducts of donor -acceptor, and net ES energies between the photoproducts and ionic groups in the protein were calculated with MDS structures. These ES energies were evaluated with charge densities of neutral radical of oAB produced after PET and anion radical of Iso obtained by semi-empirical molecular orbital (MO) method. The logarithmic PET rate did not display any clear dependence on Rc, despite the ultrafast scale of the PET rates. Most of the logarithmic PET rates in Sub A displayed in the inverted region of the standard free energy gap in the energy gap law. The transition dynamics among local excited states of Iso, CT state and the ion-pair state were discussed.