Anisotropic excited states relaxation of FAD bound with bacterial diaphorase

This paper presents the results of experimental studies of excited state dynamics of FAD in free form and FAD bound with bacterial diaphorase. The studies were carried out by recording the time-resolved polarized fluorescence using time-correlation single photon counting technique after excitation of FAD by femtosecond laser pulses. It was found that excited state dynamics of FAD-diaphorase complexes differs significantly from that of free FAD. Free FAD exhibited four fluorescence decay times ranging from a few picoseconds to a few nanoseconds, while two fluorescence decay times were observed in FAD-diaphorase complex. The analysis of fluorescence polarization decay of FAD-diaphorase complex revealed a subnanosecond decay time of $\tau_{bv}$ = 130 ps. It was shown that this fluorescence depolarization time was due to anisotropic vibrational relaxation in FAD excited state which leads to rotation of transition dipole moment due to rearrangement of the molecular nuclei configuration after excitation.