Simultaneous analyses of the rates of photoinduced charge separation and recombination between the excited flavin and tryptophans in some flavoproteins: Molecular dynamics simulation

Ultrafast transient absorption (TA) spectroscopy has been one of the most powerful experimental tools to study the mechanism of photoinduced electron transfer (ET) as in photosynthetic and flavin photoreceptor systems in plants. However, no work has been reported on their quantitative mechanisms. Apparent rates of charge separation (CS) from tryptophans (Trps) to the excited isoalloxazine (Iso*) and charge recombination (CR) from the produced ion pairs to Trps and Iso in the ground states are reported to be 0.25 ps and 3.2 ps in medium-chain acyl-CoA dehydrogenase (MCAD), and 0.15 ps and 6.6 ps in flavin mononucleotide binding protein (FMN-bp), obtained by an ultrafast TA method. The decays of the CS and CR processes were for the first time simultaneously analyzed with an ET theory and structures obtained by molecular dynamics simulation. MCAD and FMN-bp form a tetramer and dimer, respectively. The CS and CR rates of an individual donor and various related physical quantities were numerically obtained. It was found that both CS and CR rates were fastest from Trp166 among four Trps in MCAD and those from Trp106 among two Trps in FMN-bp. Logarithmic CS rates in MCAD were dependent on the donor–acceptor distance (Rc) with parabolic functions, while those of CR rates linearly decreased with Rc. Reasons why CS rates were faster than CR rates in both MCAD and FMN-bp were elucidated in terms of pre-exponential factors in the theory. The present method could be useful to understand the precise mechanisms of initial steps of biological functions of photoreceptors in plants.