Photoinduced electron transfer from aromatic amino acids to the excited isoalloxazine in flavin mononucleotide binding protein. Is the rate in the inverted region of donor–acceptor distance not real?

Abstract Mechanisms of photoinduced electron transfer (ET) from tryptophanes 32 and 106 in subunits A and B (Trp32A, Trp32B, Trp106A and Trp106B) of wild type flavin mononucleotide binding protein (FBP) dimer were studied through relations of the logarithmic ET rate (ln Rate) vs the donor-acceptor distance (Rc). The sum (GT) of standard free energy gap (SFEG) between the products and reactants, electrostatic energy (ESDA) between the photo-products and solvation reorganization energy (SROE) and electrostatic energy (NetES) between the photo-products and ionic groups inside the protein were numerically determined for the all donors with atomic coordinates obtained by molecular dynamic simulation. The GT values of Trp32A and Trp32B displayed always negative in the entire Rc range, which predicts that ET rate becomes slower as the Rc shorter. The reason of negative GT values in Trp32A and Trp32B were numerically elucidated with the mean values of SFEG, ESDA, SROE, and NetES.