Bell-shaped dependence of the rate of ultrafast photoinduced electron transfer from aromatic amino acids to the excited flavin on the donor–acceptor distance in FMN binding proteins

Flavin mononucleotide (FMN)-binding proteins (FBP) from Desulfovibrio vulgaris, Miyazaki F, contain FMN as a cofactor. Upon photo-excitation of the FBP, photoinduced electron transfer (ET) takes place from tryptophan (Trp)32, tyrosine (Tyr)35 and Trp105 residues to the excited isoalloxazine ring (Iso∗) of FMN. The ultrafast fluorescence dynamics of the wild type, and the E13K, E13R, E13T and E13Q substitution isoforms of FBP were simultaneously analyzed with molecular dynamics simulation structures and Kakitani-Mataga ET theory, to obtain the time-dependent ET rates. A bell-shaped behavior of the logarithmic ET rates (InkET) from Trp32 and Trp106 to Iso∗ versus the centre-to-centre donor–acceptor distances (Rc) was obtained in the time domain of sub-ps to ps in all five FBP isoforms. The InkET from Tyr35 to Iso∗ linearly decreased with the Rc distance, which were much slower than those of Trp32 and Trp105. These findings suggest that the nuclear term in the ET theory is important in the ultrafast time domain, whereas the electronic coupling term may be dominant in slower ET rates.