Photoinduced electron transfer in wild type and mutated flavodoxin from Desulfovibrio vulgaris, strain Miyazaki F.: Energy gap law

Time–dependent changes in the geometrical factors near the isoalloxazine (Iso) residue of FMN in three mutant isoforms [Y97F, W59F and W59F–Y97F (DM, double mutation)] of the flavodoxin (FD) from Desulfovibrio vulgaris, strain Miyazaki F., were obtained by molecular dynamic (MD) simulation. The center to center distances from Iso to Trp59 in Y97F and to Tyr97 in W59F were 0.78 nm and 0.55 nm, respectively. The remarkable fluorescence quenching in these proteins has been explained in terms of photoinduced electron transfer (ET) from the Trp59 and/or Tyr97 residues to the excited isoalloxazine (Iso*). The ultrafast fluorescence dynamics of the wild type (WT) and the Y97F, W59F and DM variant FDs reported by Mataga et al. (J. Phys. Chem. B 106 (2002) 8917–8920), were simultaneously analyzed by the electron transfer theory of Kakitani and Mataga (KM theory) and the atomic coordinates determined by MD, according to a non-linear least squares method. Agreements between the observed and calculated decays were all very good. The obtained physical constants contained in the KM theory were, for Trp and Tyr, respectively, a frequency factor (ν0) of 3.09 × 103 ps−1 and 2.46 × 103 ps−1, an ET process coefficient (β) of 55.6 nm−1 and 9.64 nm−1, a critical transfer distance (R0) of 0.772 nm and 0.676 nm, plus a free energy related to the electron affinity of Iso* (GIso0) of 7.67 eV. These constants were common to all three mutant FD systems. In contrast, the static dielectric constant depended on the FD systems, being 4.78, 4.04 and 2.28 in the Y97F, W59F and DM variant FDs, respectively. The mean ET rate to Iso* was fastest from Trp59 in Y97F among the three systems. The total free energy gap in the FD systems was obtained as a sum of the net electrostatic (ES) energy between ion pairs and the standard free energy gap. A plot of ln kET/λS vs. −ΔGT0/λS in all ET donors, where kET is ET rate, λS is the reorganization energy and ΔGT0 is the total free energy gap, revealed that ln kET/λS can be expressed by a parabolic function of −ΔGT0/λS and the ET process in FD took place mostly in the normal region.