Electron transfer dynamics in Rhodobacter sphaeroides reaction center mutants with a modified ligand for the monomer bacteriochlorophyll on the active side

The histidine ligand of the monomer bacteriochlorophyll molecule on the active side (B _A) of the photosynthetic reaction center from Rhodobacter sphaeroides was mutated to a number of other amino acids. Histidine (H) at the position L153 was replaced with aspartic acid (D), glutamic acid (E), glutamine (Q), glycine (G), leucine (L), phenylalanine (F), serine (S), valine (V) and tyrosine (Y). These mutations were created to investigate how the alteration of the ligand residue affects the properties of the B _A molecule and changes the dynamics of the primary charge separation in reaction centers. In all of the mutants, the changes in the ligand result in a blue-shift of the B _A absorption spectrum, in both visible and near-infrared spectral regions, with the size of the shift varying between mutants. The primary electron transfer time constants in these mutant reaction centers range from 4.5 to 18 ps, being substantially slower than the wild-type value of 3 ps. The decrease in the electron transfer rate is interpreted as being due to an increase in the free energy of the initial charge-separated state P ⁺B_A.