Comparative Study of Spectral and Functional Properties of Wild Type and Double Mutant H(L173)L/I(L177)H Reaction Centers of the Purple Bacterium Cereibacter sphaeroides

Previously, we found that in the reaction center (RC) of the purple bacterium <i>Cereibacter sphaeroides</i>, formation of heterodimeric primary electron donor (P) caused by the substitution of His-L173 by Leu, was compensated by the second mutation Ile-L177 - His. Significant changes in the spectral properties, pigment composition, and redox potential of P observed in the H(L173)L RC, are restored to the corresponding characteristics of the native RC in the RC H(L173)L/I(L177)H, with the difference that the energy of the long-wavelength Q_Y optical transition of P increases significantly (by ~75 meV). In this work, it was shown using light-induced difference FTIR spectroscopy that the homodimeric structure of P is preserved in the RC with double mutation with partially altered electronic properties: electronic coupling in the radical-cation of the P⁺ dimer is weakened and localization of the positive charge on one of its halves is increased. Results of the study of the triple mutant RC, H(L173)L/I(L177)H/F(M197)H, are consistent with the assumption that the observed changes in the P⁺ electronic structure, as well as considerable blue shift of the Q_Y P absorption band in the RC H(L173)L/I(L177)H, are associated with modification of the spatial position and/or geometry of P. Using femtosecond transient absorption spectroscopy, it was shown that the mutant H(L173)L/I(L177)H RC retains a sequence of reactions P* → P⁺B_A^- → P⁺H_A^- → P⁺Q_A^- with electron transfer rates and the quantum yield of the final state P⁺Q_A^- close to those observed in the wild-type RC (P* is the singlet-excited state of P; B_A, H_A, and Q_A are molecules of bacteriochlorophyll, bacteriopheophytin, and ubiquinone in the active A-branch of cofactors, respectively). The obtained results, together with the previously published data for the RC with symmetrical double mutation H(M202)L/I(M206)H, demonstrate that by introducing additional point amino acid substitutions, photochemical activity of the isolated RC from <i>C. sphaeroides</i> could be maintained at a high level even in the absence of important structural elements - axial histidine ligands of the primary electron donor P.