PROTEIN MODIFICATIONS AFFECTING TRIPLET ENERGY-TRANSFER IN BACTERIAL PHOTOSYNTHETIC REACTION CENTERS

The efficiency of triplet energy transfer from the special pair (P) to the carotenoid (C) in photosynthetic reaction centers (RCs) from a la rge family of mutant strains has been investigated. The mutants carry substitutions at positions L181 and/or M208 near chlorophyll-based cof actors on the inactive and active sides of the complex, respectively. Light-modulated electron paramagnetic resonance at 10 K, where triplet energy transfer is thermally prohibited, reveals that the mutations d o not perturb the electronic distribution of P. At temperatures greate r than or equal to 70 K, we observe reduced signals from the carotenoi d in most of the RCs with L181 substitutions. In particular, triplet t ransfer efficiency is reduced in all RCs in which a lysine at L181 don ates a sixth ligand to the monomeric bacteriochlorophyll B-B. Replacem ent of the native Tyr at M208 on the active side of the complex with s everal polar residues increased transfer efficiency. The difference in the efficiencies of transfer in the RCs demonstrates the ability of t he protein environment to influence the electronic overlap of the chro mophores and thus the thermal barrier for triplet energy transfer.