The Mechanism of Triplet Energy Transfer from the Special Pair to the Carotenoid in Bacterial Photosynthetic Reaction Centers

Abstract

The kinetics of triplet energy transfer from the primary electron donor P, a bacteriochlorophyll dimer, to the carotenoid were measured in the M182HL mutant of Rhodobacter sphaeroides photosynthetic reaction centers. In this mutant, the accessory bacteriochlorophyll that is bound in the BM position in wild-type reaction centers is changed to a bacteriopheophytin. By using phosphorescence data for P in the reaction center and for bacteriochlorophyll a and bacteriopheophytin a monomers in simple solvents, the activation energy for triplet energy transfer from the special pair to the bacteriopheophytin at BM in M182HL is predicted to be 910 ± 50 cm-1. Analysis of the decay kinetics of 3P as a function of temperature gives an experimental activation energy of 950 ± 50 cm-1. Thus, the M182HL mutant allows for prediction and quantitative verification of the energetics of triplet energy transfer by a two-step mechanism from the special pair to the carotenoid through the chromophore in the BM binding site.