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 B-M 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 hi simple solvents, the activation energy for triplet energy transfer from the special pair to the bacteriopheophytin at B-M in M182HL is predicted to be 910 +/- 50 cm(-1). Analysis of the decay kinetics of P-3 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 B-M binding site.