effects of asp residues near the lside pigments in bacterial reaction centers the primary photochemistry in rhodobacter capsulatus reaction centers rcs containing the phe to asp mutation at l polypeptide residue 121 near the photoactive bacteriopheophytin bphl is characterized using ultrafast transient absorption spectroscopy at 285 k initial charge separation from p proceeds with essentially unity quantum yield in approximately 6 ps to form a transient denoted pi this transient is proposed to involve pbphl and probably pbchll as well bchll is the lside bacteriochlorophyll molecule pi decays in approximately 150 ps both by electron transfer to give pqa approximately 78 yield and by charge recombination to the ground state approximately 22 yield these results indicate that the fl121d mutant is closely related in terms of its electron transfer properties to previously reported rcs in which bphl is replaced with a bacteriochlorophyll betatype rcs or a pheophytin however the native bphl pigment is retained in the fl121d mutant we propose that the asp at l121 raises the free energy of pbphl thereby giving rise to the altered photochemistry at 77 k the pi lifetime is shortened slightly to approximately 120 ps and the yield of pqa is increased to approximately 88 this result is somewhat different from that obtained for betatype rcs at low temperature where the pi lifetime lengthens and the yield of pqa diminishes or stays about the same compared to the values near room temperature we exploit these differences in developing a model for the charge separation process in the fl121d mutant the effects of introducing an asp near bphl are compared to those obtained previously in two mutants in which an asp is introduced near bchll