ELECTRONIC-ENERGY TRANSFER WITHIN THE HEXAMER COFACTOR SYSTEM OF BACTERIAL REACTION CENTERS

Flow of excitation energy within the bacteriochlorin cofactor system o f bacterial reaction centers has been studied by multicolor transient absorption spectroscopy using differently shaped excitation pulses of 30 fs, both at room temperature and at 15 K. This approach, which incl udes the analysis of free induction decay signals, helps to disentangl e the processes of electronic dephasing, energy transfer, internal con version Part of the excitations (estimated at 80-90% at room temperatu re) flow via the scheme H --> B* --> P+* --> P-*, in which H* and B* represent excited bacteriopheophytin and bacteriochlorophyll monomers and P+ and P-* the upper and lower excited states of the bacteriochlo rophyll dimer P, respectively. H --> B* takes less than 100 fs. B* -- > P+ (similar to 200 fs) energy transfer is a slower process than the internal conversion process within P (50-100 fs) and therefore is ra te limiting for P- formation. At low temperature, electronic dephasin g associated with the B and P+* states takes place on a similar time scale as P internal conversion. Upon excitation with pulses centered at 820 nm, estimated to spectrally overlap the close-lying B and P+ ba nds to equal extent, more than 90% of the P- band bleaches instantaneo usly and B --> P* transfer occurs for less than 10%. This might be in dicative of an unexpectedly strong contribution of P+. to the 800 nm b and. Alternatively, we propose that under these conditions the strongl y coupled B and P+* states are coherently excited. This possibility i s consistent with B --> P+* electronic energy transfer occurring in t he strong coupling regime under conditions where B is more selective populated. The observed time scales are temperature-insensitive and fu rthermore similar in Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis, which eliminates the possibility of direct B --> P+* energy transfer. At room temperature, part of the excitation energy flow devi ates from the above scheme, resulting in excitation-wavelength-depende nt distributions of excited and/or radical pair states on the picoseco nd time scale. In general agreement with previous low-temperature work on a mutant reaction center we found that this deviation is particula rly strong (up to similar to 50%) at low temperature. Upon excitation in H and B, a high quantum yield is observed of radical pairs involvin g H-L, with characteristics in the H-L Q(Y) region, differing from tho se of P+HL- and suggesting that they can be identified as BL+HL-.