Low-Temperature Femtosecond Spectroscopy of the Initial Step of Electron Transfer in Reaction Centers from Photosynthetic Purple Bacteria

The initial step of charge separation at 10 K has been monitored with 100-fs time resolution in reaction centers from Rhodopseudomonas viridis and Rhodobacter sphaeroides as well as in reaction centers from the latter species in which one of the two monomeric bacteriochlorophyll (B) molecules has been removed by treatment with borohydride. Upon excitation at 870 nm, the absorbance changes measured at several wavelengths in the near-infrared absorption bands of the pigments, and notably at the absorption maximum of the B molecule(s), give no indication of a detectable concentration of B- Instead, the appearance of the cation radical of the dimeric primary electron donor (P) and of the bacteriopheophytin anion develops in concert with the decay of P*. An initial bleaching of the 850-nm band in reaction centers from Rhodopseudomonas viridis is consistent with an assignment of at least a large fraction of this band to the high-energy exciton component of P. Upon excitation of the B molecule(s) around 600 nm in the three types of reaction centers investigated, ultrafast energy transfer leads to the formation of P* in less than 100 fs. Under these conditions, a fast transient bleaching decaying with a 400-fs time constant is observed within the absorption band of B. This transient is also present upon preferential excitation of the bac-teriopheophytins in the reaction center of Rhodopseudomonas viridis.