Fluorescence excitation spectra of membrane-bound photosynthetic reaction centers of Rhodobacter sphaeroides in which the tyrosine M210 residue is replaced by tryptophan: evidence for a new pathway of charge separation

The BA and HA absorption bands of membrane bound reaction centers of Rhodobacter sphaeroides, in which the tyrosine M210 residue has been replaced by tryptophan appear to be only weakly present in the flourescence excitation spectrum of P∗ emission. In contrast, these contributions are clearly observable in the P+QA− excitation spectrum. Upon reduction of QA, the BA and HA bands in the excitation spectrum are largely recovered. These findings are discussed in terms of a model in which, following excitation of the BA or HA pigments a direct path exists for charge separation that does not involve the excited low-exciton state of P. Similar observations were made in two other mutants in which tyrosine M210 was replaced by leucine or phenylalanine.