Blue light drives B-side electron transfer in bacterial photosynthetic reaction centers

Abstract

The structure of the photosynthetic reaction center from purple nonsulfur bacteria is quasi-symmetric, providing two potential pathways for transmembrane electron transfer. Yet past measurements have demonstrated that only one of the two pathways (the A-side) is used to any significant extent upon direct excitation of the lowest excited singlet state of the reaction center initial electron donor. Here, it is shown that excitation with blue light into the Soret band of the reaction center gives rise to electron transfer along the alternate or B-side pathway. B-side charge separation occurs within 250 fs, probably forming BB+HB- initially and then P+HB- in about 1 ps. P+HB- decays with a 5 ps time constant at 295 K, forming the normal A-side charge-separated state, P+HA-. At low temperature P+HB- is trapped, suggesting that the conversion of P+HB- to P+HA- occurs via a higher energy intermediate state. It is proposed that the physiological function of B-side electron transfer is to rapidly quench and productively conserve the excited state energy from blue and UV light absorption by the reaction center. Further, a closely arranged network of tryptophan residues surrounds the reaction center cofactors and rapidly transfers energy into the Soret band. Thus, the B-side of the reaction center appears to process the blue and UV light absorbed by the reaction center cofactors and protein, serving to protect the reaction center cofactors from damage while generating productive charge separated states.