Environmental control of primary photochemistry in a mutant bacterial reaction center

The core structure of the photosynthetic reaction center is quasisymmetric with two potential pathways (called A and B) for transmembrane electron transfer. Both the pathway and products of light-induced charge separation depend on local electrostatic interactions and the nature of the excited states generated at early times in reaction centers isolated from Rhodobacter sphaeroides. Here transient absorbance measurements were recorded following specific excitation of the Q _y transitions of P (the special pair of bacteriochlorophylls), the monomer bacteriochlorophylls (B _A and B _B), or the bacteriopheophytins (H _A and H _B) as a function of both buffer pH and detergent in a reaction center mutant with the mutations L168 His to Glu and L170 Asn to Asp in the vicinity of P and B _B. At a low pH in any detergent, or at any pH in a nonionic detergent (Triton X-100), the photochemistry of this mutant is faster than, but similar to, wild type (i.e. electron transfer occurs along the A-side, 390 nm excitation is capable of producing short-lived B-side charge separation (B _B ⁺H _B ^-) but no long-lived B _B ⁺H _B ^- is observed). Certain buffering conditions result in the stabilization of the B-side charge separated state B _B ⁺H _B ^-, including high pH in the zwitterionic detergent LDAO, even following excitation with low energy photons (800 or 740 nm). The most striking result is that conditions giving rise to stable B-side charge separation result in a lack of A-side charge separation, even when P is directly excited. The mechanism that links B _B ⁺H _B ^- stabilization to this change in the photochemistry of P in the mutant is not understood, but clearly these two processes are linked and highly sensitive to the local electrostatic environment produced by buffering conditions (pH and detergent).