Photo-accumulation of the P +Q B− radical pair state in purple bacterial reaction centres that lack the Q A ubiquinone

Abstract

Photo-excitation of membrane-bound Rhodobacter sphaeroides reaction centres containing the mutation Ala M260 to Trp (AM260W) resulted in the accumulation of a radical pair state involving the photo-oxidised primary electron donor (P). This state had a lifetime of hundreds of milliseconds and its formation was inhibited by stigmatellin. The absence of the Q A ubiquinone in the AM260W reaction centre suggests that this long-lived radical pair state is P +Q B −, although the exact reduction/protonation state of the Q B quinone remains to be confirmed. The blockage of active branch (A-branch) electron transfer by the AM260W mutation implies that this P +Q B − state is formed by electron transfer along the so-called inactive branch (B-branch) of reaction centre cofactors. We discuss how further mutations may affect the yield of the P +Q B − state, including a double alanine mutation (EL212A/DL213A) that probably has a direct effect on the efficiency of the low yield electron transfer step from the anion of the B-branch bacteriopheophytin (H B −) to the Q B ubiquinone.