Unidirectionality of Charge Separation in Reaction Centers of Rb. sphaeroides and Chloroflexus aurantiacus

Abstract

Electron transfer (ET) in reaction centers (RCs) has been shown to proceed predominantly along one of the two cofactor/protein branches, A and B, revealed in the X-ray structure analysis of R.viridis [1] and Rb,sphaeroides [2]. In particular, in RCs of Rb.sphaeroides, ET from the primary donor ¹P^* leading to the reduction of bacteriopheophytin (H) is unidirectional with a branching ratio of rates k_A/k_B > 5 [3]. In this paper we reinvestigate the limit of this branching ratio probing the Q_x-absorption bands of H_A and H_B in low temperature picosecond difference spectroscopy and using RCs where the ET along the A-branch is not blocked by photochemical or chemical reduction of H_A. Reaction centers of Rb.sphaeroides and Chloroflexus aurantiacus were chosen for the following reasons: Both RCs contain bacteriochlorophyll- and bacteriopheophytin-a and are free of carotenoids and cytochromes, which might interfere with a sensitive measurement in the 520nm-600nm region by overlap or electrochromic shifts of absorption bands. However, with respect to unidirectionality the most intriguing difference between the two species is the replacement of the bacteriochlorophyll monomer (B_B) which bridges ¹P^* and H_B in Rb.sphaeroides by a bacteriopheophytin in Chloroflexus aurantiacus. In this context the observation of the lower quantum yield of charge separation of 0.6 in Chloroflexus [4] as compared to unity in Rb.sphaeroides seemed to be consistent with some ET along the B-branch, since in the absence of Q_B recombination losses would be unavoidable. Therefore, this comparative unidirectionality study is supplemented by quantum yield measurements on the bleaching of P which are performed on the nanosecond time scale in contrast to the steady state measurements [4].