One Step Electron Transfer to P⁺H⁻ in Reaction Centers of Rhodobacter sphaeroides Derived from Dichroic Excitation Spectra of Electric Field Modulated Fluorescence Yield

Abstract

Based on the X-ray structure analysis of reaction centers (RCs) of R.viridis [1] and Rb.sphaeroides [2,3] essentially two mechanisms of primary charge separation are discussed. These differ in the role of the monomer bacteriochlorophyll (B) locatedbetween the primary donor (¹P*) and a bacteriopheophytin molecule (H). In the two-step mechanism according to

$$ ^1P*BH\mathop \to \limits^{{k_1}} {P^ + }{B^ - }H\mathop \to \limits^{{k_2}} {P^ + }B{H^ - } $$

(1)

P⁺B⁻H is the first radical pair formed. It develops with a rate constant k₁ and decays with the rate k₂ in the subsequent electron transfer (ET) from P⁺B⁻H to P⁺BH⁻. In the unistep mechanism the radical ion pair P⁺BH⁻ is formed directly from ¹P* BH with the rate k according to 1P*BH With the rate k according to

$$ ^1P*BH\mathop \to \limits^k {P^ + }B{H^ - } $$

(2)

where B mediates superexchange interaction between ¹P*BH and P⁺BH⁻ [4]. The latter mechanism is favoured byfemtosecond absorption spectroscopy at 10K [5], whereas recent measurements at room temperature support the formation of P⁺B⁻H in RCs of Rb. sphaeroides [6]. Other workers detecting the wavelength dependence of the transient absorption signals suggest an inhomogeneous distribution of primary electron transfer rates [7]. Under such circumstances an unambiguous interpretation of transient absorption data in the frame of a global fitting procedure becomes increasingly difficult.