Electron transfer in rhodobacter sphaeroides reaction centers containing Zn-bacteriochlorophylls : a hole-burning study

Abstract

Nonresonant and resonant transient, photochemical hole-burned (HB) spectra are presented for primary electron donor states of a novel bacterial reaction center (Zn-RC) of Rhodobacter sphaeroides, containing six Zn-bacteriochlorophylls (Zn-BChls). A "Zn-β-RC" in which the Zn-BChl in the bacteriopheophytin (BPhe)-binding site on the A side (Hᴀ) has the Zn penta-coordinated, was also studied. The fifth ligand comes from a histidine introduced by site-directed mutagenesis. Formation of the P⁺Qᴀ⁻ state was observed in both types of RC, although under identical experimental conditions a significantly deeper P_ band band (corresponding to the lower-energy, special pair, excitonic component) was revealed in the Zn-RC. Assuming a similar lifetime of the P⁺Qᴀ⁻ state, the quantum yield of P⁺Qᴀ⁻ formation decreased by ~60% in the Zn-β-RC (compared to the Zn-RC), as was seen in a comparison of analogous (Mg) BChl-containing wild type and β-RCs of Rb. sphaeroides [Kirmaier et al. Science1991, 251, 922]. However, the average (weakly frequency-dependent) low-temperature electron transfer (ET) rates of the Zn-RC and Zn-β-RC (measured from zero phonon holes in resonant transient HB spectra) were both ~1 ps and similar to a rate previously measured in the Rb. sphaeroides native RC [Johnson et al. J. Phys. Chem. 1989, 93, 5953]. Electron transfer rates observed in this work on the Zn-RC yielded a P870* decay rate in good agreement with recent room-temperature, time-domain data [Lin et al. Proc. Natl. Acad. Sci.2009, 106, 8537]. A lack of correlation observed between the holes near 810 and 883 nm, accounting for electrochromically induced shifts of the Zn-BChl transitions in the Bᴀ,ʙ and Hᴀ,ʙ binding sites, produced by formation of the P⁺BHQᴀ⁻ state, indicates that the 810 nm bleach does not correspond to the P₊ (upper excitonic component of the dimer) band and is mostly contributed to by a shift of the Bʙ absorption band. ZPH-action spectra indicated inhomogeneous broadening (Γinh) of ~110 cm⁻¹ (Zn-RC) and ~130 cm⁻¹ (Zn-β-RC). Experimentally determined Γinh decreased the number of variables in theoretical fits of the absorption and frequency-dependent shapes of resonant HB spectra, leading to more reliable Huang—Rhys factors for both low-frequency phonons and a pseudolocalized phonon, ωSP, often referred to as the special pair marker mode.10 page(s