Electron Transfer in Rhodobacter sphaeroides Reaction Centers Containing Zn-Bacteriochlorophylls: A Hole-Burning Study

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• 作者：Bhanu Neupane ; Paul Jaschke ; Rafael Saer ; J. Thomas Beatty ; Mike Reppert ; Ryszard Jankowiak
• 刊名：The Journal of Physical Chemistry B
• 出版年：2012
• 出版时间：March 15, 2012
• 年：2012
• 卷：116
• 期：10
• 页码：3457-3466
• 全文大小：438K
• 年卷期：v.116,no.10(March 15, 2012)
• ISSN：1520-5207

文摘

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 鈥淶n-尾-RC鈥?in which the Zn-BChl in the bacteriopheophytin (BPhe)-binding site on the A side (H_A) has the Zn penta-coordinated, was also studied. The fifth ligand comes from a histidine introduced by site-directed mutagenesis. Formation of the P⁺Q_A^{鈥?/sup> state was observed in both types of RC, although under identical experimental conditions a significantly deeper P_{鈥?/sub> band (corresponding to the lower-energy, special pair, excitonic component) was revealed in the Zn-RC. Assuming a similar lifetime of the P+QA鈥?/sup> state, the quantum yield of P+QA鈥?/sup> formation decreased by 60% in the Zn-尾-RC (compared to the Zn-RC), as was seen in a comparison of analogous (Mg) BChl-containing wild type and 尾-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-尾-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 BA,B and HA,B binding sites, produced by formation of the P+BHQA鈥?/sup> 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 BB absorption band. ZPH-action spectra indicated inhomogeneous broadening (螕inh) of 110 cm鈥? (Zn-RC) and 130 cm鈥? (Zn-尾-RC). Experimentally determined 螕inh decreased the number of variables in theoretical fits of the absorption and frequency-dependent shapes of resonant HB spectra, leading to more reliable Huang鈥揜hys factors for both low-frequency phonons and a pseudolocalized phonon, 蠅SP, often referred to as the special pair marker mode.}}