Electron−Nuclear and Electron−Electron Double Resonance Spectroscopies Show that the Primary Quinone Acceptor QA in Reaction Centers from Photosynthetic Bacteria Rhodobacter

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• 作者：Marco Flores ; Anton Savitsky ; Mark L. Paddock ; Edward C. Abresch ; Alexander A. Dubinskii ; Melvin Y. Okamura ; Wolfgang Lubitz ; Klaus Mbius
• 刊名：Journal of Physical Chemistry B
• 出版年：2010
• 出版时间：December 23, 2010
• 年：2010
• 卷：114
• 期：50
• 页码：16894-16901
• 全文大小：270K
• 年卷期：v.114,no.50(December 23, 2010)
• ISSN：1520-5207

文摘

Reaction centers (RCs) from the photosynthetic bacterium Rhodobacter (Rb.) sphaeroides R-26 exhibit changes in the recombination kinetics of the charge-separated radical-pair state, P^•+ Q_A^•−, composed of the dimeric bacteriochlorophyll donor P and the ubiquinone-10 acceptor Q_A, depending on whether the RCs are cooled to cryogenic temperatures in the dark or under continuous illumination (Kleinfeld et al. Biochemistry 1984, 23, 5780−5786). Structural changes near redox-active cofactors have been postulated to be responsible for these changes in kinetics and to occur in the course of light-induced oxidation and reduction of the cofactors thereby assuring a high quantum yield. Here we investigated such potential light-induced structural changes, associated with the formation of P^•+ Q_A^•−, via pulsed electron−nuclear double resonance (ENDOR) at Q-band (34 GHz) and pulsed electron−electron double resonance (PELDOR) at W-band (95 GHz). Two types of light excitation have been employed for which identical RC samples were prepared: (a) one sample was frozen in the dark and then illuminated to generate transient P^•+ Q_A^•−, and (b) one was frozen under illumination which resulted in both trapped and transient P^•+ Q_A^•− at 80 K. The hyperfine interactions between Q_A^•− and the protein were found to be the same in RCs frozen in the dark as in RCs frozen under illumination. Furthermore, these interactions are completely consistent with those observed in RC crystals frozen in the dark. Thus, Q_A remains in its binding site with the same position and orientation upon reduction. This conclusion is consistent with the result of our orientation-resolving PELDOR experiments on transient P^•+ Q_A^•− radical pairs. However, these findings are incompatible with the recently proposed 60° reorientation of Q_A upon its photoreduction, as deduced from an analysis of Q-band quantum-beat oscillations (Heinen et al. J. Am. Chem. Soc. 2007, 129, 15935−15946). Such a large reorientation appears improbable, and our objections against this proposition are substantiated here in detail. Our results show that Q_A is initially in an orientation that is favorable for its light-driven reduction. This diminishes the reorganization requirements for fast electron reduction and high quantum efficiency.