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Protein-Cofactor Interactions in Bacterial Reaction Centers from Rhodobacter sphaeroides R-26: I. Identification of the ENDOR Lines Associated with the Hydrogen Bonds to the Primary Quinone Q_A^•–

M. Flores ^* , R. Isaacson ^*, E. Abresch ^*, R. Calvo ^* , W. Lubitz  and G. Feher ^*

^* Department of Physics, University of California at San Diego, La Jolla, California; Max-Planck Institut für Bioanorganische Chemie, Mülheim an der Ruhr, Germany; and Departamento de Física, Facultad de Bioquímica y Ciencias Biológicas and INTEC, Universidad Nacional del Litoral and CONICET, Santa Fe, Argentina

Correspondence: Address reprint requests to George Feher, E-mail: gfeher{at}ucsd.edu.

Hydrogen bonds are important in determining the structure and function of biomolecules. Of particular interest are hydrogen bonds to quinones, which play an important role in the bioenergetics of respiration and photosynthesis. In this work we investigated the hydrogen bonds to the two carbonyl oxygens of the semiquinone in the well-characterized reaction center from the photosynthetic bacterium Rhodobacter sphaeroides R-26. We used electron paramagnetic resonance and electron nuclear double resonance techniques at 35 GHz at a temperature of 80 K. The goal of this study was to identify and assign sets of ¹H-ENDOR lines to protons hydrogen bonded to each of the two oxygens. This was accomplished by preferentially exchanging the hydrogen bond on one of the oxygens with deuterium while concomitantly monitoring the changes in the amplitudes of the ¹H-ENDOR lines. The preferential deuteration of one of the oxygens was made possible by the different ¹H ²H exchange times of the protons bonded to the two oxygens. The assignment of the ¹H-ENDOR lines sets the stage for the determination of the geometries of the H-bonds by a detailed field selection ENDOR study to be presented in a future article.

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				M. Flores, R. Isaacson, E. Abresch, R. Calvo, W. Lubitz, and G. Feher Protein-Cofactor Interactions in Bacterial Reaction Centers from Rhodobacter sphaeroides R-26: II. Geometry of the Hydrogen Bonds to the Primary Quinone Formula by 1H and 2H ENDOR Spectroscopy Biophys. J., January 15, 2007; 92(2): 671 - 682. [Abstract] [Full Text] [PDF]