Protein-cofactor Interactions in Bacterial Reaction
Centers from Rhodobacter sphaeroides R-26: II.
Geometry of the Hydrogen Bonds to the Primary Quinone
QA.- by 1H and
2H ENDOR Spectroscopy
Marco Flores 1, Roger A Isaacson 2, Edward C Abresch 2, Rafael Calvo 3, Wolfgang Lubitz 4 and George Feher 2*
1 UC San Diego, Max-Planck Institut fur Bioanorganische Chemie
2 UC San Diego
3 UNL & INTEC (CONICET-UNL)
4 Max-Planck-Institut fur Bioanorganische Chemie
* To whom correspondence should be addressed. E-mail: gfeher{at}ucsd.edu.
Submitted on June 29, 2006
Revised on August 29, 2006
Accepted on 27 September 2006
 |
Abstract |
|---|
The geometry of the hydrogen bonds to the two carbonyl oxygens of the semiquinone QA•- in the reaction center (RC) from the photosynthetic purple bacterium Rb. sphaeroides R-26 were determined by fitting a spin Hamiltonian to the data derived from 1H and 2H ENDOR spectroscopies at 35 GHz and 80 K. The experiments were performed on RCs in which the native Fe2+ (high spin) was replaced by diamagnetic Zn2+ to prevent spectral line broadening of the QA•- due to magnetic coupling with the iron. The principal components of the hyperfine coupling and nuclear quadrupolar coupling tensors of the hydrogen bonded protons (deuterons) and their principal directions with respect to the quinone axes were obtained by spectral simulations of ENDOR spectra at different magnetic fields on frozen solutions of deuterated QA•- in H2O buffer and protonated QA•- in D2O buffer. Hydrogen bond lengths were obtained from the nuclear quadrupolar couplings. The two hydrogen bonds were found to be non-equivalent, having different directions and different bond lengths. The H-bond lengths rO...H are 1.73 ± 0.03 Å and 1.60 ± 0.04 Å, from the carbonyl oxygens O1 and O4 to the NH group of Ala M260 and the imidazole nitrogen N
of His M219, respectively. The asymmetric hydrogen bonds of QA•- affect the spin density distribution in the quinone radical and its electronic structure. It is proposed that the H-bonds play an important role in defining the physical properties of the primary quinone, which affect the electron transfer processes in the RC.
Key Words:
Bacterial Photosynthesis, Characterization of hydrogen bonds to QA.-, Semiquinone radical, Simulation of ENDOR spectra
