Endor Spectroscopy Shows That Q(A) Remains In The Same Orientation Upon Reduction In Reaction Centers From Rhodobacter Sphaeroides

It has been, recently, proposed that QA moves and rotates ∼60° upon its reduction.1 Here we investigated possible changes using ENDOR spectroscopy that provides a very sensitive (to ∼0.01 A) probe for the binding site of QA•-.2 Identical RC samples were made - (a) one frozen in the dark (ground state) and then illuminated generating D•+QA•- and (b) one frozen under illumination (excited state) which trapped D•+QA•- in ∼70% of the RCs at 80 K. Figure 1 shows the resultant 1H ENDOR spectra of QA•-. The peaks labeled L1, L2 and L3 correspond to the two H-bonds to QA•-.2,3 Essentially no differences in the ENDOR spectra were observed indicating that the interactions of QA•- with the protein are the same in the ground state as in the excited state. These results are irreconcilable with the proposed rotation.1 Thus, QA is preset in an environment that favors its reduction.1 Heinent et al. (2007), J. Am. Chem. Soc. 129, 15935. 2 Flores et al. (2007), Biophys. J. 92, 671. 3 Sinnecker et al. (2006), Phys. Chem. Chem. Phys. 8, 5659.View Large Image | View Hi-Res Image | Download PowerPoint Slide