Genetically introduced hydrogen bond interactions reveal an asymmetric charge distribution on the radical cation of the special-pair chlorophyll P680.

The special-pair chlorophyll (Chl) P680 in photosystem II has an extremely high redox potential ( E _m ) to enable water oxidation in photosynthesis. Significant positive-charge localization on one of the Chl constituents, P _D1 or P _D2 , in P680 ⁺ has been proposed to contribute to this high E _m To identify the Chl molecule on which the charge is mainly localized, we genetically introduced a hydrogen bond to the 13 ¹ -keto C=O group of P _D1 and P _D2 by changing the nearby D1-Val-157 and D2-Val-156 residues to His, respectively. Successful hydrogen bond formation at P _D1 and P _D2 in the obtained D1-V157H and D2-V156H mutants, respectively, was monitored by detecting 13 ¹ -keto C=O vibrations in Fourier transfer infrared (FTIR) difference spectra upon oxidation of P680 and the symmetrically located redox-active tyrosines Y _Z and Y _D , and they were simulated by quantum-chemical calculations. Analysis of the P680 ⁺ /P680 FTIR difference spectra of D1-V157H and D2-V156H showed that upon P680 ⁺ formation, the 13 ¹ -keto C=O frequency upshifts by a much larger extent in P _D1 (23 cm ^-1 ) than in P _D2 (<9 cm ^-1 ). In addition, thermoluminescence measurements revealed that the D1-V157H mutation increased the E _m of P680 to a larger extent than did the D2-V156H mutation. These results, together with the previous results for the mutants of the His ligands of P _D1 and P _D2 , lead to a definite conclusion that a charge is mainly localized to P _D1 in P680^{.
(© 2017 by The American Society for Biochemistry and Molecular Biology, Inc.)}