Hydrogen Bond Interactions of the Pheophytin Electron Acceptor and Its Radical Anion in Photosystem II As Revealed by Fourier Transform Infrared Difference Spectroscopy

The primary electron acceptor pheophytin (PheO(D1)) plays a crucial role in regulation of forward and backward electron transfer in photosystem II (PSII). It is known that some cyanobacteria control the Pheo(D1) potential in high-light acclimation by exchanging the DI proteins from different copies of the psbA genes. To clarify file mechanism of the potential control of PheODh we studied the hydrogen bond interactions of Pheo(D1) in the neutral and anionic States using light-induced Fourier transform infrared (FTIR) difference spectroscopy. FTIR difference spectra of Pheol:)l upon its photoreduction were obtained using three different PSII preparations, PSII core complexes from Thermosynehococcus elongatus possessing PsbA1 as a D1 subunit (PSII-PsbA1), those with PsbA3 (PSII-PsbA3), and PSII membranes from spinach. The DI-Gln130 side chain, which is hydrogen bonded to the 13(1)-keto C=O group of Pheo(D1) PSII membranes from spinach. The D1-Gln130 in PSII-PsbA3 and spinach PSIL The spectrum of PSII-PsbA1 exhibited 131-keto C=O bands at 1682 and 1605 cm(-1) in neutral Pheol), and its anion, respectively, while the corresponding bands were observed at frequencies lower by 1-3 and 18-19cm(-1), respectively, in the latter two preparations. This larger frequency shift in PhcoD(1)(-) than Plleol), by the change of the hydrogen bond donor Was well reproduced by density functional theory (DFT) calculations for the Pheo models hydrogen bonded with acetamide and acetic acid. The DFT calculations also exhibited a higher redox potential for Pheo reduction in the model with acetic acid than that with acetamide, consistent with previous observations for the D1-Gln130GIu mutant of Synechocystis. It is thus concluded that a stronger hydrogen bond effect oil the Pheo(-) anion than the neutral Pheo causes the shift in the redox potential, which IS utilized In the photoprotection mechanism of PSII.