Effects of hydrogen bonding interactions on the redox potential and molecular vibrations of plastoquinone as studied using density functional theory calculations

Abstract

The effects of H-bonding on the redox potential and molecular vibrations of plastoquinone (PQ) that functions as a primary and a secondary quinone electron acceptor (Q_A and Q_B, respectively) in photosystem II (PSII) in plants and cyanobacteria were investigated using density functional theory calculations. Calculations were performed on the neutral and semiquinone anion forms of PQ and its H-bonded complexes, which form H-bonds with water molecules, or using amino acid models mimicking the interactions of Q_A and Q_B. The calculated redox potential (E^o) of PQ showed a linear relationship with the number of H-bonds, and the E^o increased by +100–200 mV with the addition of one H-bond. Vibrational analysis of the model PQ complexes showed that the CO stretching vibrations of neutral PQ are sensitive to the number and symmetry of H-bonding interactions, providing criteria to determine the H-bonding structure. Although no specific trend in the H-bonding dependency was found for anionic PQ, complex spectral features in the CO stretching region due to significant couplings with other PQ vibrations and the vibrations of H-bonding amino acids are useful monitors of the change in the H-bonding structure of anionic PQ in proteins. The calculated E^o values and infrared spectra of the Q_A and Q_B models are consistent with the view that one additional H-bond to Q_B from D1-Ser264 largely contributes to the redox potential gap between Q_A and Q_B in PSII.