Effects of hydrogen bonding interactions on the redox potential and molecular vibrations of plastoquinone as studied using density functional theory calculationsElectronic supplementary information (ESI) available: Optimized geometries of semiquinone anions in plastoquinone complexes H-bonded to water molecules and amino acid models. See DOI: 10.1039/c3cp54742f

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. The redox potential of plastoquinone, an electron acceptor in photosystem II, is controlled by its H-bonding interactions, which can be monitored by detecting the CO stretching vibrations.