Impact of molecular flexibility on the site energy shift of chlorophylls in Photosystem II

Light harvesting from the Sun by antenna complexes surrounding the reaction center of Photosystem II represents the first step of the natural oxygenic photosynthesis performed by plants, algae and cyanobacteria. The excitation energy derived from the sunlight is absorbed by the chlorophylls of the antenna and subsequently conveyed to the reaction center of Photosystem II through resonant energy transfer mechanisms. In the special pair of chlorophylls of the reaction center the charge separation occurs, eventually leading to the oxidation of water molecules into protons, electrons and molecular oxygen. The adsorption properties of the antenna chlorophylls are ad hoc modulated by the protein environment to guarantee fast energy transfer and minimize side and back reactions. At the same time these properties are influenced by the molecular fluctuations occurring at physiological temperature. In the present work, combining classical molecular dynamics simulations with the Charge Density Coupling method, we estimated the impact of the thermal fluctuations on the site energy shift of the chlorophylls embedded in the Photosystem II complex. Our results show how the effect of the molecular fluctuations is not homogeneous throughout the complex, although the symmetry of the homodimer is maintained. Many peripheral chromophores undergo fluctuations larger then 10 kJ/mol around the average values. Possible physiological roles of such fluctuations are discussed.