The dynamics of the special pair of chlorophylls of Photosystem II

Cholophylls are at the basis of the photosynthetic energy conversion mechanisms in algae, plants and cyanobacteria. In Photosystem II, the photo-produced electrons leave a special pair of chlorophylls (namely PD1 and PD2) that becomes cationic. This oxidizing pair [PD1,PD2]+ in turn, triggers a cascade of oxidative events, eventually leading to water splitting and oxygen evolution. In the present work, using Quantum Mechanics/Molecular Mechanics calculations, we investigate the electronic structure and the dynamics of the PD1PD2 special pair in both its oxidised and reduced states. In agreement with previously reported static calculations, the symmetry between the two chlorophylls was found to be broken, the positive charge being preferentially located on PD1. Nevertheless, this study reveal for the first time that large charge fluctuations occur along dynamics, temporarily inverting the charge preference for the two branches. Finally, a vibrational analysis pinpointed that such charge fluctuations are strongly coupled to specific modes of the special pair.