Theoretical Assessment of Hinge-Type Models for Electron Donors in Reaction Centers of Photosystems I and II as Well as of Purple Bacteria

Hinge-type molecular models for electron donors in reaction centers of Photosystem I, II, and purple bacteria were investigated using a two-state computational approach based on Frozen-Density Embedding. This methodology, dubbed FDE-diab, is known to avoid consequences of the self-interaction error as far as intermolecular phenomena are concerned, which allows to predict qualitatively correct spin densities for large bio-molecular systems. The calculated spin density distributions are in a good agreement with available experimental results and demonstrated a very high sensitivity to changes in relative orientiation of co-factors and amino-acid protonation states. This allows to validate the previously proposed hinge-type models and make predictions on protonation states of axial histidine molecules. Contrary to the reaction centers in Photosystem I and purple bacteria, the axial histidines from Photosystem II were found to be deprotonated. This fact might shed some light on remarkable properties of Photosystem II reaction centers.