Mixed Exciton-Charge-Transfer States in Photosystem II: Stark Spectroscopy on Site-Directed Mutants

We investigated the electronic structure of the photosystem II reaction center (PSII RC) in relation to the light-induce d charge separation process using Stark spectroscopy on a series of site-directed PSII RC mutants from the cyanobacterium Synechocystis sp. PCC 6803. The site-directed mutations modify the protein environment of the cofactors involved in charge separation (P-D1, P-D2, Chl(D1), and Phe(D1)). The results demonstrate that at least two different exciton states are mixed with charge-transfer (CT) states, yielding exciton states with CT character: (P(D2)(delta+)P(D1)(delta-)Chl(D1))(673nm)* and Chl(D1)(delta+)Phe(D1)(delta 1))(681nm)* (where the subscript indicates the wavelength of the electronic transition). Moreover, the CT state PD2+PD1- acquires excited-state character due to its mixing with an exciton state, producing (PD2+PD1-)(684nm)(delta)*. We conclude that the states that initiate charge separation are mixed exciton-CT states, and that the degree of mixing between exciton and CT states determines the efficiency of charge separation. In addition, the results reveal that the pigment-protein interactions fine-tune the energy of the exciton and CT states, and hence the mixing between these states. This mixing ultimately controls the selection and efficiency of a specific charge separation pathway, and highlights the capacity of the protein environment to control the functionality of the PSII RC complex.