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

• Published in: Biophysical journal Vol. 103; no. 2; pp. 185 - 194
• Main Authors: Romero, Elisabet, Diner, Bruce A., Nixon, Peter J., Coleman, Wiliam J., Dekker, Jan P., van Grondelle, Rienk
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 18-07-2012; Biophysical Society; The Biophysical Society
• Subjects: Absorption; Bacteria; Cell Biophysics; Chlorophyll - metabolism; energy; mixing; Models, Molecular; Molecular Conformation; Mutagenesis, Site-Directed; Mutant Proteins - chemistry; Mutant Proteins - metabolism; mutants; Mutation; Mutation - genetics; Phase transitions; photosystem II; Photosystem II Protein Complex - chemistry; Photosystem II Protein Complex - metabolism; Proteins; spectroscopy; Spectrum analysis; Spectrum Analysis - methods; Synechocystis; Synechocystis - metabolism; wavelengths
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1016/j.bpj.2012.06.026

We investigated the electronic structure of the photosystem II reaction center (PSII RC) in relation to the light-induced 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 (PD1, PD2, ChlD1, and PheD1). The results demonstrate that at least two different exciton states are mixed with charge-transfer (CT) states, yielding exciton states with CT character: (PD2δ+PD1δ−ChlD1)∗673nm and (ChlD1δ+PheD1δ−)∗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−)δ∗684nm. 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.