Mixed exciton-charge-transfer states in photosystem II: Stark spectroscopy on site-directed mutants

Abstract

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 (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)(δ)(+)P(D1)(δ)(-)Chl(D1)) (673 nm) and (Chl(D1)(δ)(+)Phe(D1)(δ)(-)) (681 nm) (where the subscript indicates the wavelength of the electronic transition). Moreover, the CT state P(D2)(+)P(D1)(-) acquires excited-state character due to its mixing with an exciton state, producing (P(D2)(+)P(D1)(-))(δ) (684 nm). 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.

Figure 1

Relative positions of the central cofactors in the PSII RC complex together with the positions of the mutated residues (D₁-Gln130, D₁-His198, D₂-His197, and D₁-Thr179) and the chemical structures of the introduced residues (Glu, Gln, Asn, Ala, and His; adapted from Umena et al. (5), Protein Data Bank ID: 3ARC). Water molecules are depicted in red; for the cofactors, the magnesium atoms are in green; for the residues, carbon atoms are in green, oxygen is in red, and nitrogen is in blue.

Figure 2

Simultaneously recorded 77 K absorption and Stark spectra for the Synechocystis PCC 6803 RC complex WT and D₁-Gln130Glu mutant, and the spinach RC complex WT. Left panel: Chl-Phe Q_Y absorption region (inset: spinach RC WT 681 nm Stark minimum expanded). Right panel: Phe Q_X absorption region. The absorption and Stark spectra are normalized to the RC content. The Stark spectra were recorded at χ = 54.7° and at an external electric field strength of 2.25 × 10⁵ V cm⁻¹.

Figure 3

Simultaneously recorded 77 K absorption and Stark spectra of the Synechocystis PCC 6803 core complex in the Q_Y absorption region for the WT, D₁-His198Gln, and D₂-His197Gln mutants (top left panel); WT, D₁-His198Asn, and D₂-His197Asn mutants (top-right panel); WT, D₁-His198Ala, and D₂-His197Ala mutants (bottom-left panel); and WT and D₁-Thr179His mutant (bottom right panel). The absorption and Stark spectra are normalized to the cofactor content. The absorption difference spectra have been multiplied by 5 to facilitate visualization. The Stark spectra were recorded at χ = 54.7° and an external electric field strength of 2.25 × 10⁵ V cm⁻¹.