Electric Field Effects on the Chlorophylls, Pheophytins, and β-Carotenes in the Reaction Center of Photosystem II

Abstract

We present an electric field modulated absorption spectroscopy (Stark effect) study of isolated photosystem II reaction center complexes, including a preparation in which the inactive pheophytin H_B was exchanged for 13¹-deoxo-13¹-hydroxy-pheophytin. The results reveal that the Stark spectrum of the Q_x and Q_y transitions of the pheophytins has a second-derivative line shape, indicating that the Stark effect is dominated by differences in the dipole moment between the ground and the electronically excited states of these transitions (Δμ). The Δμ values for the Q_x and Q_y transitions of H_B are small (Δμ = 0.6−1.0 D f^-1), whereas that of the Q_x transition of the active pheophytin H_A is remarkably large (Δμ = 3 D f^-1). The Stark spectrum of the red-most absorbing pigments also shows a second-derivative line shape, but this spectrum is considerably red-shifted as compared to the second derivative of the absorption spectrum. This situation is unusual but has been observed before in heterodimer special pair mutants of purple bacterial reaction centers [Moore, L. J., Zhou, H., and Boxer, S. G. (1999) Biochemistry 38, 11949−11960]. The red-shifted Stark spectra can be explained by a mixing of exciton states with a charge-transfer state of about equal energy. We conclude that the charge transfer state involves H_A and its immediate chlorophyll neighbor (B_A), and we suggest that this (B_A^δ+H_A^δ-) charge transfer state plays a crucial role in the primary charge separation reaction in photosystem II. In contrast to most other carotenes, the two β-carotene molecules of the photosystem II reaction center display a very small Δμ, which can most easily be explained by excitonic coupling of both molecules. These results favor a model that locates both β-carotene molecules at the same side of the complex.