Electric field effects on the chlorophylls, pheophytins, and beta-carotenes in the reaction center of photosystem II

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 HB was exchanged for 131-deoxo-131-hydroxy-pheophytin. The results reveal that the Stark spectrum of the Qx and Qy 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 Qx and Qy transitions of HB are small (Δμ = 0.6−1.0 D f-1), whereas that of the Qx transition of the active pheophytin HA 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 mutant...