STEADY-STATE SPECTROSCOPY AT 6-K OF THE ISOLATED PHOTOSYSTEM-II REACTION CENTER - ANALYSIS OF THE RED ABSORPTION-BAND

The red absorption band of the isolated photosystem II reaction centre at 6 K has been analysed by measurements of linear and circular dichroism, absorbance-detected magnetic resonance and absorbance differences induced by photoaccumulation of reduced pheophytin. The data are complementary to those of van Kan et al. (Biochim. Biophys. Acta, 1020 (1990) 146-152) and were measured using the same preparations. The spectral analysis by van Kan et al., assuming negligible interactions between pigments, is extended to accommodate the four accessory chlorophylls now known to be present. They would therefore have Q(y) transitions at 667.0 and 671.1 nm, both nearly perpendicular to the membrane plane, and at 669.7 and 673.7 nm, both at a small angle (35-degrees or less) with that plane. The assignment of two bands at 679.6 and 683.6 nm to a special pair of chlorophyll molecules constituting the primary electron donor P-680 is confirmed, but these bands appear to reflect a spectral heterogeneity of P-680 in the preparation rather than exciton splitting of a single spectral form of P-680. The proposed interpretation of absorbance changes associated with the photoaccumulation of reduced pheophytin as a pheophytin bleaching centred at 676.5 nm and a blue shift of the P-680 absorbance requires additional assumptions.