PRESSURE EFFECTS ON ABSORPTION-SPECTRA OF THE ISOLATED REACTION-CENTER OF PHOTOSYSTEM-II

Absorption spectra of the D1-D2-cytochrom b(559) complex at 4 degrees C were investigated at pressures up to 300 MPa. Pressure effects were mostly reversible and independent of the detergent used (CHAPS or dode cyl-beta-D-maltoside). Red-shifts were observed under pressure for the chlorophyll Q(y) - and the beta-carotene S-0 --> S-2 bands. The relat ively small Q(y)-shift of approximately 0.15 cm(-1)/MPa is an indicati on for the absence of strongly coupled chlorophyll dimers within the r eaction center and supports earlier reports from low-temperature measu rements (Chang HC, Jankowiak R, Reddy NRS and Small GJ (1995) Chem Phy s 197: 307-321). The carotene red-shift (seen in CHAPS) is much larger (0.5 - 0.6 cm(-1)/MPa) and within the range observed for excitonicall y coupled chlorophylls. However, since carotenes are more sensitive to changes of refractive index, we do not consider this evidence for exc itonically coupled carotenes. Varying the pH and the detergent induced only small effects. Pigment exchange using high pressure instead of e levated temperature was not possible under the conditions tested.