Pigment organization and their interactions in reaction centers of photosystem II: Optical spectroscopy at 6 K of reaction centers with modified pheophytin composition

Photosystem II reaction centers (RC) with selectively exchanged pheophytin (Pheo) molecules as described in [Germano, M., Shkuropatov, A. Ya., Permentier, H., Khatypov, R. A., Shuvalov, V. A., Hoff, A. J., and van Gorkom, H. J. (2000) Photosynth. Res. 64, 189-198] were studied by low-temperature absorption, linear and circular dichroism, and triplet-minus-singlet absorption-difference spectroscopy. The ratio of extinction coefficients is an element of (Pheo)/is an element of (Chl) for Q(y) absorption in the RC is similar to0.40 at 6 K and similar to0.45 at room temperature. The presence of 2 beta -carotenes, one parallel and one perpendicular to the membrane plane, is confirmed. Absorption at 670 mn is due to the perpendicular Qy transitions of the two peripheral chlorophylls (Chl) and not to either Pheo. The "core" pigments, two Pheo and four Chl absorb in the 676-685 nm range. Delocalized excited states as predicted by the "multimer model" are seen in the active branch. The inactive Pheo and the nearby Chl, however, mainly contribute localized transitions at 676 and 680 nm, respectively, although large CD changes indicate that exciton interactions are present on both branches. Replacement of the active Pheo prevents triplet formation, causes an LD increase at 676 and 681 nm, a blue-shift of 680 nm absorbance, and a bleach of the 685 nra exciton band. The triplet state is mainly localized on the Chl corresponding to BA in purple bacteria. Both Pheo Qy transitions are oriented out of the membrane plane. Their Q(x) transitions are parallel to that plane, so that the Pheos in PSII are structurally similar to their homologues in purple bacteria.