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., Perment ier, 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 abs orption, linear and circular dichroism, and triplet-minus-singlet absorptio n-difference spectroscopy. The ratio of extinction coefficients is an eleme nt of (Pheo)/is an element of (Chl) for Q(y) absorption in the RC is simila r to0.40 at 6 K and similar to0.45 at room temperature. The presence of 2 b eta -carotenes, one parallel and one perpendicular to the membrane plane, i s confirmed. Absorption at 670 mn is due to the perpendicular Qy transition s of the two peripheral chlorophylls (Chl) and not to either Pheo. The "cor e" pigments, two Pheo and four Chl absorb in the 676-685 nm range. Delocali zed excited states as predicted by the "multimer model" are seen in the act ive branch. The inactive Pheo and the nearby Chl, however, mainly contribut e localized transitions at 676 and 680 nm, respectively, although large CD changes indicate that exciton interactions are present on both branches. Re placement of the active Pheo prevents triplet formation, causes an LD incre ase at 676 and 681 nm, a blue-shift of 680 nm absorbance, and a bleach of t he 685 nra exciton band. The triplet state is mainly localized on the Chl c orresponding to BA in purple bacteria. Both Pheo Qy transitions are oriente d out of the membrane plane. Their Q(x) transitions are parallel to that pl ane, so that the Pheos in PSII are structurally similar to their homologues in purple bacteria.