SITE-SELECTION SPECTROSCOPY OF THE REACTION-CENTER COMPLEX OF PHOTOSYSTEM-II .1. TRIPLET-MINUS-SINGLET ABSORPTION DIFFERENCE - SEARCH FOR A 2ND EXCITON BAND OF P-680

The D1-D2-Cytb559 reaction center complex of photosystem II was studied at 4 K with triplet-minus-singlet (TS) absorption difference spectroscopy using selective laser excitation. The TS spectrum is mainly due to the bleaching of P-680 and significantly differs from the TS spectrum of monomeric Chi alpha in detergent. The absorption spectrum ascribed to the primary electron donor, P-680, was interpreted in terms of two distinguishable distributions, around 681 and 684 nm, respectively. These spectral forms could be preferentially selected by relatively blue (lambda(exc) = 661-677 nm) and extremely red excitation (lambda(exc) = 691 nm), respectively. In addition to the bleaching at 681-684 nm, a small band at 667 nm was also observed, which had opposite polarization. We attributed the latter to an exciton band, not associated with a single pigment. Thus upon triplet formation a system consisting of N excitonically coupled pigments (N > 1) is reduced by one due to localization of the triplet state on one of the pigments. The simplest, although not the only, case is given by N = 2, for which P-680 is a dimer with exciton bands at 681-684 and 667 nm. This interpretation immediately explains the positive and negative bands observed at 679 and 669 nm in the circular dichroism spectrum of D1-D2-Cytb559 (Braun, P.; Greenberg, B. M.; Scherz, A. Biochemistry 1990, 29, 10376-10387). The relatively small amplitude of the 667-nm band indicates that the angle between the two unperturbed Q(y) transitions of a putative Chi dimer (15-21 degrees) is two times smaller than that of the special pair pigments of purple bacteria (38 degrees). We do not rule out the case that N = 3 or larger.