CHARGE RECOMBINATION REACTIONS IN PHOTOSYSTEM-II .2. TRANSIENT ABSORBENCY DIFFERENCE SPECTRA AND THEIR TEMPERATURE-DEPENDENCE

Absorbance difference spectra of the transient states in photosystem I I (PS II) have been examined in the Q(y) absorption region between 660 and 700 nm. The P680(+)Pheo(-)/P680Pheo, (3)P680/ P680, and P680(+)Q( A)(-)/P680Q(A) spectra were measured in O-2-evolving PS II core comple xes from Synechococcus and PS II-enriched membrane fragments from spin ach. The low-temperature absorbance difference spectra vary only sligh tly between both PS II preparations. The (3)P680/P680 spectrum is char acterized by a bleaching at 685 nm at 25 K and indicates weak exciton coupling with neighboring pigment(s). We conclude that P680 absorbs at 685 nm in more intact PS II preparations at cryogenic temperature. Th e difference spectra of the radical pairs are strongly temperature dep endent. At low temperature the P680(+)Q(A)(-)/P680Q(A)(-) spectrum exh ibits the strongest bleaching at 675 nm whereas the P680(+)Pheo(-)/P68 0Pheo spectra show two distinct bleaching bands at 674 and 684 nm. It is suggested that an electrochronic red shift resulting in a bleaching at 675 nm and an absorbance increase at about 682 nm dominates the sp ectral features of the charge-separated states. On the basis of the pr esent results and those in the literature, we conclude that the intera ctions between the pigments and especially the organization of the pri mary donor must be quite different in PS II compared to bacterial reac tion centers, although the basic structural arrangement of the pigment s might be similar. Spectral data obtained with samples in the presenc e of singly and doubly reduced Q(A) indicate that the primary photoche mistry in PS II is not strongly influenced by the redox state of Q(A) at low temperature and confirm the results of the accompanying paper [ Van Mieghem, F. J. E., Brettel, K., Hillmann, B., Kamlowski, A., Ruthe rford, A. W., & Schlodder, E. (1995) Biochemistry 34, 4798-4813]. The spectra of the primary radical pair and the reaction center triplet ob tained with more intact PS II preparations differ widely from those of D1/ D2/cyt b-559 complexes. In the latter sample, where (3)P680 forma tion results in a bleaching at 680 nm, the P680(+)Pheo(-)/P680Pheo spe ctrum shows only one broad bleaching band at about 680 nm, and the mai n bleaching due to photoaccumulation of Pheo(-) at 77 K appears at 682 nm instead of 685 nm in PS II core complexes. This indicates that the removal of the core antenna which is accompanied by the loss of Q(A) causes also structural changes of the reaction center.