ON THE QUESTION OF THE CHLOROPHYLL-A CONTENT OF THE PHOTOSYSTEM-II REACTION-CENTER

Isolation procedures have led to chlorophyll alpha (Chl alpha) content s of the photosystem II reaction center (RC) that range between about 4 and 6. Since this content for the bacterial RC is 4 (with two of tho se being associated with the special pair), the nature of the ''extra' ' Chl alpha in RC preparations of photosystem II containing more than 4 Chl alpha molecules is currently of much interest. So too are the dy namics of primary charge separation in the RC which are triggered by e xcitation of the primary electron donor state, P680 (where P680 indic ates that the lowest energy ground-state absorption band of the primar y donor lies at 680 nm; the asterisk indicates lowest lying (1) pi pi (Q(y)) state). We report absorption and triplet-state bottleneck hole spectra (4.2 K) for RC preparations of photosystem II containing 4, 5 , and 6 Chl alpha molecules. The spectra reveal that the extra Chl alp ha are due to 684-nm-absorbing Chl alpha, some contamination by the pr oximal antenna protein complex CP47, and, probably, also nonnative (di srupted) Chl alpha absorbing near 670 nm. The 684-nm Chl alpha were fo und to be easily disrupted by the ionic detergent Triton X-100 (much m ore so than P680). The results are inconsistent with the model that ha s the 684-nm band being the dimer (special pair) partner of P680. Nor can they be satisfactorily interpreted within the model that has the 6 84-nm band being P684 of a structurally very distinct subset of the RC ensemble. This ''mixture'' model has the ensemble comprised of P680 a nd P684 RC subsets. Importantly, the intensities of the 684-nm band ob served for the CP47 complex and the CP47-RC complex were also found to vary from preparation to preparation adn be sensitive to Triton C-100 . Two possibilities are considered: that the 684-nm Chl alpha are asso ciated with the CP47-RC complex as a whole or that both CP47 and the R C possess 684-nm-absorbing Chl alpha or, equivalently, an intrinsic (f ragile) 684-nm state. Irrespective of which of these two is correct, i t is concluded that the number of Chl alpha in the hydrophobic interio r of the RC of photosystem II is 4 and that the 684-nm Chl alpha are l ocated in the exterior region of the RC protein complex. The of P680 of the 4 Chl alpha-RC preparation, which contains very little 684-nm C hl alpha (5% on a Chl alpha basis), was determined to be 1.9 ps at 4.2 K. This is identical to our previous determination for higher Chl alp ha content RC and CP47-RC samples. Thus, the 684-nm Chl alpha do not a ffect the lifetime of P680 at low temperatures, i.e., do not serve as an efficient trap for P680. A theoretical analysis of the burn wavel ength dependence of the P680 hole spectra of the 4 Chl alpha preparati on is given. In agreement with our previous work, the electron-phonon (protein) coupling is as strong (S = 2) as that observed for P870 and P960 of the bacterial RC Rhodobacter sphaeroides and Rhodopseudomonas viridis, respectively. However, the special pair marker mode (125/145 cm(-1)) progression of P870 and P960 is essentially silent in P680. Th is, together with the observation that the weakly absorbing, upper dim er partner of P680 lies only 300 cm(-1) higher in energy, further est ablishes that the special pair of the PS II RC has a structure which i s significantly different than in the bacterial RC. Structural models for the special pair are reviewed and discussed.