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

Isolation procedures have led to chlorophyll alpha (Chl alpha) contents 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 those 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 dynamics of primary charge separation in the RC which are triggered by excitation of the primary electron donor state, P680* (where P680 indicates that the lowest energy ground-state absorption band of the primary 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 alpha are due to 684-nm-absorbing Chl alpha, some contamination by the proximal antenna protein complex CP47, and, probably, also nonnative (disrupted) Chl alpha absorbing near 670 nm. The 684-nm Chl alpha were found to be easily disrupted by the ionic detergent Triton X-100 (much more so than P680). The results are inconsistent with the model that has the 684-nm band being the dimer (special pair) partner of P680. Nor can they be satisfactorily interpreted within the model that has the 684-nm band being P684 of a structurally very distinct subset of the RC ensemble. This ''mixture'' model has the ensemble comprised of P680 and P684 RC subsets. Importantly, the intensities of the 684-nm band observed 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 associated with the CP47-RC complex as a whole or that both CP47 and the RC possess 684-nm-absorbing Chl alpha or, equivalently, an intrinsic (fragile) 684-nm state. Irrespective of which of these two is correct, it is concluded that the number of Chl alpha in the hydrophobic interior of the RC of photosystem II is 4 and that the 684-nm Chl alpha are located 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 Chl 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 alpha content RC and CP47-RC samples. Thus, the 684-nm Chl alpha do not affect the lifetime of P680* at low temperatures, i.e., do not serve as an efficient trap for P680*. A theoretical analysis of the burn wavelength dependence of the P680 hole spectra of the 4 Chl alpha preparation 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. This, together with the observation that the weakly absorbing, upper dimer partner of P680* lies only 300 cm(-1) higher in energy, further establishes that the special pair of the PS II RC has a structure which is significantly different than in the bacterial RC. Structural models for the special pair are reviewed and discussed.