On the Question of the Chlorophyll a Content of the Photosystem II Reaction Center

H. C. Chang, R. Jankowiak, N. R.S. Reddy, C. F. Yocum, R. Picorel, M. Seibert, G. J. Small

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Isolation procedures have led to chlorophyll a (Chl a) 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 a in RC preparations of photosystem II containing more than 4 Chl a 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 1ππ* (Qy) 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 a molecules. The spectra reveal that the extra Chl a are due to 684-nm-absorbing Chl a, some contamination by the proximal antenna protein complex CP47, and, probably, also nonnative (disrupted) Chl a absorbing near 670 nm. The 684-nm Chl a 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 bands observed for the CP47 complex and the CP47-RC complex were also found to vary from preparation to preparation and be sensitive to Triton X-100. Two possibilities are considered: that the 684-nm Chl a are associated with the CP47-RC complex as a whole or that both CP47 and the RC possess 684-nm-absorbing Chl a or, equivalently, an intrinsic (fragile) 684-nm state. Irrespective of which of these two is correct, it is concluded that the number of Chl a in the hydrophobic interior of the RC of photosystem II is 4 and that the 684-nm Chl a are located in the exterior region of the RC protein complex. The lifetime of P680* of the 4 Chl a-RC preparation, which contains very little 684-nm Chl a (5% on a Chl a basis), was determined to be 1.9 ps at 4.2 K. This is identical to our previous determinations for higher Chl a content RC and CP47-RC samples. Thus, the 684-nm Chl a 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 a 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.

Original languageAmerican English
Pages (from-to)7725-7735
Number of pages11
JournalJournal of Physical Chemistry
Issue number31
StatePublished - 1994

NREL Publication Number

  • NREL/JA-452-6402


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