EXCITON LEVEL STRUCTURE AND DYNAMICS IN THE CP47 ANTENNA COMPLEX OF PHOTOSYSTEM-II

Persistent nonphotochemical and population bottleneck hole-burning results obtained as a function of burn wavelength are reported for the CP47 proximal antenna protein complex of photosystem II. Attention is focused on the lower energy chlorophyll alpha Q(y) states. Results are presented for the CP47 complex from two preparations. The Chl alpha content per CP47 complex was determined, spectroscopically, to be 14 +/- 2. On the basis of the analysis of the hole spectra and the 4.2 K static fluorescence spectrum, the lowest energy state of CP47 lies at 690 nm (fluorescence origin at 691 nm). The width of the weak 690-nm absorption band from inhomogeneous broadening is 100 cm(-1). The linear electron-phonon coupling of the 690-nm state is weak with a Huang-Rhys factor (S) of about 0.2 and a mean phonon frequency (omega(m)) of 20 cm(-1), which explains why the Stokes shift (2S omega(m)) is so small. The 690-nm state is found to be excitonically correlated with a, hitherto, unobserved state at 687 nm. However, the combined absorption intensity of the 690- and 687-nm states was determined to be equivalent to only 1 Chl alpha molecule. Results are presented which illustrate that these two states are fragile (i.e., their associated chlorophyll a molecules are readily disrupted). Thus, it is possible that the correct number of Chl alpha molecules is 2, not 1. Indeed, the simplest interpretation consistent with the hole-burning data has the 687- and 690-nm states being associated with a Chl alpha dimer with the latter close to forbidden in absorption. The results indicate that the 687-nm state relaxes to the 690-nm state in about 70 fs. The zero-phonon hole widths for the higher integrity CP47 samples are used to determine the energy-transfer times for the higher energy absorbing states at 4.2 K. The absorption intensity of a previously identified state at 684 nm is found to vary from preparation to preparation. Diminution of the intensity of the 684-nm band is accompanied by increased absorption at similar to 670 nm. This speaks to the fragility of the 684-nm absorbing Chl alpha. Consideration of the nature of the 684-nm-absorbing Chl alpha of CP47 is mainly reserved for the accompanying paper on the D1-D2-cyt b(559) reaction center and CP47-D1-D2-cyt b(559) complexes.