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 a Q_y states. Results are presented for the CP47 complex from two preparations. The Chl a 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 (ω_m) of 20 cm^-1, which explains why the Stokes shift (2Sω_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 a 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 a 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 a 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 ∼670 nm. This speaks to the fragility of the 684-nm absorbing Chl a. Consideration of the nature of the 684-nm-absorbing Chl a of CP47 is mainly reserved for the accompanying paper on the D1-D2-cyt b₅₅₉ reaction center and CP47-D1-D2-cyt b₅₅₉ complexes.