Femtosecond visible/visible and visible/mid-IR pump-probe study of the photosystem II core antenna complex CP47

CP47 is one of the two core antenna proteins of Photosystem II involved in the transfer of solar energy toward the photochemically active reaction center, the D1D2cytb559 complex. We have performed vis/vis and vis/mid-IR pump-probe experiments at room temperature as a first step in linking the energy-transfer dynamics to the arrangement of the individual chlorophylls in the CP47 complex. The chlorophylls in CP47 have very similar absorption maxima (within 20 nm of each other); therefore, few spectral changes due to energy transfer can be observed at room temperature. We used the annihilation of excitation energy as a tool to enhance the spectral changes associated with energy transfer. Energy transfer was found to occur on time scales of similar to100 fs, 1-4 ps, and 12-28 ps, and our results are consistent with the presence of two red (683-nm) pools, plus an additional red-shifted one (690 nm). From the time-resolved mid-IR spectra, it follows that these red states show a keto C=O stretching frequency at 1686 cm(-1) and therefore are either in a polar environment or have a fairly weak hydrogen bond. For the more blue-absorbing states, we observe varying keto band positions between 1696 and 1664 cm(-1), and thus their hydrogen bonds strength varies between "none present" and "strong". A change in the frequency of the coordination marker mode was observed when the 690-nm state was populated, probably caused by a more planar conformation of the macrocyle of the chlorophyll responsible for the 690-nm state.