TRIPLET AND FLUORESCING STATES OF THE CP47 ANTENNA COMPLEX OF PHOTOSYSTEM-II STUDIED AS A FUNCTION OF TEMPERATURE

Fluorescence emission and triplet-minus-singlet (T-S) absorption diffe rence spectra of the CP47 core antenna complex of photosystem II were measured as a function of temperature and compared to those of chlorop hyll a in Triton X-100. Two spectral species were found in the chlorop hyll T-S spectra of CP47, which may arise from a difference in ligatio n of the pigments or from an additional hydrogen bond, similar to what has been found for Chi molecules in a variety of solvents. The T-S sp ectra show that the lowest lying state in CP47 is at similar to 685 nm and gives rise to fluorescence at 690 nm at 4 K. The fluorescence qua ntum yield is 0.11 +/- 0.03 at 4 K, the chlorophyll triplet yield is 0 .16 +/- 0.03. Carotenoid triplets are formed efficiently at 4 K throug h triplet transfer from chlorophyll with a yield of 0.15 +/- 0.02. The major decay channel of the lowest excited state in CP47 is internal c onversion, with a quantum yield of about 0.58. Increase of the tempera ture results in a broadening and blue shift of the spectra due to the equilibration of the excitation over the antenna pigments. Upon increa sing the temperature, a decrease of the fluorescence and triplet yield s is observed to, at 270 K, a value of about 55% of the low temperatur e value. This decrease is significantly larger than of chlorophyll a i n Triton X-100. Although the coupling to low-frequency phonon or vibra tion modes of the pigments is probably intermediate in CP47, the tempe rature dependence of the triplet and fluorescence quantum yield can be modeled using the energy gap law in the strong coupling limit of Engl man and Jortner (1970. J. Mol. Phys. 18:145-164) for non-radiative dec ays. This yields for CP47 an average frequency of the promoting/accept ing modes of 350 cm(-1) with an activation energy of 650 cm(-1) for in ternal conversion and activationless intersystem crossing to the tripl et state through a promoting mode with a frequency of 180 cm(-1). For chlorophyll a in Triton X-100 the average frequency of the promoting m odes for non-radiative decay is very similar, but the activation energ y (300 cm(-1)) is significantly smaller.