Red chlorophyll a antenna states of photosystem I of the cyanobacterium Synechocystis sp PCC 6803

The antenna chlorophyll a (Chl a) molecules of photosystem I of green plants and cyanobacteria that absorb further to the red than P700, the special pair of the reaction center, have long been of considerable interest. Recently, the results of nonphotochemical hole burning experiments at liquid helium temperatures, which included use of high pressure and external electric (Stark) fields, led to the conclusion that the cyanobacterium Synechocystis sp. PCC 6803 possesses two "red" antenna states whose S-0 (ground state) --> Q(y)(S-1) origin absorption bands are at 714 and approximate to 708 nm (Ratsep et al. J. Phys. Chem B 2000, 104, 836). The results indicated that the 714 nm state is due to strongly coupled Chi a molecules (C-714), a dimer or, possibly, a trimer. It was concluded that the 714 nm state is responsible for the fluorescence origin band at 720 nm. Presented here are the results of theoretical simulations of the dependence of the hole spectra on burn wavelength and burn fluence that are consistent with the conclusions of Ratsep et al. They lead to a more detailed characterization of the two states, including determination of their site distribution functions (SDF) and the electron-phonon coupling parameters of the S-0 --> Q(y) transitions. The higher energy state is found to lie closer to 706 nm than 708 nm. The electron-phonon coupling of the 714 nm transition is strong with a total Huang-Rhys factor (S-t) of 2.0 due to low-frequency modes at 18 and approximate to 70 cm(-1). The coupling of the 706 nm transition is weaker by a factor of 1.5. It is concluded that both C-714 and C-706 are, at a minimum, dimers that are not in close proximity to each other. The large widths, approximate to 300 cm(-1), of the SDF suggest that the structures of C-706 and C-714 are fragile. The spectroscopic properties of P700, C-706, and C-714 are compared and discussed in terms of excitation energy transfer at low temperatures. A new model that explains why only about half of the PS I complexes undergo irreversible charge separation in the low-temperature limit following excitation of the higher energy bulk antenna states is presented.