The red-absorbing chlorophyll a antenna states of photosystem I:: A hole-burning study of Synechocystis sp PCC 6803 and its mutants

Low temperature (4.2 K) absorption and hole-burned spectra are presented for the trimeric (wild-type, WT) photosystem I complex of the cyanobacterium Synechocystis sp. PCC 6803, its monomeric form, and mutants deficient in the PsaF, K, L, and M protein subunits. High-pressure- and Stark-hole-burning data for the WT trimer are presented as well as its temperature-dependent Q(nu)-absorption and -fluorescence spectra. Taken as a whole, the data lead to assignment: of a new and lowest energy antenna Q(y)-state, located at 714 nm at low temperatures. It is this state that is responsible for the fluorescence in the low-temperature limit and not the previously identified antenna Q(y)-state near 708 nm. The data indicate that the 714 nm state is associated with strongly coupled chlorophyll a molecules (perhaps a dimer) and possesses significant charge transfer character. The red chlorophylls absorbing at 708 and 714 nm do not appear to be directly bound to any of the above protein subunits. The results are consistent with a location close to the interfacial regions between PsaL and M and the PsaA/B heterodimeric core. It is likely that the red chlorophylls are bound to PsaA and/or PsaB.