Red antenna states of photosystem I from cyanobacterium Synechococcus elongatus: a spectral hole burning study Original Research Article

The existence of at least three low-energy chlorophyll a (Chl a) antenna states is shown for photosystem I (PS I) of Synechococcus elongatus by the use of nonphotochemical hole burning (NPHB) spectroscopy. In addition to the previously reported states at 708 and 719 nm, it is demonstrated that there is a third state at 715 nm. The responsible Chl a molecules are referred to as C-708, C-715 and C-719. For both Synechococcus and Synechocystis, the lowest energy state is shown to be characterized by strong electron–phonon coupling (large Huang-Rhys factor S), large permanent dipole moment change (f·Δμ), and large linear pressure shift rates attributable to electron exchange of dimeric Chl a. The lowest energy state of Synechocystis is at 714 nm. The properties of the 714 and 719 nm states are very similar, suggesting that their dimer structures are also similar. Although the other red antenna states of these cyanobacteria have smaller values for S, f·Δμ, and linear pressure shift rate, these are still larger than typically measured for monomeric antenna Chl a. Possible assignments of red absorption bands to particular chlorophyll dimers or trimer are discussed.