Spectroscopic studies of phycobilisome subcore preparations lacking key core chromophores: Assignment of excited state energies to the L_cm, β¹⁸ and α^AP-B chromophores

Chromophore absorption and emission characteristics of the α^AP-B, β¹⁸ and L_cm (large core-membrane linker) chromopeptides within the phycobilisome core are investigated using genetically engineered strains of Synechococcus sp. PCC 7002. Steady-state and time-resolved emission were used to examine energy transfer in subcore preparations from the wild-type organism and two mutants. Low-temperature (77 K) emission spectra were also measured for intact phycobilisomes from the wild-type and five mutant strains. Mutants retaining either the α^AP-B subunit or the unaltered L_cm chromophore resulted in only small changes in the low-temperature emission spectra, while retention of only the β¹⁸ subunit resulted in blue-shifted emission spectra. The L_cm chromophore has a room-temperature absorption maximum at 675 nm. In phycobilisomes at 77 K the α^AP-B and L_cm chromophores emit at 682-683 nm, and they are the best candidates for long-wavelength emitters also at room temperature. Overlap of these emission spectra with the absorption of chlorophyll a in the associated thylakoid membrane plays a significant role in excitation transfer from the antenna complexes in cyanobacteria.