ИСТИНА

Phycobilisomes (PBs) are water soluble megadalton light-harvesting complexes of cyanobacteria and red algae. They consist of hundreds of phycobiliproteins containing thousands of bilin pigments, which are organized in an “energy funnel” to effectively deliver excitation to photosystems. Quenching of excess excitation energy is necessary for the photoprotection of light-harvesting complexes. In cyanobacteria quenching of PBs excitation is induced by the orange carotenoid protein (OCP) which photoactivation occurs under high light conditions. However, recent studies have shown that PBs can switch among various states depending on lighting conditions without OCP, presumably due to intramolecular charge-transfer within a phycobilins, which might play an essential physiological role. In this work using time-resolved fluorescence spectroscopy with picosecond time resolution in single photon counting mode we investigate spectroscopic signatures of Synechocystis sp. PBs and their components in absence and in presence of activated OCP. Since PBs is a multilevel system with energy transfer, we decided to use low energy quanta to excite the lowest energy levels of pigments in PBs core. Analysis of anti-Stokes fluorescence showed that the maximum fluorescence intensity of PBs is shifted to longer wavelengths by about 30 nm compared to excitation to the absorption maximum of phycocyanin. These long-wavelength excited states are characterized by approximately 2 ns lifetime, which does not change when the OCP complex with the PBs is formed. We assume that investigation of the nature of excited states, selectively excited in the infrared region, can provide a better understanding of the processes of energy conversion in light-harvesting complexes and the mechanisms of photoprotective reactions. This study was supported by Russian Science Foundation (grant no. 21-44-00005).