DELAYED MICROSECOND LUMINESCENCE OF PHYCOBILISOMES

The time-resolved luminescence spectra (in the microsecond range) of phycobilisomes and biliproteins in buffer and polymer matrix were measured in the temperature range from 8 K. to 293 K. Delayed luminescence located in the same spectral region as prompt fluorescence of investigated samples (DLF) and the long-wavelength delayed emission in the720–760 nm range (DL1) was observed. The temperature and viscosity dependencies of DLF and DL1 luminescences were different, but both do not have uniexponential decays and are not quenched by oxygen. This means that delayed luminescence could be generated without the participation of the triplet states, or the chromophores could be shielded by protein against interaction with oxygen. The linear dependence of delayed luminescence on exciting light intensity shows that delayed luminescence is monophotonically induced. It seems that both DLF and DL1 are related to electron-cation recombination, which yields excited singlet states. The DLF is emitted from the first excited singlet state of biliprotein chromophores and DL1 from the same state of the excimers or from the triplet state of some groups of chromophores. Ionization energy of chromophores can be lowered as a result of their interactions with the environment. Delay of emission is due to the trapping or solvation of electrons. Every type of biliprotein consisting of phycobilisomes possesses its own “trap” and can emit the DL. In the case of native phycobilisomes a competition between the excitation energy trapping and transfer occurs.