Energy-Transfer Kinetics in Phycobilisomes

Abstract

Blue-green alga (cyanobacteria) , red alga and cryptophyceae contain phycobiliproteins as major light-harvesting pigments which gather light in the wavelength region of low Chl absorption. The chromophores in these pigments are open chain tetrapyrroles which are bound covalently to apoproteins [1]. It has been shown that the energy absorbed by phycobiliproteins feeds the small pool of Chl in these algae [2]. In cyanobacteria and red alga the phycobiliproteins form large supramolecular antenna complexes, so-called phycobilisomes (PBS) [3, 4] which are located at the outer surface of the thylakoid membrane. PBS are made up of two or three different types of phycobiliproteins which occur predominantly in hexameric aggregation. The first picosecond measurements of phycobiliprotein containing algae and isolated PBS were carried out by Porter et al. [5, 6]. Phycobiliproteins and PBS are interesting objects for time-resolved studies for several reasons. Unlike the Chl protein complexes of higher plants, different phycobiliproteins have their absorption and emission spectra fairly well separated, which more easily allows the detailed sequence of energy-transfer steps to be explored. Furthermore, the single-step transfer times seem to be significantly longer than those of Chl complexes, which puts these processes in a time-range accessible to picosecond techniques.