Exciton Annihilation in the Isolated Phycobiliproteins From the Blue-Green Alga Nostoc Sp. Using Picosecond Absorption Spectroscopy

— Phycobilisomes from the blue-green alga Nostoc sp. contain the phycobiliproteins: c-phycoerythrin (c-PE), c-phycocyanin (c-PC) and allophycocyanin (APC). The depletion and the recovery of the ground states for the individual phycobiliproteins were measured using picosecond (ps) absorption spectroscopy. In all cases the depletion time was Glops. The recovery was found to be non-exponenzial which could be fitted to a single exponential (‘fast’ component) and a second component with a relaxation time of > 300ps. The recovery times of the fast component were found to be intensity dependent and for c-PE, c-PC and APC were measured to be 19, 27 and 35ps, respectively, at intensity (I) ∼ 7 × 1020 photons/m2 and increased to 54, 55 and 67 ps, respectively, at I∼ 8 × 1019 photons/m2. The ps absorption data support the assignment of the‘fast’ component to singlet-singlet exciton annihilation.