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Purpose: To explore the photophysical and photochemical properties of allophycocyanin (APC) and contribute to the understanding of the molecular mechanism of APC photosensitization. Materials and methods: Laser-flash photolytic and pulse radiolytic techniques were used for the first time to characterize the transient intermediates involved in APC photochemistry and photophysics. The excited triplet state and radical cation of APC were identified by acetone sensitization and one-electron oxidation. Results: The 248-nm laser-flash photolysis of APC in N[sub 2]-saturated aqueous solution (pH 7.0) yields the triplet state and radical cation of APC. The APC radical cations were generated by ionization via a monophotonic process, with a quantum yield of 0.17. Conclusions: APC can undergo both photo-excitation and photo-ionization under the present experimental conditions. These new findings suggest that APC has the potential to act as both a type I and type II photosensitizer. [ABSTRACT FROM AUTHOR]