Fluorescence Properties of C-Phycocyanin Isolated from a Thermophilic Cyanobacterium

Abstract The fluorescence lifetime of purified C‐phycocyanin from the thermophilic cyanobacterium, Phormidium laminosum (strain OH‐1‐p. Cl 1), was measured as 1.48 ± 0.06 ns using the technique of time‐correlated single‐photon counting under very weak excitation pulses. The natural radiative lifetime (∼6.1 ns) of the pigment was calculated by integrating the absorption spectrum using the Strickler–Berg equation. From these two lifetimes we calculate a fluorescence quantum yield of ∼0.24 which is very close to the value ∼0.22 which we measure relative to the known value of cresyl violet in methanol. Both the fluorescence lifetime and the quantum yield of the pigment from this organism are lower than most previous values reported in the literature. We conclude that our lower values are not due to high light intensity, pH, buffer, concentration, instrumentation artifacts, aggregation effects or the thermophilic nature of the organism. Instead, we suggest that the photophysical properties of C‐phycocyanin are species dependent, perhaps due to the specific molecular environment of the tetrapyrrole.