Isolated-Chromophore and Exciton-State Photophysics inC-Phycocyanin Trimers

We have employed femtosecond pump−probe spectroscopy to examine the photophysics of C -phycocyanin, a photosynthetic light-harvesting protein located in the rods of the phycobilisome in cyanobacteria. The experiments were intended to focus on the photophysics of the paired α84 and β84 phycocyanobilin chromophores that are present in C -phycocyanin's C 3 -symmetric trimeric aggregation state. The time-resolved pump−probe absorption-difference spectrum exhibits initially narrow photobleaching/stimulated-emission holes that broaden on the 200 fs time scale but shift 250 cm -1 to lower energy on the ∼50 fs time scale. The line broadening arises primarily from intramolecular vibrational redistribution, as observed previously in preparations of the α subunit ( J. Phys. Chem. 1996, 100, 14198−14205), which contains only one phycocyanobilin chromophore. The pump−probe anisotropy in C -phycocyanin trimers exhibits decay components on the 20−60 and ∼700 fs time scales when the probe beam is centered on the 640 nm region, where a rising stimulated-emission signal is observed owing to the red shift of the time-resolved pump−probe spectrum. The initial anisotropy is significantly larger than 0.4, which strongly suggests the presence of an initially delocalized state involving the α84 and β84 chromophores. The fast red shift of the pump−probe spectrum, then, can be assigned to interexciton-state relaxation between the two exciton states of the coupled α84−β84 pairs.