Femtosecond spectroscopic study of excition-state dynamics in allophycocyanin and C-phycocyanin

Femtosecond pump-probe spectroscopy has been employed to examine the exciton-state dynamics in allophycocyanin and C-phycocyanin, photosynthetic light-harvesting proteins located in the phycobilisomes of cyanobacteria. Interexciton-state relaxation dynamics were observed in pump-probe anisotropy and transient hole-burning (THB) experiments in both systems. Anisotropy decays observed in two-color pump-probe experiments exhibit multiexponential decays from initial values higher than 0.4 with components exhibiting time constants of 10-30 fs, 280 fs, and 1 ps for allophycocyanin and 20-60 fs and 700 fs for C-phycocyanin. The fast decay reports the time scale for interexciton-state relaxation, whereas the slow component involves equilibration of population between the site states. THB spectra observed with allophycocyanin and C-phycocyanin evidence interexciton-state relaxation dynamics in terms of spectral red-shifts occurring on the 35- and 50-fs time scales, respectively; line broadening occurs on the 100-200-fs time scale owing to intramolecular vibrational redistribution and transient solvation dynamics. Vibrational relaxation in the lower exciton state occurs on the 400-fs time scale. When the ;Our results indicate that the sub-picosecond dimer photophysics in allophycocyanin and C-phycocyanin involve delocalized exciton states, but there is evidence for exciton localization on a longer time scale. Interexciton-state relaxation results in a red shift of population towards the absorption bands of chlorophyll proteins on a much shorter time scale than permitted by ordinary Forster energy-transfer in the weak-coupling limit. We suggest that this feature of the photophysics of delocalized excited states enhances the efficiency of light harvesting in photosynthetic antenna proteins.