EVIDENCE FOR COHERENT ENERGY-TRANSFER IN ALLOPHYCOCYANIN TRIMERS

We have performed two-color femtosecond pump-probe anisotropy experiments on allophycocyanin trimers, a photosynthetic light-harvesting system located at the core of the phycobilisome of cyanobacteria. The phycocyanobilin (open-chain tetrapyrrole) chromophore dimer systems in allophycocyanin trimers exhibit an unusual ground-state absorption spectrum that has been attributed to molecular dimer systems with moderately strong dipole-dipole interaction strengths. Polarized two-color pump-probe transients, obtained with 80-fs 620-nm pump and 640-nm probe pulses, were fit by a global iterative reconvolution routine to a model consisting of dichroism-free and anisotropic functions. The anisotropy decays in a multiexponential manner from an initial maximum in the 0.58-0.7 range, with components exhibiting time constants of 10-30 fs, 280 fs, and 1 ps. The observation that the initial anisotropy is significantly larger than 0.4 evidences the initial presence of coherently excited molecular dimers in allophycocyanin trimers. The 10-30-fs anisotropy component reports the time scale for interexciton state relaxation, while the slower components report the time scale for localization of excitation on one of the dimer chromophores; the rate constant for localization is expected to be comparable with that estimated for Forster energy transfer between the two chromophores in a dimer. We suggest that the initial presence and decay of dimer electronic coherence might play a role in enhancing the energy-transfer function of allophycocyanin in the phycobilisome.