Coherent energy transfer and dephasing in allophycocyanin trimers: A mechanism for directed energy transfer in photosynthesis

From a combination of femtosecond time-resolved absorption and anisotropy experiments, we have obtained evidence for a directed energy-transfer mechanism in the cyanobacterial light-harvesting protein allophycocyanin that depends on the initial coherence and dephasing of its phycocyanobilin (linear tetrapyrrole)-dimer exciton states. Interexciton-state relaxation and localization processes occur on the 10-30-fs and 300-fs time scales, respectively. These processes allow allophycocyanin to trap excitons from its energy donor in the phycobilisome, C-phycocyanin, and to direct them toward the photosystem IT reaction center.