Exciton coherence and energy transport in the light-harvesting dimers of allophycocyanin.

Femtosecond transient grating and photon echo spectroscopies with a sub-20 fs time resolution are applied to allophycocyanin (APC), a protein located at the base of the phycobilisome antenna of cyanobacteria. Coupling between pairs of phycocyanobilin pigments with nondegenerate energy levels gives rise to the four-level exciton electronic structure of APC. Spectroscopic signals obtained in multiple experiments (e.g., linear absorption, fluorescence, transient grating, 2D Fourier transform photon echo) are used to constrain the parameters of a Frenkel exciton Hamiltonian. Comparison between experiment and theory yields a robust microscopic understanding of the electronic and nuclear relaxation dynamics. In agreement with previous work, transient absorption anisotropy establishes that internal conversion between the exciton states of the dimer occurs with time constants of 35, 220, and 280 fs. The sub-100 fs dynamics are decomposed into three distinct relaxation processes: electronic population transfer, intramolecular vibrational energy redistribution, and the dephasing of electronic and nuclear coherences. Model calculations show that the sub-100 fs red-shift in the transient absorption signal spectrum reflects interference between stimulated emission (ESE) and excited state absorption (ESA) signal components. It is also established that the pigment fluctuations in the dimer are not well-correlated, although further experiments will be required to precisely quantify the amount of correlation. The findings of this paper suggest that the light harvesting function of APC is enhanced by nondegeneracy of the pigments comprising the dimer and strong vibronic coupling of intramolecular modes on the phycocyanobilins. We find that the exciton states are 96% localized to the individual molecular sites within a particular dimer. Localization of the transition densities, in turn, is suggested to promote significant vibronic coupling which serves to both broaden the absorption line shape and open channels for fast internal conversion. The dominant internal conversion channel is assigned to a promoting mode near 800 cm(-1) involving hydrogen out-of-plane (HOOP) wagging motion similar to that observed in phytochrome and retinal. This rate enhancement ensures that all photoexcitations quickly and efficiently relax to the electronic origin of the lower energy exciton state from which energy transfer to the reaction center occurs.