Dependence of energy relaxation and vibrational coherence on the location of light-harvesting chromoproteins in photosynthetic antenna protein complexes.

Phycobilisomes are antenna protein complexes in cyanobacteria and red algae. In phycobilisomes, energy transfer is unidirectional with an extremely high quantum efficiency close to unity. We investigate intraprotein energy relaxation and quantum coherence of constituent chromoproteins of allophycocyanin (APC) and two kinds of C-phycocyanin (CPC) in phycobilisomes using two-dimensional electronic spectroscopy. These chromoproteins produced by an Escherichia coli expression system have similar adjacent pairs of pigments α84 and β84, which are excited to delocalized exciton states. However, the kinetics and coherence of exciton states are significantly different from each other. Even CPCs with almost the same molecular structure display different 2D spectra when the locations in the phycobilisome are different. The spectra of the inner CPC in the phycobilisome are red-shifted relative to that of the outer one. This may promote the efficient and unidirectional energy transfer to the APC core. We observe low-frequency coherent vibrational motion of ∼200 cm-1 with large amplitude and a decay time of 200 fs. The wave packet motion involving energy relaxation and oscillatory motions on the potential energy surface of the exciton state is clearly visualized using beat-frequency-resolved 2D-ES.