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

TL;DR

This study uses two-dimensional electronic spectroscopy to explore how the position of chromoproteins in photosynthetic complexes affects energy relaxation and vibrational coherence, revealing differences that may underpin efficient energy transfer.

Contribution

It provides new insights into how chromoprotein location influences energy dynamics and coherence in photosynthetic antenna complexes, highlighting mechanisms for unidirectional energy transfer.

Findings

Different chromoprotein locations lead to distinct energy relaxation kinetics.

Vibrational coherence shows a ~200 cm$^{-1}$ mode with 200 fs decay.

Location-dependent spectral differences suggest a key role in energy transfer efficiency.

Abstract

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 (2D-ES). These chromoproteins have similar adjacent pairs of pigments $\alpha$84 and $\beta$84, which are excited to delocalized exciton states. However, the kinetics and coherence of exciton states are significantly different from each other. Even CPCs with almost the same molecular structure display significantly different spectra and kinetics when the locations in the phycobilisome are different. This difference may be one of the key mechanisms for the efficient and unidirectional energy transfer in phycobilisomes. We observe low-frequency coherent vibrational motion of approximately 200 cm$^{-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.