Observation of robust energy transfer in the photosynthetic protein allophycocyanin using single-molecule pump-probe spectroscopy

Abstract Photosynthetic organisms convert sunlight to electricity with near unity quantum efficiency. Absorbed photoenergy transfers through a network of chromophores positioned within protein scaffolds, which fluctuate due to thermal motion. The resultant variation in energy transfer has not yet been measured, and so how the efficiency is robust to this variation, if any, has not been determined. Here, we describe single-molecule pump-probe spectroscopy with facile spectral tuning and its application to the ultrafast dynamics of single allophycocyanin, a light-harvesting protein from cyanobacteria. Using the spectral dependence of the dynamics, energy transfer and energetic relaxation from nuclear motion were disentangled. For energy transfer, an asymmetric distribution of timescales was observed. For energetic relaxation, the timescales were slower and more heterogeneous due to the impact of the protein environment. Collectively, these results suggest that energy transfer is robust to protein fluctuations, a prerequisite for efficient light harvesting.