In photosynthesis, solar energy capture to conversion occurs with a remarkable near-unity quantum efficiency despite thermally-driven protein fluctuations. These fluctuations are predicted to produce sub-optimal energy transfer rates and thereby decrease overall efficiency. Understanding how high efficiency is maintained despite these fluctuations is a requisite for identifying fundamental design principles for robust light-harvesting devices. Furthermore, the knowledge of the distribution of energy transfer rates arising from protein fluctuations is crucial to understanding the high efficiency in photosynthetic organisms. We describe the further development of a novel technique, single-molecule pump-probe spectroscopy, which utilizes a pump-probe like excitation into a confocal microscope to transfer femto- to picosecond dynamics into the fluorescence emission of single molecules. This technique directly measures the distribution of timescales for ultrafast dynamics of single molecules. We apply this technique to two individual photosynthetic light-harvesting subunits from cyanobacteria, allophycocyanin and c-phycocyanin. Broad distributions of timescales were observed, addressing the variation that underlies the high quantum efficiency of photosynthetic light harvesting.

Heterogeneous ultrafast dynamics in single photosynthetic proteins

ACS Spring Meeting 2021

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