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. However, conventional experiments to measure rates rely on ensemble transient absorption spectroscopy, which is limited to average values. Single-molecule spectroscopy overcomes the limitation of ensemble averaging by measuring individual proteins, thereby revealing significant photophysical heterogeneity. However, these experiments were primarily restricted to fluorescence, which occurs on a nanosecond timescale, leaving the femto- to picosecond energy transfer steps unexplored. Herein, we report the further development of a novel technique, single-molecule ultrafast spectroscopy, which utilizes a pump-probe like excitation into a confocal microscope to report femto- to picosecond dynamics into the fluorescence emission of single molecules. This technique directly measures the distribution of energy transfer rates of single molecules, uncovering the variation in rates that underlies the high quantum efficiency of photosynthetic light harvesting. These observations will characterize how tightly the protein structure regulates energy transfer. In this direction, we present results on allophycocyanin (APC), a robust and well characterized light-harvesting complex present in the photosynthetic organisms, cyanobacteria. Initial experiments on APC are benchmarked with a photostable fluorophore, Atto647N, while varying optical pulse length and excitation wavelength to investigate the subsequent heterogeneity in energy transfer rates.

Development of single-molecule ultrafast spectroscopy to investigate the distribution of energy transfer rates in the photosynthetic light harvesting complex, allophycocyanin

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.