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 the individual energy transfer steps has not yet been measured, and so how the efficiency is robust to this variation 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. We disentangled the energy transfer and energetic relaxation from nuclear motion using the spectral dependence of the dynamics. We observed an asymmetric distribution of timescales for energy transfer and a slower and more heterogeneous distribution of timescales for energetic relaxation, which was 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.
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This work was supported by the National Institutes of Health Director’s New Innovator Award 1DP2GM128200-01 and a Beckman Young Investigator Award (G.S.S.-C.). R.M. acknowledges a National Science Foundation Graduate Research Fellowship. T.K. acknowledges a Japan Science and Technology Agency PRESTO (no. JPMJPR18G7) and a Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research (no. 19H02665). G.S.S.-C. also acknowledges a Smith Family Award for Excellence in Biomedical Research, Sloan Research Fellowship in Chemistry and a CIFAR Global Scholar Award.
The authors declare no competing interests.
Peer review information Nature Chemistry thanks Pavel Malý, Tomas Polivka and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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(a) The structure of C-phycocyanin (Protein Data Bank ID Code 1GH0) is shown with a callout of a tetrapyrole chromophore (purple). (b) The corresponding absorption (solid) and emission (dashed) spectra are shown with the 610 nm excitation shown in blue. (c-f) Representative traces for C-phycocyanin with 610 nm excitation are with values of 125 ± 2, 503 ± 124, 113 ± 29, and 270 ± 51 fs, respectively. Errors given are the standard error of the maximum likelihood estimate.
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Moya, R., Norris, A.C., Kondo, T. et al. Observation of robust energy transfer in the photosynthetic protein allophycocyanin using single-molecule pump–probe spectroscopy. Nat. Chem. 14, 153–159 (2022). https://doi.org/10.1038/s41557-021-00841-9
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