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.

光合生物以接近单位的量子效率将阳光转化为电能。吸收的光能通过位于蛋白质支架内的发色团网络传输，该支架因热运动而波动。尚未测量单个能量转移步骤中产生的变化，因此尚未确定效率如何对这种变化具有鲁棒性。在这里，我们描述了具有简单光谱调谐功能的单分子泵-探针光谱学及其在单个别藻蓝蛋白超快动力学中的应用，别藻蓝蛋白是一种来自蓝藻的光捕获蛋白。我们使用动力学的光谱依赖性从核运动中解开了能量转移和能量弛豫。我们观察到能量转移时间尺度的不对称分布和能量松弛时间尺度的更慢和更不均匀的分布，这是由于蛋白质环境的影响。总的来说，这些结果表明能量转移对蛋白质波动是稳健的，这是有效光收集的先决条件。