Photosynthesis begins when a network of pigment-protein complexes captures solar energy and transports it to the reaction center, where charge separation occurs. When necessary (under low light conditions), photosynthetic organisms perform this energy transport and charge separation with near unity quantum efficiency. Remarkably, this high efficiency is maintained under physiological conditions, which include thermal fluctuations of the pigment-protein complexes and changing local environments. These conditions introduce multiple types of heterogeneity in the pigment-protein complexes, including structural heterogeneity, energetic heterogeneity, and functional heterogeneity. Understanding how photosynthetic light-harvesting functions in the face of these fluctuations requires understanding this heterogeneity, which, in turn, requires characterization of individual pigment-protein complexes. Single-molecule spectroscopy has the power to probe individual complexes. In this review, we present an overview of the common techniques for single-molecule fluorescence spectroscopy applied to photosynthetic systems and describe selected experiments on these systems. We discuss how these experiments provide a new understanding of the impact of heterogeneity on light harvesting and thus how these systems are optimized to capture sunlight under physiological conditions.
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