Single-Molecule Fluorescence Spectroscopy of Photosynthetic Systems

Abstract

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.

Microsecond and millisecond dynamics in the photosynthetic protein LHCSR1 observed by single-molecule correlation spectroscopy

“…The relationship between structure and function is often studied based on well-resolved, static structural models, such as those from Xray crystallography and electron microscopy. However, protein conformational dynamics lead to time-varying structures, which emerge in response to environmental and thermal perturbations and therefore, are critical to survival under natural conditions (1,2). These dynamics occur across spatial, temporal, and energetic scales (3).…”

“…Most commonly, the dynamics are fast and thus, local. Local dynamics can take place simultaneously and asynchronously across several discrete sites in an individual protein (2,3), providing multifunctionality to the system if individually controlled. These dynamics are obscured in conventional structural measurements due to ensemble averaging.…”

Waveguide-based chemo- and biosensors: complex emulsions for the detection of caffeine and proteins

“…3 The amplitude of the evanescent field decreases exponentially with distance from the surface with a penetration depth, typically of about 200 nm, which allows for sensing interactions with the environment immediately surrounding the waveguide. 4 Evanescent field sensing has been widely used in devices targeting physical, 5 chemical, 6 or biological events 7 by detecting changes in evanescent fluorescence, 8 refractive index, 9 or spectroscopic shifts. 10,11 Popular examples include waveguide sensors for the detection of pathogens, 12,13 total internal reflection fluorescence spectroscopy and dual-polarization interferometry, 14,15 as well as fingerprint identification scanners.…”

From isolated light-harvesting complexes to the thylakoid membrane: a single-molecule perspective

“…In the blinking case these can be described by rapid switching between several protein conformational states with a different energy, represented by various PESs [44,48]. The connection to the excitonic model, which qualitatively describes the spectral diffusion, can be made by assuming that the dark state arises from energy transfer to a short-lived state with fast non-radiative recombination.…”