Elucidation of the Photodynamics of Single Photosynthetic LH2 Complexes in Solution

Abstract

Photosynthetic organisms function under low light by converting photoenergy to chemical energy with near-unity quantum efficiency and under high light by dissipating unused photoenergy to prevent formation of deleterious photoproducts. There is widespread interest in how this dual functionality is achieved, because this balance enables efficient light harvesting under the fluctuating intensities of sunlight at the earth's surface. One obstacle to characterizing the molecular mechanisms responsible for this balance is that the excited-state properties of photosynthetic proteins vary drastically between individual proteins (due to static heterogeneity) and even within a single protein over time (due to dynamic heterogeneity). In ensemble measurements, these excited-state properties appear as a static, average value. To overcome this averaging, we investigate light-harvesting complex 2 (LH2), the primary antenna in purple bacteria, at the single-molecule level. Using a novel technique, the Anti-Brownian ELectrokinetic (ABEL) trap, we study individual LH2 complexes in a solution-phase environment to eliminate perturbations due to immobilization schemes, which can alter the protein structure and function. Furthermore, we perform the first simultaneous measurements of fluorescence intensity, lifetime, and emission spectra from individual proteins. We identify three distinct functional conformations of LH2, two of which correspond to a quenched and an unquenched form, and observe transitions occurring between these forms on a timescale of seconds. Our results reveal that individual LH2 complexes undergo photoactivated switching to the quenched state, and thermally revert to the ground state. This is a previously unknown, reversible mechanism for dissipation of excitation energy activated by high light conditions, and may be one component by which photosynthetic organisms flourish under varying light intensities.