Photo-induced conformational flexibility in single solution-phase peridinin-chlorophyll-proteins.

The peridinin-chlorophyll-protein (PCP) is an accessory light-harvesting complex found in red-tide dinoflagellates. PCP absorbs photons primarily in the blue-green spectral region via peridinin (Per) carotenoid pigments which then transfer excitations to chlorophyll (Chl) and ultimately downstream to photosystem II (PSII). Whereas the ultrafast dynamics of PCP are well-studied, much less is known about slower protein dynamics on time scales of milliseconds and seconds. Previous single-molecule studies of spectral emission and intensity have attached PCP to surfaces, but the native environment of PCP is in the lumen, meaning that a surface-attached environment could perturb its native conformations. To address this concern, we use the anti-Brownian electrokinetic (ABEL) trap to study single PCP monomers in solution for several seconds each. We measure, for the first time, simultaneous single-molecule intensity, lifetime, and spectral emission shifts for each trapped PCP monomer. The rate of reversible spectral redshifts depends linearly on irradiance over a factor of 30, indicating a light-induced mechanism which we attribute to a protein conformational change. Independent of these spectral shifts, our measurements of intensity and lifetime show reversible Chl quenching. In contrast to previous work, we show that this quenching cannot result from isolated photobleaching of Chl. These independent mechanisms arise from distinct conformational changes which maintain relatively stable fluorescence emission.