Spectrally Resolved Anti-Brownian ELectrokinetic (ABEL) Trapping of Single Peridinin-Chlorophyll-Proteins in Solution

We use an Anti-Brownian ELectrokinetic (ABEL) trap to probe spectral emission shifts in solution-phase single Peridinin-Chlorophyll-Proteins (PCPs). The ABEL trap allows localization of single biomolecules in solution in a small volume for extended observation without immobilization. The essential idea combines fluorescence-based position estimation with fast electrokinetic feedback in a microfluidic geometry to counter the Brownian motion of a single nanoscale object, hence maintaining its position in a sub-micron-sized field of view for hundreds of milliseconds to seconds. Peridinin-chlorophyll-protein is a water-soluble antenna protein found in dinoflagellates which uses peridinins (carotenoids) as accessory light harvesting pigments to absorb sunlight in the green region of the spectrum before transferring electronic excitation to chlorophyll. PCP is simpler than many other antenna complexes in that there are only two chlorophyll pigments per monomer which do not form an exciton. We use the ABEL trap to study single PCP monomers in solution for several seconds each. A significant fraction of the molecules show slow spectral shifts (spectral diffusion) relative to the bulk PCP spectrum. This is the first spectral emission measurement conducted in the ABEL trap.