Fig. 1.

FCS measurements of a fluorescent switch built into the structure of SR1 molecules. SR1 molecules diffuse in and out of a focused laser beam. In the T conformation, the fluorescent dye (Atto655), which labels a cysteine at position 327, is close enough to a tryptophan residue at position 44 (W44) to form a ground-state complex that quenches it. In the R conformation, the dye is too far from the tryptophan for quenching to occur, and it fluoresces more strongly. Fluorescence is collected by a microscope, and registered by a single-photon detector. Correlation functions of the fluorescence are generated by a hardware correlator.