Abstract In the present investigation, fluorescence con-elation spectroscopy (FCS) was used to measure the molecular motion of the pteridine derivative neopterin. However, technical limitations in the present optical setup precluded the identification of ,single neopterin molecules. FCS measurements with a fluorophore were also can-ied out for comparison. Exemplified by rhodamine green, we have introduced a concept that allows the detection, identification and analysis of assays in solution at the single-molecule level in tenns of bulk concentration. This concept is based on FCS and Poisson distribution analysis of assay sensitivity. The molecules had not to be quantified in a more concentrated fonn, or in flow and trapping experiments. The study demonstrated an ultrasensitive, reliable, rapid and direct tool for analytics and diagnostics in solution. We discuss a possible application of our new concept in activation control of cell-mediated immunity via neopterin determination.
Kinetic Characterization of Nonmuscle Myosin IIB at the Single Molecule Level