Fluorescence Correlation Spectroscopy: A Tool for Measuring Dynamic and Equilibrium Properties of Molecules in Cells

Fluorescence correlation spectroscopy (FCS) measures dynamic processes of fluorescent molecules including diffusion, convection, and chemical reaction and equilibrium properties such as molecular concentrations and aggregation states. The measurements are carried out on low concentrations of fluorescent molecules (nanomolar), in tiny volumes (femtoliter), and over a wide time range (microseconds to seconds). FCS measures correlations of fluorescence intensity from individual fluorescent molecules and therefore inherently is a single molecule technique. FCS is well suited to measurements of molecular processes in cells and has been used to study diffusion, molecular associations, and assembly of cellular structures. A complementary, closely related approach, fluorescence photobleaching recovery (FRAP) is suited for high concentrations of fluorescent molecules and slower times (100ms to seconds).