Fluorescence correlations, single molecule detection and large number screening. Applications in biotechnology.

Fluorescence correlation spectroscopy (FCS), when carried out under conditions with low background as obtained in very small volume elements, is a powerful tool for examining molecular interactions as well as their time dependence. Interactions of biological importance which can be analyzed are hybridization between nucleic acid primers and DNA or RNA targets, between peptide ligands and isolated as well as cell-bound receptors, between antigen and antibodies. Since the interaction can be analyzed rapidly in small volumes without the need for separating unbound from bound ligand, an important application of FCS is envisaged in large-scale drug screening. The sensitivity has been advanced to the point that detection of single dye molecules is possible in the submillisecond range. This opens up the possibility for detecting rare events such as the appearance of pathogens in the early phase of infection or mutants exhibiting unusual properties when screening combinatorial libraries.