Ligands for fluorescence correlation spectroscopy on g protein-coupled receptors.

G protein-coupled receptors (GPCRs) comprise a large protein family of transmembrane receptors involved in many physiological processes. They are engaged in various transduction processes of extracellular signals into intracellular responses. Due to their involvement in numerous diseases they represent an important pharmacological target. Fluorescence correlation spectroscopy (FCS) poses a very sensitive analytical technique well-suited for the investigation of GPCRs. It is minimally invasive and operates on a single molecular level. It further provides detailed pharmacological information on receptor kinetics and quantities of activated receptors on the cell membrane. In addition, FCS allows distinguishing between different receptor states based on different diffusion time constants. In order to be applicable for FCS, the molecule of interest has to be fluorescently labeled. This review focuses on the physical requirements for dyes intended for FCS, their influence on the binding characteristics of coupled ligands and strategies to generate dye labeled ligands, exemplified on GPCR ligands.