1,8-ANS is a fluorescent dye that binds with high affinity to hydrophobic surfaces of proteins. The emission maximum of 1,8-ANS undergoes a blue shift and fluorescence intensity increases significantly upon binding to low polarity regions of a protein surface. These properties make 1,8-ANS well suited for determining the affinity of hydrophobic ligands to their corresponding binding proteins, such as the binding of free fatty acids to fatty acid binding proteins (FABPs). 1,8-ANS binds to intestinal FABP (FABP2) with a Kd value of ~9.7 µM at 24.5°C.