Phenyl Peri Acid, also known as 8-aniline-1-naphthalenesulfonic acid, is a dye intermediate, which is used to produce weak acid black BR, weak acid dark blue 5R, sulfur dyes and azo dyes, as well as fluorescent probes for protein research. Phenylperi acid is a fluorescent dye with high affinity for hydrophobic surface of protein. After a blue shift and a significant increase in fluorescence intensity, the phenylperi acid binds to the protein surface 1,2 in the low polar region. These properties make phenylperiacids very suitable for the determination of affinity of hydrophobic ligands and their corresponding binding proteins.