Fluorescence origin of 6,P-toluidinyl-naphthalene-2-sulfonate (TNS) bound to proteins

6,P-toluidinylnaphthalene-2-sulfonate (TNS) is a highly fluorescent molecule when dissolved in a low polarity medium or when bound to proteins. The aim of the present work is to explain origin of this fluorescence, to find out how the medium (solvent, protein matrix) affects fluorescence observables such as lifetimes and spectra and finally to put into evidence possible relation that exists between these observables and fluorophore structure. To achieve our goal we performed studies on TNS dissolved in ethanol, at high concentrations in water (aggregated form) and bound to proteins. Our experiments allowed us to find out that TNS in the three environments has different structures. Presence of three lifetimes observed in proteins and in water instead of one lifetime found in ethanol can be assigned to the high contact between TNS molecules. Our results are discussed in terms of solvent polarity and interaction within fluorophore molecules bound to proteins.