Abstract
The effects of varying concentrations of monovalent cation chlorides on the fluorescence of nine proteins were studied. These are discussed in terms of “direct” or “indirect” interactions with the aromatic amino acid residues. Cs^+^ is the only cation that quenches fluorescence of proteins due to “direct” interaction with aromatic amino acid residues. Quenching is due to collisional processes. An agreement with the Stern‐Volmer relationship is shown and the values of [(K~Q~)eff] and [(f~a~)eff] are calculated. These values confirm that the fraction of fluorescence accessible to Cs^+^ belongs to the “exposed” fluorophors. The mechanism of quenching by Cs^+^ is due to the heavy‐atom effect because phosphorescence enhancement is also seen at the same time.
The chlorides of Na^+^, K^+^, Rb^+^, NH~4~^+^, and Li^+^ do not have a similar effect on the fluorescence of all proteins. For a given protein a gradation of the same effect (i.e., quenching or dequenching) is seen. Interactions with factors that “inderectly” affect fluorescence of any protein are involved and the structural features of the protein are responsible for such “indirect” effects. The results indicate that neutral salts can act in more than one manner. The changes in fluorescence are indicative of electrostatic and lyotropic effects of ions. Only electrostatic interactions which occur in the vicinity of tryptophan in proteins are reflected. Li^+^ shows strong interactions with proteins. In 4 M LiCl, BSA, papain, and trypsin show fluorescence changes that are indicative of changes in protein structure.