Quenching of tyrosine fluorescence in proteins by phosphate

The tyrosine fluorescence of ribonuclease, insulin, and human serum albumin is quenched to different degrees by phosphate ions. Quenching and “association” constants have been determined for the interaction of phosphate ion (HPO 4 −−) with tyrosine in these proteins and free in solution. These constants, evidence from fluorescence polarization measurements, and the increase in quenching at higher temperatures show that the quenching is mainly collisional (dynamic), with a smaller static component due probably to proton transfer from tyrosine to phosphate. Various considerations are presented to support the concept that the degree of quenching of tyrosine fluorescence in proteins by added anions is related to accessibility of certain tyrosyl residues.