Exposure of tryptophanyl residues and protein dynamics

The acrylamide quenching reaction is shown to be very discriminating in sensing the exposure of fluorescing tryptophanyl residues in globular proteins. The quenching rate constants for some proteins, such as aldolase and human serum albumin, are reported to be independent of the solvent viscosity, indicating that the reaction is limited by penetration of the quencher through the protein matrix. Temperature-dependent studies are performed to determine the activation energy and entropy for the penetration of acrylamide into these proteins. The tryptophanyl residues in aldolase are shown to be shielded by a large activation energy barrier, while the single residue in human serum albumin is shielded by a large activation entropy barrier. These parameters characterize the nature of the protein matrix enveloping the fluorophors.