Measurement of specific surface energy of the protein hydration shell using spin label EPR

An approach to measure the microscopic surface tension in a water-protein matrix using spin label EPR is proposed. The approach utilizes the parameters of the viscosity isotherms for the correlation time of the spin labels bound to a protein macromolecule. The pattern of changes in the specific surface energy in the protein hydration shell depending on the protein concentration, structure of reporter group, ligand state of protein, and the presence of salts, water-soluble polymers, and D₂O in solution was studied using serum albumin, hemoglobin, and antibodies as examples. Possible causes underlying the observed changes in surface tension are discussed.