Electrochemical Studies of the Interfacial Behavior of Insulin

The interfacial behavior of insulin and chain A and chain B of insulin was investigated at the platinum electrode in a phosphate buffer, pH 7.0, using cyclic voltammetry. The enthalpy of adsorption, DeltaHADS, calculated from a linear van't Hoff relationship over the temperature range 299 to 333 K gave values of -22 ± 1, -17 ± 1, and -9 ± 1 kJ mol-1 for insulin, chain B and chain A, respectively. Above these temperatures denaturation of insulin occurs, and for all three molecules the surface adsorption measured by the surface charge densities showed an immediate decrease followed by a slight increase. The surface concentrations of insulin of 2.9 ± 0.2 mg m-2 at 299 K and 3.2 ± 0.3 mg m-2 at the physiological temperature of 310 K agreed well with the calculated value determined from geometrical dimensions. Similar calculations from experimental surface charge densities for chain A and chain B indicated that a more efficient packing prevailed with the individual polypeptides. From a consideration of the mechanism of adsorption based on cyclic voltammetric measurements, an estimation of the number of carboxylate groups on insulin was determined to be 6 ± 2, which agrees with the known number of acidic residues on the protein. Similar calculations for chain A gave 2 ± 0.4, which indicates that this peptide remains as a monomer in the phosphate buffer. However, the value obtained for chain B under similar experimental conditions was 6 ± 2, indicating that this peptide appears to dimerize in the phosphate buffer. Dimer formation of insulin is known to occur through hydrophobic interactions and four hydrogen bonds between the B chains.