Dielectric properties of insulin in solution

Values of the relative permittivity of a 5% solution of despentapeptide insulin (DPI) are given at six temperatures over a frequency range 0.2-50 MHz. The solvent was a mixture of water and ethylene glycol at pH = 3. From the dispersion curves a dipole moment of 72 +/- 1 was obtained for the insulin molecule at 25 degrees C. By combining the dielectric measurements with values of solvent viscosity the volume of the insulin molecule in solution was found to be only slightly larger than that in the crystal. This indicates that insulin possesses a markedly lower quantity of water of hydration than has been observed for globular proteins containing a higher proportion of hydrophilic groups on the surface of the molecule.