Light scattering and viscosity study of heat aggregation of insulin

Aggregation behavior and hydrodynamic parameters of insulin have been determined from static and dynamic light scattering experiments and intrinsic viscosity measurements carried out at pH 4.0, 7.5, and 9.0 in the temperature range 20-40 degrees C in aqueous solutions. The protein aggregated extensively at elevated temperatures in the acidic solutions. Intermolecular interactions were found to be attractive and to increase with temperature. The measured intrinsic viscosity [eta], diffusion coefficient D0, molecular weight M, and radius of gyration Rg exhibited the universal behavior: M[eta] = (2.4 +/- 02) x 10(-27) (Re, eta/Re, D)3(D0 eta 0/T)-3 and (D0 square root of n)-1 approximately equal to (square root of pi eta 0 xi beta/kBT) [1 + 0.201)(v/beta 3) square root of n], where n is the number of segments in the polypeptide. The effective hydrodynamic radii deduced from [eta], (Re, eta) and the same deduced from D0, (Re, D) showed a constant ratio, (Re, eta/Re, D = 1.1 +/- 0.1). Re, D/Rg = xi was found to be (0.76 +/- 0.07). From the known solvent viscosity eta 0, the segment length beta was deduced to be (10 +/- 1) A. The excluded volume was deduced to be (5 A)3 regardless of pH. The Flory-Huggins interaction parameter was found to be chi = 0.45 +/- 0.04, independent of pH and temperature.