Aggregation behavior of zinc-free insulin studied by small-angle neutron scattering: analysis by use of a thermodynamic equilibrium model

Abstract

The aggregation behavior of zinc-free insulin has been studied by small-angle neutron scattering as a function of protein concentration, pH, and ionic strength of the solution. The analysis of the data shows that the weight-averaged molecular mass and the z-average radius of gyration varies systematically with the experimental conditions. They increase with decreasing pH (decreasing charge) and with increasing ionic strength (increasing charge screening). The radius of gyration scales as a power law of the weight-average mass with the exponent 0.44. A similar scaling is found for a set of oligomer structures based on the crystal structure of zinc-free insulin. The scattering data were fitted by a model based on these oligomer structures and on the equilibrium model recently introduced by Kadima et al.. The model takes into account the variation of the effective charge of the monomer with pH and ionic strength. The model has only three parameters which were fitted simultaneously to the neutron scattering data for 12 different experimental conditions.