Self-association of Zn-insulin at neutral pH: Investigation by concentration gradient-static and dynamic light scattering

Equilibrium self-association of Zn-insulin at pH 7.0 was characterized over the range 0.3-5 mg/mL by simultaneous measurement of static and dynamic light scattering. Analysis of static light scattering yielded a concentration-dependent weight-average molecular weight, and analysis of dynamic light scattering yielded a concentration-dependent intensity-average diffusion coefficient. The concentration dependence of both quantities may be accounted for to within experimental precision by a simple model, according to which the basic structural unit of Zn-insulin at concentrations exceeding 0.3 mg/mL is a hexamer H. With increasing total protein concentration, hexameric protomers may self-associate in accordance with an isodesmic scheme in which a protomer may add to any prexisting oligomer Hn to form Hn + 1 with an invariant stepwise equilibrium association constant.