Photon correlation spectroscopy of interacting and dissociating hemoglobin

Abstract

Experimental and theoretical analysis of the effect of both intermolecular interactions and dissociation on the diffusional properties of carbon–monoxide bovine hemoglobin in solution are reported here. When performing accurate photon correlation spectroscopy measurements versus protein concentration, even on dilute solutions, one finds that the first cumulant diffusion coefficient of the macromolecule has a relevant dependence upon pH (5⩽pH⩽9.5), ionic strength (10–100 mM), and, particularly, on hemoglobin concentration (1⩽c⩽20 mg/ml). The results cannot be interpreted by considering the occurrence of either protein dissociation or intermolecular interactions only. As a consequence a simple theoretical expansion of the first cumulant diffusion coefficient, to first order in concentration, is derived here with the inclusion of protein interactions and dissociation. A fit procedure based on this expression leads to a good description of the dissociation and an accurate estimate of the native protein charge and of its single particle diffusion coefficient in the full range of solution parameters.