Surface thermodynamic properties and chromatographic and salting-out behavior of IgA and other serum proteins

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By means of contact angle determinations with two liquids, on hydrated as well as on dried protein layers, the long-range and the short-range contributions to the protein surface tensions, and from these the protein (ΔG131) and the protein-ligand (ΔG132) free energies of interaction in aqueous media, were determined. For human serum albumin (HSA), human IgG, and human IgA, the differences between ΔG 131 HYDRATED and ΔG 131 DRY were connected with the behavior of these proteins in low concentrations of (NH4)2SO4 versus saturated (NH4)2SO4 solutions. By interpolation, intermediate points are found that correlate well with the known salting-out properties of these three proteins. On the basis of the data, it is predicted that the precipitation of IgG by 1/3 saturated (NH4)2SO4 is preventable, or reversible, by the admixture of 15% dimethylsulfoxide; both predictions are confirmed experimentally. From the ΔG132 values found, it is shown that HSA and IgG should attach to phenyl ligands under physiological conditions, but that IgA is so hydrophilic that it only can adhere to phenyl ligands after partial dehydration brought about when admixed with 1 M (NH4)2SO4. Closer analysis of the values obtained for the long-range and short-range components of the surface tensions of HSA, IgG, and IgA allow deeper insight into their functional, chemical, and physicochemical properties.