Equilibrium and non-equilibrium complexes between bovine serum albumin and dextran sulfate—I. Complexing conditions and composition of non-equilibrium complexes

Abstract

The complexing between bovine serum albumin (BSA) and dextran sulfate (DS) with ionic strengths of 0.01–0.1 was studied at pH 2.7–5.6. At pHs above the BSA isoelectric point (IEP), BSA sorption by DS is reversible and non-cooperative. The swing to cooperative BSA sorption is observed over the pH range 5.6–5.0. At pHs below the BSA IEP, BSA sorption becomes irreversible. When the sorption is irreversible the insoluble complex composition (regardless of the method of macroreagent mixing) depends on the initial ratio between protein and polysaccharide concentration. The macroreagent concentration ratio allowing 100% yield of insoluble complexes corresponds to equality of molar concentrations of BSA cationic groups and DS sulfate groups. Precipitation of non-equilibrium complexes takes place even in the case of low protein content (about 150 monosaccharide units per BSA molecule). Possible reasons for stabilization of non-equilibrium states of the studied complexes are considered.