Water-insoluble triple complexes: Bovine serum albumin-bivalent metal cation-alginate

Abstract

Formation of water-insoluble complexes between bovine serum albumin (BSA) and sodium alginate in the presence of bivalent metal cations was studied. Complexing was observed to take place close to the BSA isoelectric point and above only for cations having a high affinity for BSA (Cu(II), Cd(II) and Zn(II)) and not other divalent ions (Mg(II), Mn(II) and Ca(II)). Addition of NaCl suppressed precipitation. The quantity and composition of the precipitate formed in the system BSA-Cu(II)-alginate was analysed with reference to the initial protein/polysaccharide weight concentration ratio (C P/C M) and Cu(II) concentration. In the precipitate the Cu(II)/alginate monosaccharide molar ratio (Cu/A) is constant and equal to 0·7±0·1. When C P/C M > 8, the Cu/BSA molar ratio is also practically constant, amounting to 18–20 and being equal to the number of binding sites for copper cations (II) in the BSA molecule. It is believed that insoluble triple complexes BSA-Cu(II)-alginate form due to bridging between copper-binding sites of BSA and alginate carboxyl groups via Cu(II) cations. At [Cu]/[A] > 1 and C P/C M > 8, the coordination sphere of Cu(II) cations is probably characterized by a constant structure. The change in this structure at [Cu]/[A] < 1 and/or C P/C M < 8 is accompanied by a decrease in the yield of the precipitated complex and also the protein content of the complex.