Fluorescence study of Cu2+-induced interaction between albumin and anionic polyelectrolytes

The Cu2+-induced complex formation of bovine serum albumin (BSA) with anionic polyelectrolytes (PEs) (polyacrylic acid (PAA), poly(N-isopropylacrylamide) [poly[NIPAAm]], and copolymers of N-isopropylacrylamide (NIPAAm) and acrylic acid) in aqueous solution was studied by a fluorescence technique and high-performance liquid chromatography analysis. The character of the interactions depends on the monomer composition(r = [COOH]/[NIPAAm]), [Cu2+]/[PE], and [BSA]/[PE] ratios and solution pH. Two types of ternary polycomplex (polymer + Cu2+ + BSA) particles are formed depending on the monomer composition r of the copolymer. At r from 1/3 to 1/1, the protein molecules in the structure of ternary polycomplex particles are densely covered by the shell of a polymer coil and practically "fenced off' from the water environment. At r greater than or equal to 3/1 ternary polycomplex PAA-Cu2+-BSA particles have more friable structures in which protein molecules are practically exposed to the solution. At low polymer concentration, an intrapolymer ternary polycomplex is formed. This complex aggregates to an interpolymer species upon increase in polymer concentration. Fluorescence data indicate that in ternary complex polymer interacts through Cu2+ ions with BSA preferentially at the site close to the location of "cleft" tryptophan residue. This leads to static quenching of this tryptophan fluorescence. Cu2+-induced complex formation is an equilibrium reaction.