Adsorption of bilirubin by amine-containing polyacrylamide resins

Polymeric sorbents for bilirubin have been synthesized by the reaction of lightly cross-linked poly(methyl acrylate) beads with di-, tri- or tetramines. The sorbents (or resins) have the general structure P{(CH2)nN+R2Cl−}mR, where P represents a polyacrylamide backbone, R represents H or CH3, and n and m are integers between 2–6 and between 1–3, respectively. The resins have high water swellability and amine functionalities of 2 to 8 meq/g. Isotherms (4°C) for the adsorption of bilirubin in 0.050 M aqueous phosphate buffer, pH = 7.8, show an increased capacity as the number of amino groups per pendant group is increased. The capacity is also increased by the quaternization of the amino groups in the resins. Thus, electrostatic interaction between the positively charged amino group of the polymer and the carboxyl anion of bilirubin is the main driving force for the adsorption. As indicated by the effect of changes in the length of the methylene spacer, i.e., in the value of n, hydrophobic interactions are also of importance in the binding. Isotherms with an S-shape, obtained for some of the protonated adsorbents, suggest positive cooperativity in binding, similar to that obtained for resins with lysine-containing functional groups.