Serum albumin-alginate coated microspheres: Role of the inner gel in binding and release of the KRFK peptide

In continuation with our previous study using fluorescein-isothiocyanate (FITC)–Lys-Arg-Phe-Lys (KRFK) peptide, the aim of this work was to study the interaction of the unlabelled KRFK with calcium alginate gel microspheres coated with a serum albumin (HSA)-alginate membrane prepared using a transacylation method. Coated microspheres were prepared with two main sizes and two gel strengths. Control microspheres made of cross-linked alginate-HSA without calcium alginate gel were also prepared. A series of loading and release assays conducted with methylene blue showed the requirement of inner gel for binding the cationic molecule. Release experiments were performed in different media using unlabelled KRFK and coated microspheres. A plateau was reached within 1 h, in contrast with the slow release of the FITC-peptide observed in our previous work. This discrepancy was attributed to modified properties of the labelled peptide. Adsorption assays of KRFK on coated microspheres were performed in the presence of growing concentrations of NaCl or imidazole. The ions were able to displace the peptide from the particles, which demonstrated ionic interactions, probably involving carboxylate groups of alginate. Adsorption isotherms showed that gel strength influenced affinity (4 × 105 L/mol or 8 × 105 L/mol for gelation with 5% or 20% CaCl2, respectively). Binding site number doubled (from 2.6 × 10−7 mol/mg to more than 5 × 10−7 mol/mg) when microsphere size decreased from 450 μm to 100 μm. Binding sites were assumed to be located in the gel underneath the membrane.