ELECTROSTATIC EFFECTS ON THE ADSORPTION AND CARBODIIMIDE-MEDIATED COUPLING OF FERRICHROME-A TO AMINE-MODIFIED SILICA

The adsorption behavior and carbodiimide-mediated coupling were examined for a carboxyl-containing cyclic hexapeptide, ferrichrome A (FeA), on amine-modified silica beads. Adsorption decreased with increasing salt concentration and no adsorption was observed in 100 mM sodium chloride. Ferrichrome A in water adsorbs to amine-modified silica beads to a maximum surface concentration of 1.69 mg/m(2), corresponding to an area per molecule of 104 Angstrom(2); this adsorption is fully reversed by addition of salt. Carboxyl groups on the FeA were coupled to amine groups on silane-modified silica using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) to promote formation of a peptide bond. Ferrichrome A coupled in this way to the silica beads is not washed off in high salt. The isotherms of FeA coupled irreversibly to the beads as a function of solution concentration of FeA were measured using a constant EDC:FeA mole ratio of 50:1 and a constant EDC concentration of 20 mM. There were significant differences between the coupling and adsorption isotherms, particularly at low solution concentrations of FeA. Electrophoretic mobility measurements of the silica beads to which varying amounts of FeA had been covalently bound showed the surface was positively charged at all peptide surface concentrations. The results are consistent with a model of the surface in which FeA is coupled to a porous layer of oligomeric aminosilane through a reaction which is strongly influenced by electrostatic interactions between the surface and reagents. (C) 1995 Academic Press, Inc.