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

The adsorption behavior and carbodiimide-mediated coupling were examin ed for a carboxyl-containing cyclic hexapeptide, ferrichrome A (FeA), on amine-modified silica beads. Adsorption decreased with increasing s alt concentration and no adsorption was observed in 100 mM sodium chlo ride. 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 reverse d by addition of salt. Carboxyl groups on the FeA were coupled to amin e groups on silane-modified silica using 1-ethyl-3-(3-dimethylaminopro pyl)carbodiimide (EDC) to promote formation of a peptide bond. Ferrich rome A coupled in this way to the silica beads is not washed off in hi gh salt. The isotherms of FeA coupled irreversibly to the beads as a f unction of solution concentration of FeA were measured using a constan t EDC:FeA mole ratio of 50:1 and a constant EDC concentration of 20 mM . There were significant differences between the coupling and adsorpti on isotherms, particularly at low solution concentrations of FeA. Elec trophoretic mobility measurements of the silica beads to which varying amounts of FeA had been covalently bound showed the surface was posit ively charged at all peptide surface concentrations. The results are c onsistent with a model of the surface in which FeA is coupled to a por ous layer of oligomeric aminosilane through a reaction which is strong ly influenced by electrostatic interactions between the surface and re agents. (C) 1995 Academic Press, Inc.