Counterion Exchange in Peptide-Complexed Core–Shell\nMicrogels

Abstract

The\ncomplexation of polyvalent macroions with oppositely charged polyelectrolyte\nmicrogels can lead to core–shell structures. The shell is believed\nto be highly deswollen with a high concentration of counter-macroions.\nThe core is believed to be relatively free of macroions but under\na uniform compressive stress due to the deswollen shell. We use cryo-scanning\nelectron microscopy (SEM) with X-ray microanalysis to confirm this\nunderstanding. We study poly­(acrylic acid) (PAA) microgels which form\na core–shell structure when complexed with a small cationic\nantimicrobial peptide (L5). We follow the spatial distribution of\npolymer, water, Na counterions, and peptide based on the characteristic\nX-ray intensities of C, O, Na, and N, respectively. Frozen-hydrated\nmicrogel suspensions include buffers of known composition from which\ncalibration curves can be generated and used to quantify both the\nmicrogel water and sodium concentrations, the latter with a minimum\nquantifiable concentration less than 0.048 M. We find that as-synthesized\nPAA microgels are enriched in Na relative to the surrounding buffer\nas anticipated from established ideas of counterion shielding of electrostatic\ncharge. The shell in L5-complexed microgels is depleted in Na and\nenriched in peptide and contains relatively little water. Our measurements\nfurthermore show that shell/core interface is diffuse over a length\nscale of a few micrometers. Within the limits of detection, the core\nNa concentration is the same as that in as-synthesized microgels,\nand the core is free of peptide. The core has a slightly lower water\nconcentration than as-synthesized controls, consistent with the hypothesis\nthat the core is under compression from the shell.