Counterion Exchange in Peptide-Complexed Core-Shell Microgels.

Area of Science:

• Materials Science
• Polymer Chemistry
• Biophysics

Background:

• Complexation of polyvalent macroions and polyelectrolyte microgels can form core-shell structures.
• The shell is expected to be deswollen with high counter-macroion concentration, and the core to be compressed.

Purpose of the Study:

• To confirm the core-shell structure hypothesis using cryo-scanning electron microscopy (SEM) and X-ray microanalysis.
• To investigate the spatial distribution of polymer, water, sodium (Na) counterions, and peptide (L5) within poly(acrylic acid) (PAA) microgels.

Main Methods:

• Utilized cryo-scanning electron microscopy (SEM) coupled with X-ray microanalysis.
• Analyzed characteristic X-ray intensities of Carbon (C), Oxygen (O), Sodium (Na), and Nitrogen (N) to map element distribution.
• Quantified microgel water and sodium concentrations using calibration curves from known buffer compositions.

Main Results:

• As-synthesized PAA microgels showed Na enrichment, consistent with counterion shielding.
• L5-complexed microgels exhibited a shell depleted in Na, enriched in peptide, and with low water content.
• The core remained Na-rich (similar to controls), peptide-free, and slightly water-depleted, indicating compression.

Conclusions:

• Cryo-SEM with X-ray microanalysis validates the core-shell model for PAA-L5 microgel complexes.
• The shell's deswollen state and peptide enrichment, along with the core's peptide absence and compression, are confirmed.
• The diffuse nature of the shell/core interface was observed over a few micrometers.