Atomic force microscopy of gastric mucin and chitosan mucoadhesive systems

Atomic force microscopy has been utilized to probe, at a molecular level, t he interaction between purified pig gastric mucin (PGM) and a mucoadhesive cationic polymer, chitosan (sea cure 210 +), with a low degree (approx. 11 %) of acetylation. Images were produced detailing the structures of both PG M and chitosan in 0.1 M acetate buffer (pH 4.5), followed by the complex of the two structures in the same buffer. PGM in 0.1 M acetate buffer reveale d long linear filamentous structures, consistent with earlier electron micr oscopy and scanning tunnelling micoscopy studies. The chitosan molecules al so adopted a linear conformation in the same buffer, although with a smalle r average length and diameter. They appeared to adopt a stiff-coil conforma tion consistent with earlier hydrodynamic measurements. The complexes forme d after mixing PGM and chitosan together revealed large aggregates. In 0.1M ionic strength buffer they were of the order of 0.7 mu m in diameter, cons istent with previous electron microscopy studies. The effect of ionic stren gth of the buffer on the structure of the complex was also studied and, tog ether with molecular hydrodynamic data, demonstrates that the interaction i s principally electrostatic in nature.