Structure and supramolecular ordering of chitosan L- and D-aspartates

Abstract

Chitosan (CS) with a viscosity-average molecular weight of 200 kDa and a deacetylation degree of 82 mol%, produced by Bioprogress Ltd. (RF) has been used in this work. Aqueous solutions of enantiomeric salt complexes of CS with L- and D-aspartic acid (AspA) have been obtained at an equimolar CS:AspA ratio, in terms of amino groups. Powders of CS·L-(D-) AspA salts have been isolated from the corresponding solutions by evaporation of water and stored in a desiccator at zero humidity. It has been established that under such conditions a water-soluble salt form of the polymer with lamellar light beige particles 0.05–1.0 mm in size is formed. Using the methods of elemental analysis, IR and NMR spectroscopy, and X-ray diff ractometry, the chemical interaction of CS with L-(D-)AspA in aqueous solution and condensed state has been evaluated, and the chemical structure and supramolecular ordering of these enantiomeric salts have been studied. It has been established that, according to the classifi cation of K. Ogawa et al., the formula unit CS·L-(D-)AspA corresponds to non-hydrated salts of type I, in which water molecules are replaced by acid anions. IR spectroscopy confi rmed the donor--acceptor polymer–acid interaction and revealed a developed system of intermolecular and intramolecular contacts. One- and two-dimensional NMR spectroscopy showed the interaction of pairs of atomic nuclei between H3–H6 of the polymer and 2Hβ or Hα of the acid, H1 or H2 and Hα, H1 and H3–H6, due to the spatial proximity of protons in repeating monomer units, in the “bend” chain segments removed along the chain, and in neighboring macromolecules. For CS·L-AspA, additional resonances have been identifi ed between H2 and H3–H6 of the polymer, for CS·D-AspA these have been between H1 or H2. X-ray diff ractometry has revealed a highly ordered orientation of macrochains and a high crystallinity degree, untypical for CS salts. The salt complex CS·D-AspA, in contrast to CS·L-AspA, is characterized by a smaller amount of crystallization water, a more ordered supramolecular structure, and a more developed system of intermolecular and intramolecular contacts.

Keywords

• chitosan
• l- and d-aspartic acid
• salt complexes
• interaction
• elemental analysis
• ir and nmr spectroscopy
• x-ray diff ractometry