Some chemical and analytical aspects of polysaccharide modifications. IV. Electron spin resonance studies of nitroxide-labelled chitin and chitosan derivatives

Abstract

Nitroxide spin labels have been attached to chitin, chitosan, and their derivatives in order to investigate the solution and gel characteristics of these polymers. Thus, chitin was nonselectively acylated using the chloroacetamide label 3 to afford. the corresponding nitroxide derivative 4. Selective and nonselective labelling of chitosan was accomplished either via reductive alkylation using the keto label 5 and sodium cyanoborohydride or via a chitosan sulfate intermediate using labels 3 or 5. Two branched water soluble chitosan derivatives, carrying C-6 oxidized, terminal galactose side chains were selectively labelled by reductive animation using the amine label 13 to yield derivatives 14 and 15. Motional correlation values of the covalently-linked nitroxide moieties were shown to reflect the nature of the linkage between spin label and polymer, as well as structural differences between the branch residues of derivatives 14 and 15. The spin-probe – spin-label method was employed to demonstrate structural heterogeneities for the chitosan derivatives 7 and 15.

Spin-labeled carbohydrates

“…63 These results can be considered to depend on microscopic changes in the gel structure63 as well as on changes from a threefold double helix of the solid state to larger aggregates in the gel state arising from inter helix packing and formation of bundles.49,71 Some interesting ESR results were obtained for the spin-labeled chitin 214, chitosan 207 and 215, and their water-soluble analogues 204, 205, and 208. 54,58 The rc value for the chitin derivative 214 (Table V) was considerably smaller than those obtained for the chitosan derivatives. This result was attributed to the fact that chitin was labeled under heterogeneous conditions in which the ds was lower (0.04), but the label was attached mostly to the surface of the biopolymer.58 Consequently this surface labeling allows more rotational mobility for the spin label while the polymer motion (tc > 100 ns) would make no contribution on the ESR time scale.…”

“…Studies with gels of compound 207 revealed a complex ESR spectrum that seemed to indicate two nitroxyl populations.54,58 When 207 was subjected to a watermethanol-water solvent cycle over a 2-week period, the ESR changed to a simple three-line spectrum. 58 In this case the mobile nitroxyl population completely predominated, indicating a structural reorganization allowing more spin label to have access to the solvent.…”

“…In a number of instances the ESR spectra of labeled polysaccharides were complex and seemed to indicate the presence of two nitroxyl populations, one more mobile and probably located at the exterior surface, the other less mobile and located in interior pockets or cores, resulting from folding and/or aggregation of polymer chains. 55,58 Spin labeling has been used to study the rheological properties of polysaccharides. This technique is useful for studying conformational changes of these biopolymers in solution, affecting properties such as viscosity and gel-sol transitions.…”

“…However, in the case of the chitosan analogues 207, 215, 204, and 205, the overall tumbling rate includes the motion of the large polymer chain, within either a gel matrix or solution aggregate, and so the observed rc values are larger. 58 Compound 207 is less mobile than 215 because the C-6 position of 215 enjoys more motional freedom than the 2-amino function of 207. Also, the nitroxyl group of 215 is freer to rotate because of the spacer of three atoms separating it from the polymer chain.…”

“…Also, the nitroxyl group of 215 is freer to rotate because of the spacer of three atoms separating it from the polymer chain. 58 Compound 205 has a lower tc value (more mobile) than 204, and this result was explained by their different structures in solution.58 Compound 205 has a 1-*-6 glycosidic linkage in the side chain which causes it to be more extended and less sterically restricted, allowing greater motional freedom for the spin label.…”