Correlation of 13C chemical shifts with torsional angles from high-resolution, 13C-C.P.-M.A.S. N.M.R. studies of crystalline cyclomalto-oligosaccharide complexes, and their relation to the structures of the starch polymorphs

Abstract

The chemical shifts and multiplicities of the resonances in high-resolution, 13C-c.p.-m.a.s. n.m.r. spectra of cyclomalto-oligosaccharide inclusion complexes are characteristic of the crystalline structure of the different complexes. In particular, the 13C chemical shifts of C-1 and C-4 correlate with the torsion angles φ' 2and φ 1 respectively (related to Ψ and φ, respectively, in an alternative terminology), which describe the orientation of the D-glucosyl residues about the α-D-(1→ 4) glycosidic linkage. The 13C chemical shift of C-6 correlates with the torsion angle x, which describes the orientation of O-6 about the exocyclic, C-5-C-6 bond. The cyclomalto-oligosaccharide inclusion complexes are good models for the interpretation of the characteristic chemical shifts and multiplicities previously observed in the 13C-c.p.-m.a.s. n.m.r. spectra of the natural starch polymorphs. From these chemical-shift correlations, values for the torsion angles φ' 2, φ 1 and X are predicted for starches that crystallize as “A” and “B” structures. These predicted values are in agreement with the limited data currently available from X-ray fiber diffraction studies.