Structural Investigation of Amylose Complexes with Small Ligands: Inter- or Intra-helical Associations?

Highly crystalline amylose complexes with menthone (1) and linalool (2) were analysed by wide-angle X-ray diffraction and solid-state nuclear magnetic resonance (NMR). The complexes, after partial water desorption in a controlled atmosphere (aw=0.75), displayed a typical V-isopropanol structure, showing the presence of ligand inside or between the helices in the crystalline domains. Sequential washing of the powdered complexes with ethanol before and after desorption permitted probing the intra- and inter-helical inclusions. High resolution magic angle spinning (HRMAS) recordings were used to compare the chemical shifts of free and bound aroma which allowed a proposal that some hydrogen bonding is involved in the amylose complexing. Moreover, it showed that free aroma was completely removed by ethanol washing. Using cross polarization magic angle spinning (CPMAS) and X-ray scattering experiments, it was demonstrated that the V-isopropanol type was retained for linalool whatever the treatment used. On the contrary, the measurement shifts toward the V-6I amylose hydrate (V-h) type for menthone after ethanol washing before the desorption step, reflecting the disappearance of inter-helical associations between menthone and amylose. The stability of the complex prepared with linalool shows that this ligand is more strongly associated to amylose helices. The discrepancies observed in the chemical shifts attributed to carbons C1 and C4 in CPMAS spectra of V-isopropanol and V-h forms could be attributed either to a deformation of the single helix (with possible inclusion of the ligand inside) or to the presence of the ligand between helices (only water molecules are present in the V-h form).