Mobility of lipid in complexes of amylose–fatty acids by deuterium and C solid state NMR

Palmitic and lauric acid complexes with amylose were studied by solid state methods: ¹³C CP/MAS NMR, deuterium NMR, X-ray powder diffraction and differential scanning calorimetry (DSC). The crystalline amylose complexes were found to be in a V-type sixfold single chain helix. The melting points of the complexes were over 100°C, at least 40–50°C higher than the melting points of the free fatty acids. CP/MAS ¹³C NMR spectra revealed two resonance peaks at 33.6 and 32.4 ppm for the palmitic acid, which were assigned as free and complexed fatty acid, respectively. A single resonance peak at 32.4 ppm was found for the lauric acid and assigned to the complex. The chemical shift of 32.4 ppm for the complexed fatty acids suggests a combined trans and gauche conformation for the fatty acid chain in the complex. T₁ relaxation measurements on the two palmitic acid resonances show different behavior: a very slow relaxation for the 33.6 ppm and a much faster relaxation (1.2 s) for the 32.4 ppm resonances. The latter was similar to the relaxation of the single resonance of the lauric acid (1.1 s). Temperature dependent deuterium spectra of the amylose–lauric acid and amylose–palmitic acid complexes suggest a complete complexation for the amylose–lauric acid, whereas the amylose–palmitic acid complex is partially disassociated by the thermal treatment. Based on the overall data, a partially disordered model is proposed: an imperfect helix with the fatty acid partly inside and partly out, depending on crystallization conditions and the necessity of placing the carboxyl head outside the V-helix.