Physical Characterization of Amylose-Fatty Acid Complexes in Starch Granules and in Solution

Previous differential scanning calorimetry studies of native starch have shown an endotherm at ∼ 100°C on heating, which is attributed to melting of an amylose-lipid complex. In the present work we have shown that this transition is fully reversible (ΔH ≈ 0.4 ± 0.1 cal g⁻¹ on heating and cooling) and displays substantial thermal hysteresis (T_m≈76°C on cooling). On removal of lipid by extraction with methanol the transition is lost, but re-appears on addition of sodium palmitate to de-fatted granules. Closely similar thermal behaviour, including hysteresis, is observed on addition of palmitate to amylose. Further evidence of complex formation between amylose and palmitate is the almost total disappearance of high resolution ¹H n.m.r. signal, indicating loss of conformational mobility by the polysaccharide chain. Formation of the complex is also accompanied by a large reduction in optical activity. Using an established correlation between D -line optical rotation and the dihedral angles between adjacent residues we have compared the observed optical rotation with values calculated for specific ordered structures of amylose. The measured optical rotation values indicate adoption in solution of a novel right-handed V-amylose structure (calculated [α] D =104°; found = 100±5°) rather than the expected left-handed form characterized in the solid state (calculated [α] D =157°). Analysis of optical rotatory dispersion curves shows that in each case the changes in optical rotation on complex formation arise predominantly from changes in the intensity of the known polysaccharide backbone transition at 161 nm.