Inclusion complexes of V-amylose with undecanoic acid and dodecanol at atomic resolution: X-ray structures with cycloamylose containing 26 d-glucoses (cyclohexaicosaose) as host Original Research Article

Crystal structures are reported of cycloamylose containing 26 d-glucose residues (CA26, cyclohexaicosaose, C156H260O130) in complexes with undecanoic acid (CA26·2C10H21COOH·34.95H2O, orthorhombic P212121, one CA26 and two bound undecanoic acids F1 and F2 in the asymmetric unit, resolution 0.95 Å) and with dodecanol ((CA26)0.5·C12H25OH·32.0H2O, monoclinic C2, half a CA26 binding one dodecanol, A, in the asymmetric unit, resolution 1.0 Å). The macrocycle of CA26 is folded like the figure `8ʹ into two 10 d-glucoses long left-handed V-amylose helices forming ∼5 Å wide V-channels that are occupied by undecanoic acid (F1, F2) or dodecanol (A) as guest molecules. The functional head groups of the guests near the O(6) ends of the V-channels are hydrogen bonded with d-glucose O(6)n–H; the aliphatic termini beyond C(9) protrude from the O(2), O(3) ends. Parts of the aliphatic chains enclosed in the V-channels are all-trans except for one torsion angle each (∼130°) in undecanoic acid molecules F1 and F2. There are several (guest)C–H⋯O hydrogen bonds to O(4) and O(6) of CA26 in both complexes, and H⋯H van der Waals interactions with d-glucose C(3)–H and C(5)–H dominate. C(5)–H determine the position of the aliphatic chains of undecanoic acid F1 and of dodecanol A in contrast to F2 where both C(3)–H and C(5)–H contribute equally, probably because the V-channel is narrower than in F1 and in dodecanol. Complexes of polymeric V-amylose with fatty acids and alcohols studied by X-ray fiber diffraction could not provide the here described high resolution.