Octa-O-propanoyl-β-maltose: crystal structure, acyl stacking, related structures, and conformational analysis

The crystal structure of β-maltose octapropanoate (1) was solved to improve understanding of di-, oligo-, and polysaccharide conformations. The O6 and O6′ atoms are in gg and gt orientations, respectively. Extrapolation of the coordinates of the non-reducing residue and observed linkage bond and torsion angles of 1(τC1′–O4–C4=116.0°,ϕO5′–C1′–O4–C4=77.1°,ψC5–C4–O4–C1′=-157.5°) yields a left-handed helix similar to amylose triacetate I. The ϕ and ψ values of 1 are also similar to those of other crystalline, acylated maltose compounds as well as some hydroxyl-bearing molecules. Acylated maltose moieties are often stabilized by stacking of the carbonyl groups and α-carbons on O3 and O2′ as well as by the exo-anomeric effect. The conformation of 1 is within the 1-kcal/mol contour on a hybrid energy map built with a dielectric constant of 7.5, but corresponds to higher energies on maps made with lower dielectric constants. In one region of ϕ,ψ space, both hydroxyl-bearing and derivatized maltose moieties are found but no inter-residue, intramolecular hydrogen-bonding occurs. In another region, only hydroxyl-bearing molecules crystallize and O2′⋯O3 hydrogen bonds are always found. In agreement with the energy surfaces, amylose helices extrapolated from available linkage geometries were almost all left-handed.

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