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

The crystal structure of P-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 (iota Cl-04-c4,, (C4) 116.0 degrees, phi(O5'-Cl-O4-C4) 77.1., psi(C5-C4-O4-Cl)' -157.5 degrees) yields a left-handed helix similar to amylose triacetate 1. The 0 and values of I 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 alpha-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 phi,psi 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. (C) 2007 Elsevier Ltd. All rights reserved.