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

摘要

The crystal structure of beta-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(tau_(C1'-O4-C4)= 116.0°.phi_(O5'-C1'-O4-C4)= 77.1°,psi _(C5-C4-O4-C1')=-157.5°)yields a left-handed helix similar to amylose triacetate I.The phi and psi 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 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.