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Title: Octa-O-propanoyl-B-maltose: crystal structure, acyl stacking, related structures and conformational anaylsis

item Johnson, Glenn
item French, Alfred - Al

Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2007
Publication Date: 7/2/2007
Citation: Johnson, G.P., Stevens, E.D., French, A.D. 2007. Octa-O-propanoyl-B-maltose: crystal structure, acyl stacking, related structures and conformational anaylsis. Carbohydrate Research. v. 342(9). p. 1210-1222.

Interpretive Summary: The properties of carbohydrate materials depend on their three-dimensional structures. One especially effective way to study the structure is x-ray crystallography, which furnishes the exact locations and types of the various atoms in the molecule. This molecule is a chemically modified product of degraded starch, and the experimental results furnish new information not only on the properties of starch but on other agricultural carbohydrates such as the cellulose molecules in cotton. The results are also important because they furnish data for comparisons with computer modeling, a still-developing technique with potential to substantially reduce actual laboratory work and guide scientists in new chemical or biological modifications of agricultural materials.

Technical Abstract: The crystal structure of beta-maltose octapropanoate (1) was solved to increase knowledge of the influences on conformations of di-, oligo- and polysaccharides. The O6 and O6' atoms are in gg and gt conformations, 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 deg., psi (C5–C4–O4–C1') = –157.5 deg.) yields a left-handed helix with –4.63 residues per turn and an advance per residue of 3.99 A, similar to amylose triacetate I. The phi and psi values of 1 are also similar to those of the other crystalline, acylated maltose compounds as well as some hydroxylated molecules. Apparently, acylated maltose moieties are often stabilized by stacking of the carbonyl groups on O3 and O2' as well as the exo-anomeric effect. The conformation of 1 is near the global minima on HF/6-31G(d) energy surfaces for several maltose analogs and an MM3 surface for permethylated maltose. 1 is also within the 1-kcal/mol contour on a hybrid energy map for maltose itself built with a dielectric constant of 7.5. On such maps made with lower dielectric constants, 1 corresponds to higher energy values. There is one region of phi,psi space where both hydroxylated and substituted maltose molecules are found but no inter-residue, intramolecular hydrogen bonding occurs. In another region, only hydroxylated molecules crystallize and O2'- -O3 hydrogen bonds are always found. Cyclodextrins are considered in a separate article.