Interresidue linkages

Fig. 11. Interresidue linkages in carbohydrates chains (illustrated for some glucose dimers). Linkage conformation is defined by the dihedral angles 4>, ij/) or (4>, j/, o>). The intramolecular hydrogen bonds are also indicated.

Determination of saccharide sequences and interresidue linkage positions ... 

A GC/MS method, commonly called linkage analysis or methylation analysis, has also proven to be a successful approach to determining interresidue linkage and branch points [14]. It involves complete alkylation of all free hydroxyl groups. The fully methylated oligosaccharides are hydrolyzed into monomers, which are further reduced and acetylated to provide partially methylated alditol acetates (PAMAs). Acetylation takes place at those positions that were newly exposed by hydrolysis and reduction. The GC retention times and fragmentation patterns can reveal the mode of attachment in the native sugar. 

Monosaccharides, di- and oligosaccharides, and polysaccharides have cumulative conformational descriptors. Because polysaccharides contain as subunits the monomers and dimers, the information on the subunits is usually of interest in the description of the larger molecules as well. However, once the issue of the ring conformation can be regarded as settled, the variables of primary interest for a disaccharide become the torsion angles about the bonds that form the interresidue linkages. 

One reason for studying the conformation of the disaccharide maltose is that it contains the same interresidue linkage as found in amylose and amylopectin, the polymers of starch. Therefore, conformations of the maltosyl linkages found in maltotriose or panose would certainly be of interest in the comparison with a calculated energy surface. It would be obvious to include a-maltose, -maltose, and methyl- -maltoside, molecules that differ only at the anomeric center, which is as distant as possible from the disaccharide linkage and therefore unlikely to affect the linkage conformation very much. 

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