Acetyl Group Migration in Xylan and Glucan Model Compounds as Studied by Experimental and Computational Methods

Abstract

It was recently demonstrated by us that acetyl groups in oligosaccharides can migrate not only within one saccharide unit but also between two different saccharide units. Kinetics of this phenomenon were previously investigated in both mannan model compounds and a naturally occurring polysaccharide. In addition to mannans, there are also several other naturally acetylated polysaccharides, such as xyloglucans and xylans. Both xyloglucans and xylans are some of the most common acetylated polysaccharides in nature, displaying important roles in the plant cells. Considering the various biological roles of natural polysaccharides, it could be hypothesized that the intramolecular migration of acetyl groups might also be associated with regulation of the biological activity of polysaccharides in nature. Consequently, a better understanding of the overall migration phenomenon across the glycosidic bonds could help to understand the potential role of such migrations in the context of the biological activity of polysaccharides. Here, we present a detailed investigation on acetyl group migration in the synthesized xylan and glucan trisaccharide model compounds by a combination of experimental and computational methods, showing that the migration between the saccharide units proceeds from a secondary hydroxyl group of one saccharide unit toward a primary hydroxyl group of the other unit.

Mechanism of Acyl Group Migration in Carbohydrates

“…The same mechanism was applied to acetyl group migration over the glycosidic bond in both glycan and mannan trisaccharides, with good results considering the challenge of calculating the pK a s and the conformations of such flexible trisaccharides. [17] The mechanism in aqueous media differs from those in other solvents, where the possibility for migration under acidic or neutral conditions becomes more viable. One such example was studied by Crout and co-workers.…”

“…Most of the earlier studies on acyl group migration have been performed in monosaccharides [2] . Recently, however, also investigations on acyl group migration in oligo‐ and polysaccharides have been reported [17,22] . These studies have demonstrated that an acetyl group may also migrate across the glycosidic linkage, from a secondary to a primary hydroxyl group in both oligosaccharides (Scheme 2) and a native polysaccharide.…”

“…[2] Recently, however, also investigations on acyl group migration in oligo-and polysaccharides have been reported. [17,22] These studies have demonstrated that an acetyl group may also migrate across the glycosidic linkage, from a secondary to a primary hydroxyl group in both oligosaccharides (Scheme 2) and a native polysaccharide. These studies were carried out on hemicellulose model compounds, where the glycosidic linkage is β-(1!4).…”

“…Energies corresponding to the observed rate constants were obtained by considering the pK a s when calculating the energies of the acetyl group migration in several structurally different carbohydrate molecules. The same mechanism was applied to acetyl group migration over the glycosidic bond in both glycan and mannan trisaccharides, with good results considering the challenge of calculating the pK a s and the conformations of such flexible trisaccharides [17] …”

“…For example, until recently, the deprotonation step in the base catalyzed mechanism has not been included in the computational models. In this concept article, based on both literature by others and our recent own work in the subject, [15,17] we address the current understanding of the acetyl group migration mechanism, in carbohydrates in particular, and the acyl group migration process on more general level.…”