Glycan Stability and Flexibility: Thermodynamic and Kinetic Characterization of Nonconventional Hydrogen Bonding in Lewis Antigens

Abstract

We provide evidence for CH-based nonconventional hydrogen bonds (H-bonds) for 10 Lewis antigens and two of their rhamnose analogues. We also characterize the thermodynamics and kinetics of the H-bonds in these molecules and present a plausible explanation for the presence of nonconventional H-bonds in Lewis antigens. Using an alternative method to simultaneously fit a series of temperaturedependent fast exchange nuclear magnetic resonance (NMR) spectra, we determined that the H-bonded conformation is favored by ∼1 kcal/mol over the non-H-bonded conformation. Additionally, a comparison of temperature-dependent 13 C linewidths in various Lewis antigens and the two rhamnose analogues reveals H-bonds between the carbonyl oxygen of the N-acetyl group of N-acetylglucosamine and the OH2 group of galactose/fucose. The data presented herein provide insight into the contribution of nonconventional H-bonding to molecular structure and could therefore be used for the rational design of therapeutics.

Dissecting the Conformational Stability of a Glycan Hairpin

“…The largest chemical shift deviation was detected for 9mer-IV . This observation indicated a higher conformational stability for the 9mer-IV analogue, in agreement with the reported conformational stability of Le a vs Le x . Analysis of the MD trajectories indicated a twisted conformation of the two strands with opposite directionality for 9mer-III and 9mer-IV (Figure S12).…”

“…A nonconventional CH···O hydrogen bond between the H-5 of l -Rha (Rha-5) and the O-5 pyranose oxygen of Glc-B stabilizes the turn, forming a 10-membered ring . This weak interaction is complemented by stereoelectronics of the glycosidic bonds (i.e., exoanomeric effect), the van der Waals and hydrophobic interactions between the methyl group of Rha and the hydrophobic face of Glc, and a steric contribution connected to the amide moiety of GlcNAc. , This turn proved suitable to hold two cellulose branches in an ideal parallel orientation resulting in a folded hairpin conformation …”

“…In 9mer-IV , the position of the branches ( l -Rha and d -Glc) is inverted to promote a different orientation of the two stacking strands. The inversion of l -Rha and d -Glc should preserve the close conformation of the turn, as shown for the trisaccharide regio-isomers Le x and Le a …”

Glycan Shape, Motions, and Interactions Explored by NMR Spectroscopy

“…Nonconventional hydrogen bonding has also been proposed in linear oligosaccharides containing the structural element α- d -Gal p -(1 → 4)-α- d -Gal p in which the inter-residue C5′-H5′···O3 interaction makes a favorable contribution of ∼2 kcal·mol –1 based on natural bond orbital calculations . By NMR experiments, the hydrogen bonded conformation of Lewis antigens has been shown to be favored by ∼1 kcal·mol –1 over the one lacking the nonconventional hydrogen bond for the two conformations in rapid exchange . By combining natural carbohydrate motifs with β-(1 → 4)-linked cellulose oligomers as strands with a turn unit in the form of a Lewis-type trisaccharide structure based on nonconventional hydrogen bonding in conjunction with classical hydrogen bonding and hydrophobic interactions between strands it was possible to form glycan hairpin structures having a folded conformation .…”

Advances in glycoside and oligosaccharide synthesis

“…Pure, well-defined glycans are essential tools to advance the glycosciences as they enable molecular investigations into the interactions between glycans and proteins, such as antibodies 11,12 and carbohydrate-active enzymes (CAZy); 13,14 facilitate the creation of well-defined microarrays; 10,15,16 allow for the examination of glycan secondary structure; 17–20 and contribute to the development of vaccines 21–23 and chemical biology tools. 24,25 Synthetic glycans can play many other roles in environmental, material and health research.…”