The Solution Conformation of Native α-1,6-Mannobiose as Determined by NMR Spectroscopy

The biochemical functions of oligosaccharides with multiple mannose sugars (high-mannose glycans) are utilized in numerous biological processes. For example, they have been shown to assist in protein folding of the nascent amino acid chain during translation. The first step in exploring how these sugars behave in nature is to elucidate their three-dimensional structure and conformation. Magnetic Resonance (NMR) spectroscopy is a powerful instrumental technique uniquely suited to reveal structural details of molecules. As proof of concept, the solution conformation of 5α-cholestan-3β-ol was studied with NMR spectroscopy. Specifically, the 1H- and 13C-spectra as well as advanced 2D experiments (COSY, HSQC, HMBC, and NOESY/ROESY) were utilized to determine the specific structural identities of all NMR peaks, the coupling constants between proton-proton and proton-carbon pairs, and the spatial proximity of protons to each other. The so-established NMR experiments were subsequently applied to α-1,6-mannobiose. Whereas the structure of 5α-cholestan-3β-ol was naturally rigid because of multiple fused rings, the structure of α-1,6-mannobiose was surprisingly rigid as well. This was an unexpected finding because the 1,6-glycosidic linkage has potential for significant flexibility. Consequently, structural rigidity of the α-1,6-mannobiose unit in larger natural oligosaccharides may make them reliable templates for protein folding