NMR spectroscopic studies of chitin oligomers - Resolution of individual residues and characterization of minor amide cis conformations

Chitin is the second most abundant biopolymer in nature after cellulose and is composed of N-acetylglucosamine (GlcNAc) connected via β(1 → 4)-glycosidic bonds. Despite its prominence in nature and diverse roles in pharmaceutical and food technological applications, there is still a need to develop methods to study structure and function of chitin and its corresponding oligomers. Efforts have been made to analyse chitin oligomers by NMR spectroscopy, but spectral overlap has prevented any differentiation between the interior residues. In this study, chitin oligomers up to hexaose with natural abundance of ¹⁵N were analysed with NMR spectroscopy in aqueous solution. Different ¹H,¹⁵N-HSQC pulse sequences were evaluated to obtain the best resolution and sensitivity. Interior residues were resolved in the ¹⁵N dimension and detailed chemical shifts of amide proton and nitrogen are reported for the first time. Additionally, all oligomers were analysed for the presence of the amide cis form and its corresponding chemical shifts were assigned. This study exploits the information that can be obtained from chitin oligomers with NMR spectroscopy and depicts methods for detailed analysis of glycans containing oligomers of N-acetylglucosamine.