Glucan Phosphorylase-Catalyzed Enzymatic Synthesis of a New Unnatural Heteroaminopolysaccharide, Glucosamino-2-deoxyglucan, and Its N-Benzylation for pH Responsive Property

Abstract

Based on the fact that the enzymatic approach has been identified as a powerful tool to prepare polysaccharides with well-defined structure, this study investigates the enzymatic synthesis of a new unnatural heteroaminopolysaccharide, that is, α($1\to$)-linked glucosamino-2-deoxyglucan. The desired heteroaminopolysaccharide is obtained by thermostable glucan phosphorylase (from Aquifex aeolicus VF5)-catalyzed enzymatic copolymerization of α-d-glucosamine 1-phosphate/d-glucal as comonomers from maltotriose as a primer in 0.10 m ammonia buffer containing MgCl₂ at 40 °C. The MALDI-TOF MS and ¹H NMR spectrum of the isolated product fully supported the heteroaminopolysaccharide structure. The effects of comonomer feed ratios and reaction times on unit ratios and degrees of polymerization (molecular weights), respectively, are precisely investigated. As a representative derivatization using reactive amino groups in the produced heteroaminopolysaccharide, its N-benzylation is attempted using benzyl bromide in DMSO. The N-benzylated derivative exhibited pH responsive property, where the polymeric molecules assembled by hydrophobic interaction among benzyl groups under the neutral aqueous condition, whereas the assemblies are dissociated by electrostatic repulsion via protonation on nitrogen atoms under the acidic condition. The result indicates the potential of the present heteroaminopolysaccharide as biomedical materials in the practical application field.