Synthesis and X-ray crystallographic investigation of N-(β-D-glycosyl)butanamides derived from GlcNAc and chitobiose as analogs of the conserved chitobiosylasparagine linkage of N-glycoproteins.

The linkage region, GlcNAcβAsn, is conserved in all eukaryotic N-glycoproteins. As a logical extension of a research endeavor aimed at understanding the structural significance of GlcNAc and Asn as the linkage region constituents, the newer analogs GlcNAcβNHBu and (GlcNAcβ(1-4)GlcNAc)alkanamides have been synthesized to assess the influence of aglycon as well as additional GlcNAc on the linkage region. X-ray crystallographic analysis of the GlcNAcβNHBu and (GlcNAcβ(1-4)GlcNAc)βNHBu is described. Comparative analysis of these structures with those of reported models and analogs shows that the deviation in N-glycosidic torsion, ϕN among the GlcNAc alkanamides is negligible (<2°) whereas (GlcNAcβ(1-4)GlcNAc)βNHBu deviates by ∼15° as compared to GlcNAcβNHBu. Under the influence of the molecular packing, the conformation around the C1'-C2' bond deviates from anti to gauche in (GlcNAcβ(1-4)GlcNAcβNHBu. Interestingly, C2-acetamido group in (GlcNAcβ(1-4)GlcNAc)NHBu orients differently as compared to GlcNAc alkanamides and this orientation was found to be almost similar to β-N,N'-diacetylchitobiose trihydrate. The bifurcated anti-parallel pattern involving N-H⋯O and C-H⋯O hydrogen bonds, a hallmark feature of the N-glycoprotein models, GlcNAcβNHAc and GlcNAcβAsn, is absent in both the title alkanamides. This is the first report on the crystal structure analysis of chitobiosyl alkanamide as analog of the N-glycoprotein linkage region, (GlcNAcβ(1-4)GlcNAc)βAsn.