Probing the Glycosidic Linkage:  UV and IR Ion-Dip Spectroscopy of a Lactoside

The β(1→4) glycosidic linkage found in lactose is a prevalent structural motif in many carbohydrates and glycoconjugates. Using UV and IR ion-dip spectroscopies to probe benzyl lactoside isolated in the gas phase, we find that the disaccharide unit adopts only a single, rigid structure. Its fully resolved infrared ion-dip spectrum is in excellent agreement with that of the global minimum structure computed ab initio. This has glycosidic torsion angles of φ_H (H1−C1−O−C4‘) ≈ 180° and ψ_H (C1−O−C4‘−H4‘) ≈ 0° which correspond to a rotation of ∼150° about the glycosidic bond compared to the accepted solution-phase conformation. We discuss the biological implications of this discovery and the generality of the strategies employed in making it.