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 φH (H1−C1−O−C4‘) ≈ 180° and ψH (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.