Computational and experimental NMR definition of differences in the conformational behavior of free and lectin-bound glycomimetic aza/carba-lactosides

Versatile synthetic procedures make saccharide derivatives with substitution at various sites available. Because of the particular involvement of galactosides in biorecognition processes, with lectins affecting growth regulation and adhesion, lactose has become a popular starting point for systematic structure variation. Since shape and flexibility are key parameters for ligand binding, we studied the conformational behavior of aza/carba-C/O analogues of lactose (compound 1, aza-C; compound 2, carba-O). Computational calculations and NMR spectroscopy revealed conspicuous differences between 1 and in the extents of accessible conformational area and flexibility. The aza-C derivative also revealed clear shifts in population density relative to lactose, attributable to the exo-anomeric effect. The assumption that the derivatives can act as glycomimetics was substantiated by binding studies with a plant lectin. The aza-C analogue 1 maintained a higher degree of flexibility when bound than the O-glycosidically linked carbadisaccharide 2. The experimental detection of exclusive NOEs characteristic of different conformers of bound 1 [i.e., H1'/H4 (syn-(Phi), H1'/H3 (anti-Psi), and H2'/H4 (anti-(Phi) NOE cross-peaks] argues in favor of the ligand still harboring a certain degree of flexibility when bound to mistletoe lectin (VAA). In contrast, there is unambiguous evidence for binding of the major exo-anomeric conformer of 2 by VAA, as indicated by the presence of the exclusive NOE of H7eq'/H4. Evidently, structural variations between closely related compounds can translate into clear differences in the lectin-bound states of glycomimetics. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)