Evidence for two populated conformations for the dimeric Le(x) and Le(a)Le(x) tumor-associated carbohydrate antigens.

The conformational behavior of tumor-associated carbohydrate antigens (TACAs) dimLe(x) and Le(a)Le(x) was studied using a combination of NMR experiments and molecular dynamics simulations. It is shown that within the hexasaccharides, the Le(x) and Le(a) branched trisaccharide fragments adopt the rigid "stacked" conformation known for the isolated trisaccharide antigens. In contrast, the β-D-GlcNAc-(1→3)-D-Gal glycosidic bond that connects the two Le(x) trisaccharides in dimLe(x), and the Le(a) trisaccharide to the Le(x) trisaccharide in Le(a)Le(x), was found to be very flexible in both hexasaccharides.

Stochastic searches and NMR experiments on four Lewis A analogues: NMR experiments support some flexibility around the fucosidic bond.

We have compared the conformational behavior of three Le(a) analogues with that of Le(a) using stochastic searches (MOE2005) and selective ROESY experiments. In the analogues either or both the β-d-Gal and α-l-Fuc residues were replaced by β-d-Glc and α-l-Rha units, respectively. All compounds showed similar behavior and even though four conformational families were identified, the calculations and NMR experiments support that the 'stacked conformation' known for Le(a) is predominant for all analogues.

How the substituent at O-3 of N-acetylglucosamine impacts glycosylation at O-4: a comparative study.

An assessment of the relative reactivities of the 4-OH of N-acetylglucosamine acceptors bearing simple protecting groups, beta-linked or alpha-linked D or L sugars at O-3 is presented, using a per-O-acetylated alpha-D-glucosyl trichloroacetimidate donor under activation by BF(3) x OEt(2). The presence of either an acyl or carbonate protecting group at O-3 did not impact the reactivity at O-4 with all glycosylations proceeding successfully. On the other hand, the presence of peracetylated sugars at O-3 of N-acetylglucosamine acceptors did impact the reactivity of the 4-OH.

The flexibility of the Le(a)Le(x) tumor associated antigen central fragment studied by systematic and stochastic searches as well as dynamic simulations.

The solution conformational behavior of the Tumor-Associated Carbohydrate Antigen Le(a)Le(x) central fragment: methyl alpha-L-fucopyranosyl-(1-->4)-2-acetamido-2-deoxy-beta-D-glucopyranosyl-(1-->3)-beta-D-galactopyranoside was studied using three computational methods: a rigid systematic search as implemented in Sybyl, a stochastic search as implemented in MOE2004, and dynamics simulations using the SANDER module of AMBER9. Our results illustrate the complementarity of these methods to identify energetically relevant conformations and flexible linkages. In particular, the beta-GlcNAc-(1-->3)-Gal linkage was shown to be extremely flexible adopting a wide range of orientations around two energy minima.