Further justification for the exo-anomeric effect. Conformational analysis based on nuclear magnetic resonance spectroscopy of oligosaccharides

Abstract

. Can. J. Chem. 60, 44 (1982). Hard-sphere (HS) calculations predict three conformers for the branched B human blood group antigenic determinant (arFuc(l+ 2)[c(~Gal(l+ 3 ) I p~G a l ) and two conformers for the linear tetrasaccharide ( a~R h a ( l + 2 ) a~R h a ( l + 3)arRha(l+ 3)-p~G l c N A c ) , which constitutes a part of the repeating unit of the Shigella jexnrri 0-antigen, which differ in conformational energy by less than 0.7 and 0.2 kcal/mol, respectively. However, a detailed 'H nmr study of specific interunit deshielding effects, quantitative treatment of spin-lattice relaxation times. and nuclear Overhauser enhancements requires that only one of the conformers thus predicted be, in fact, conformationally preferred in solution. These conformers are those predicted by hard-sphere e.uoanomeric (HSEA) calculations. [Traduit par le journal] Introduction We have recently published evidence (1) that the addition of the torsion-angle dependent exo-anomeric effect to the ground state energy of an oligosaccharide allowed the conformational preferences about the glycosidic linkages to be assessed by hard-sphere molecular modelling in good agreement with physical evidence based on nmr spectroscopy. The evidence was based mainly on specific interunit deshieldings of protons and interunit nuclear Overhauser enhancements (nOe's) (1, 2). Since then, we have gained more ready access to a

The conformational properties of the gangliosides GM2 and GM1 based on 1H and,13C nuclear magnetic resonance studies

“…In addition, we report the first detailed 'H nmr study of the Gh,l-oligosaccharide (9) including 'H nuclear Overhauser enhancement experiments in the difference mode (7) on this and related compounds that firmly establish its conformational preference in aqueous solution. The results obtained are shown to be in agreement with the molecular model for GMl predicted by hard-sphere, exo-anomeric (HSEA) calculations (8).…”

“…In this communication, we report a reinvestigation of the "C nmr spectra of GM1 (8) and the GMl-oligosaccharide (9) and extend these studies to include the I3C nmr spectrum of the GM,-oligosaccharide (7). In addition, we report the first detailed 'H nmr study of the Gh,l-oligosaccharide (9) including 'H nuclear Overhauser enhancement experiments in the difference mode (7) on this and related compounds that firmly establish its conformational preference in aqueous solution.…”

“…HSEA calculations were conducted in the usual manner (8) to estimate the conformational preference for the oligosaccharide portion of asialo-GMl. Since all the interactions are between only two sugar units about each of the three intersugar glycosidic bonds, the calculations were first made for the three constituent disaccharide units, namely, PDGalb(l+4)P~Glc" (I), P D G~~N A c ' (~+~) P D G~~~ (2), and PDGald(l+3)PDGalNAcC (3).3 As expected, the results did not change when the calculations were made using the intact tetrasaccharide.…”

“…These compounds are related to the asialoforms of the gangliosides GM, (6) and GM, (8), respectively. Following the trivial nomenclature introduced by Sillerud and Yu (I), these haptens are referred to as ASGM,-Hp (4) and ASGM,-Hp (5). '…”

“…We would only suggest that the assignments for C-lb and C-1' be interchanged. Since we have high confidence in the assignments for the hapten 5, these are compared in Table 5 to those for GMIOS (9) and GMl (8).…”