Dilute liquid crystals used to enhance residual dipolar couplings may alter conformational equilibrium in oligosaccharides.
The solution structures of a trisaccharide and a pentasaccharide containing the Lewis(x) motif were determined by two independent approaches using either dipolar cross-relaxation (NOE) or residual dipolar coupling (RDC) data. For the latter, one-bond 13C[bond](1)H RDC enhanced by two different mineral liquid crystals were used alone. Home-written programs were employed firstly for measuring accurately the coupling constants in the direct dimension of non-decoupled HSQC experiments, secondly for transforming each RDC data set into geometrical restraints. In this second program, the complete molecular structure was expressed in a unique frame where the alignment tensor is diagonal. Assuming that the pyranose rings are rigid, their relative orientation is defined by optimizing the glycosidic torsion angles. For the trisaccharide, a good agreement was observed between the results of both approaches (NOE and RDC). In contrast, for the pentasaccharide, strong discrepancies appeared, which seem to result from interactions between the pentasaccharide and the mesogens, affecting conformational equilibrium. This observation is of importance, as it reveals that using simultaneously NOE and RDC can be hazardous as the former represent 99% of the molecules free in solution, whereas the latter correspond to less than 1% of the structure bound to the mesogen.
Berthault
Patrick
P
Laboratoire Commun de R.M.N., DSM/DRECAM/Service de Chimie Moléculaire, URA 331 CNRS, C.E.A./Saclay, F-91191 Gif sur Yvette, France.
Jeannerat
Damien
D
Camerel
Franck
F
Alvarez Salgado
Francisco
F
Boulard
Yves
Y
Gabriel
Jean-Christophe P
JC
Desvaux
Hervé
H
D000465:Algorithms; D002240:Carbohydrate Sequence; D003461:Crystallography; D017666:Deuterium Oxide; D006027:Glycosides / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D009844:Oligosaccharides / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry
D006027:Glycosides; D009844:Oligosaccharides; D012996:Solutions; D017666:Deuterium Oxide