Use of 13C chemical shift surfaces in the study of carbohydrate conformation. Application to cyclomaltooligosaccharides (cyclodextrins) in the solid state and in solution

Abstract

The anomeric carbon chemical shifts of free cyclomaltohexaose, -heptaose, -octaose, -decaose, and -tetradecaose (α-, β-, γ-, ε-, and η-cyclodextrin, respectively), and of α-cyclodextrin inclusion complexes, both in the solid state and in solution, were computed using ab initio 13C chemical shift surfaces for the d-Glc p-α-(1 → 4)- d-Glc p linkage as a function of the glycosidic bond 〈 Φ, Ψ〉 dihedral angles. Chemical shift calculations in the solid state used 〈 Φ, Ψ〉 angle pairs measured from cyclodextrin X-ray structures as input. For estimations in the liquid state two different approaches were employed to account for dynamic averaging. In one, the computed solid-state anomeric carbon chemical shifts for each cyclodextrin d-Glc p monomer were simply averaged to obtain an estimate of the 13C shifts in solution. In the other, chemical shifts for the anomeric carbons were determined by averaging back-calculated 13C shift trajectories derived from a series of 5 ns molecular dynamic simulations for the oligosaccharides with explicit representation of water. Good agreement between calculated and experimental 13C shifts was found in all cases. Furthermore, our results show that the ab initio 13C chemical shift surfaces are sufficiently sensitive to reproduce the small variations observed for the anomeric 13C shifts of the different cyclodextrin d-Glc p units in the solid state with excellent accuracy. The use of chemical shift surfaces as tools in conformational studies of oligosaccharides is discussed.