Hydroxy Protons in Structural Analysis of Carbohydrates by NMR Spectroscopy and Computational Methods

“Hydroxy Protons in Structural Analysis of Carbohydrates by NMR Spectroscopy and Computational Methods” S. Bekiroglu, Doctoral thesis, 2003. ISBN 91-576-6442-0 This thesis describes the use of hydroxy protons in structural analysis of carbohydrates in aqueous solution by NMR spectroscopy. For aqueous solutions of carbohydrates, using H2O as the solvent of choice instead of D2O makes it possible to observe exchangeable protons provided that the proton exchange with bulk water is slow enough. Thus, additional data from exchangeable protons can be acquired in terms of chemical shifts, vicinal coupling constants, temperature coefficients, exchange rates, and NOEs. The application of the method on the Lewis b, X, and Y oligosaccharides supplied further information about the rigidities of the molecules. Regarding the chemical shift differences, extent of interaction with bulk water molecules and being located around amphiphilic regions were anticipated to play role on magnetic shieldings of hydroxy protons (Articles I-II). The hydrogen bonds between O(2)H and O(3)H groups on adjacent glucose units in CDs were proved to exist in water solution (Article III), as they had been reported in solid state and DMSO solutions. A weak and transient interaction was also observed between O(2')H and O(3)H in maltose. Using hydroxy protons, the study of intermolecular interactions on the cyclodextrin complexes proved to be useful in providing structural information as chemical shift, temperature coefficient and line-shape of the hydroxy proton signals. The intermolecular interactions between carbohydrates and nucleotides as revealed by IR studies could not be detected by NMR spectroscopy (Article IV). However, a drastic improvement in the intensity and line-shape of the hydroxy signals from saccharides were encountered upon addition of small amount of purine nucleos(t)ides and nucleobases. If the reason for the observed upfield and downfield shifts (positive and negative ∆δ values) is contemplated, hydration turns out to be the keyword (Article V). When the hydration of a hydroxy proton is hampered by either interactions with acetal oxygens or structural formations (steric effects or perturbed water interactions in amphiphilic regions), the chemical shift of that proton reads an upfield-shifted value in comparison with the hydroxy proton in the corresponding monomeric unit. Likewise, provided that the hydration state is kept the same, a hydroxy proton becomes deshielded when it forms hydrogen bond interaction with another hydroxy group.