THE USE OF CHEMICAL-SHIFTS OF HYDROXY PROTONS OF OLIGOSACCHARIDES AS CONFORMATIONAL PROBES FOR NMR-STUDIES IN AQUEOUS-SOLUTION - EVIDENCE FOR PERSISTENT HYDROGEN-BOND INTERACTION IN BRANCHED TRISACCHARIDES

The H-1 NMR chemical shifts, vicinal coupling constants, temperature c oefficients, exchange rates with solvent and NOEs have been measured f or the hydroxy protons of a series of 15 branched trisaccharides in aq ueous solution. These compounds are methyl alpha-D-galactopyranosides substituted at the 3- and 4-positions with either L-fucosyl or D-gluco syl groups. While most of the hydroxy protons in the trisaccharides ha ve chemical shifts Similar to those in the corresponding methyl monosa ccharides (Delta delta less than or equal to +/-0.20 ppm), some hydrox y protons are found to exhibit large upheld shifts. The NMR data toget her with HSEA and MM2 calculations show a correlation between these la rge upheld shifts and the proximity of the hydroxy group to oxygen ato ms. The largest upheld shifts are observed for hydroxy protons which a re in close proximity to ring oxygens O(5). This dependency of chemica l shifts of hydroxy protons on the electronic environment might be use d as a conformational probe to improve the determination of conformati on of oligosaccharides. The NMR data also show that in three branched trisaccharides, alpha-D-Glcp-(1 --> 3)-[alpha-D-Glcp-(1 --> 4)]-alpha- D-Galp-OMe, beta-L-Fucp-(1 --> 3)-[alpha-D-Glcp-(1 --> 4)]-alpha-D-Gal p-OMe and alpha-D-Glcp-(1 --> 3)-[beta-L-Fucp-(1 --> 4)]alpha-D-Galp-O Me, there is a persistent hydrogen bond interaction between the O(2')H of the 3-linked sugar residue and the O(5 '') of the Clinked sugar re sidue. In the three compounds, a large upfield shift relative to the c onstituent methyl monosaccharide is observed for the hydroxy proton O( 2')H involved in hydrogen bonding with the O(5 '') oxygen. Additional information about the conformation could also be obtained from the int er-residue NOEs involving the exchangeable hydroxy protons. These addi tional NOEs are in good agreement with a conformation in which the O(2 ')H proton and the O(5 '') oxygen atoms are involved in a hydrogen ban d interaction.