Definition of Angular Dependence of 1H-15N Coupling Constants in Amino Sugars

点击翻译按钮获取中文摘要

The one-dimensional (1D) 1H spectra and 1D 1H-15N Carr-Purcell-Meiboom-Gill heteronuclear single quantum, multiple bond correlation (CPMG-HSQMBC) NMR spectra of the subject amino sugars (with 15N at natural abundance) have been analyzed completely, and the resulting 3JHCCN coupling constants correlated with molecular geometry. These amino sugars comprise those that are commonly present in bacterial polysaccharides, including D-glucosamine, D-mannosamine, and D-galactosamine, in this study as their N-acetyl and hydrochloride derivatives. The geometries of the alpha and beta pyranose anomers of N-acetyl-D-glucosamine, N-acetyl-D-mannosamine, and N-acetyl-D-galactosamine have been defined by molecular dynamics (MD) calculations with energy minimization by molecular mechanics (MM), and implicit solvent. This molecular modeling study yielded values of the vicinal nitrogen-hydrogen dihedral angles (phi) that have been correlated with the measured values of the 3JHCCN coupling constant. Gauche arrangements of the hydrogen and nitrogen-15 nuclei in the common N-acetyl amino sugars were found by MD/MM to have phi values in the range 52-66, which corresponded to 3JHCCN values of 0.8-2.5 Hz. Trans orientations of the hydrogen and nitrogen-15 nuclei displayed phi values in the range 175-178 in MD/MM, which correlated with larger 3JHCCN values in the range 3.2-4.0 Hz. However, if the coupling involves the anomeric proton H-1, then the magnitude of the trans coupling constant is reduced by the electronegativity of the pyranose ring oxygen to 2.7-2.8 Hz. Thus, it appears that the magnitude of the 3JHCCN coupling constant could be used to distinguish gauche and trans orientations of vicinal nitrogen-15proton pairs in amino saccharides, thus offering the possibility of defining the stereochemical configuration (axial or equatorial) of the nitrogen atom in unknown structures. Although these common amino sugar models contain only limited ranges of 15N-1H dihedral angles, the results for their 3JHCCN coupling constants are in general agreement with those measured for the organic-soluble amino sugar derivatives, from which we previously formulated a Karplus-type equation: 3JHCCN = 3.1 cos2 phi - 0.6 cos phi + 0.4