Exploring the Analytical Potential of NMR Spectroscopy in Chiral Anisotropic Media for the Study of the Natural Abundance Deuterium Distribution in Organic Molecules

Abstract

The deuterium/hydrogen (D/H)(i) ratio measurement by quantitative (2)H NMR spectroscopy is a method of choice for the analysis of kinetic isotopic effects associated with enzyme-catalyzed reactions during a biosynthetic pathway. However, the efficiency of the current isotropic (2)H-[(1)H] NMR can be limited by the rather small chemical shift dispersion of deuterium nuclei. In addition, this method does not allow the enantiotopic deuterons in prochiral molecules to be spectrally discriminated, hence precluding the quantification of isotopic fractionation on methylene prostereogenic sites. In this work, we explore another analytical strategy able to circumvent these disadvantages. This approach is based on the use of natural abundance (2)H 2D NMR experiments on solutes embedded in polypeptidic, chiral liquid crystalline solvent. Thus, we show that NMR in these oriented phases is a powerful way to separate deuterium signals on the basis of the quadrupolar interactions, providing a promising alternative to overcrowded (2)H NMR spectra obtained in liquid state. To illustrate our purpose, we have experimentally investigated the case of 1,1'-bis(phenylthio)hexane derived by cleavage from methyl linoleate of safflower. The (2)H NMR results in chiral liquid crystals are presented and discussed. We show, for the first time, that (D/H)(pro-R) and (D/H)(pro-S) can be measured at the same methylene position of a fatty acid chain.