Conformational Distribution Function of a Disaccharide in a Liquid Crystalline Phase Determined Using NMR Spectroscopy

Abstract

A new approach is proposed for analysis of NMR parameters in general, and residual dipolar couplings in particular. The method, which enables construction of the conformational distribution function, is applied to a disaccharide dissolved in a dilute liquid crystal. Our approach rests on two models that have been frequently used for interpretations of dipolar couplings in liquid crystals: (i) the additive potential model (AP) and (ii) the maximum entropy method (ME). These models suffer, however, from serious limitations: the AP model requires an a priori knowledge of the functional form of the torsional potential, while the ME approach gives the flattest possible distribution, which results in an incorrect description of systems with low orientational order. The procedure presented herein (APME) is valid in the low-order limit. The intraresidue dipolar couplings were used to determine the orientational order, while the conformational distribution function is constructed from the interresidue dipolar- and J couplings, together with NOEs. Very good agreement between experimental and calculated NMR parameters was observed. An attempt to analyze the experimental data using a single molecular conformation resulted in significantly larger errors. This study demonstrates that the APME method can be used as a tool in structure determination of flexible molecules in dilute liquid crystals.