Relative configuration of micrograms of natural compounds using proton residual chemical shift anisotropy

Abstract

3D molecular structure determination is a challenge for organic compounds or natural products available in minute amounts. Proton/proton and proton/carbon correlations yield the constitution. J couplings and NOEs oftentimes supported by one-bond H-1,C-13 residual dipolar couplings (RDCs) or by C-13 residual chemical shift anisotropies (RCSAs) provide the relative configuration. However, these RDCs or carbon RCSAs rely on 1% natural abundance of C-13 preventing their use for compounds available only in quantities of a few 10's of mu gs. By contrast, H-1 RCSAs provide similar information on spatial orientation of structural moieties within a molecule, while using the abundant H-1 spin. Herein, H-1 RCSAs are accurately measured using constrained aligning gels or liquid crystals and applied to the 3D structural determination of molecules with varying complexities. Even more, deuterated alignment media allow the elucidation of the relative configuration of around 35 mu g of a briarane compound isolated from Briareum asbestinum. Determination of 3D molecular structures remains challenging for natural products or organic compounds available in minute amounts. Here, the authors determine the structure of complex molecules, including few micrograms of briarane B-3 isolated from Briareum asbestinums, through measurement of H-1 residual chemical shift anisotropy.