Structural studies of two capsaicinoids: dihydrocapsaicin and nonivamide. 13C and 15N MAS NMR supported by genetic algorithm and GIAO DFT calculations

Abstract

Capsaicinoids are alkaloid type capsaicin analogs with prospective pharmacological activity. However their solid state conformations have not been studied yet. As part of the study, cross polarization (CP) magic angle spinning (MAS) solid state ¹³C and ¹⁵N NMR spectra of dihydrocapsaicin (DHCAP) and nonivamide (NVA) were recorded. Solid state chemical shifts differ from their solution counterparts; remarkable differences occur for carbons C2′, C6′ and C7′ linked to C1′ in DHCAP and with methylene carbons C4–C8 in NVA. The doubling of some resonances in the spectra of solid NVA indicates that there are two molecules in the crystallographic asymmetric unit. DFT GIAO calculations of shielding constants were performed for several geometric isomers, including molecules with different orientations of aliphatic chain with respect to aromatic ring. Low-energy conformers were found by genetic algorithm methodology. Comparison of experimental ¹³C chemical shifts with theoretical (GIAO DFT) shielding parameters was helpful in predicting the most reliable geometry in the solid state. Cross polarization time constants T_CP and relaxation times in the rotating frame T^H_1ρ were obtained from variable-contact cross-polarization experiments. T^H_1ρ are longer in the order: NVA < CAP < DHCAP.