Discovery of a Remarkable Methyl Shift Effect in the Vanilloid Activity of Triterpene Amides

As part of a study on triterpenoid conjugates, the dietary pentacyclic triterpenoids oleanolic ( 2a ) and ursolic acids ( 3a ) were coupled with vanillamine, and the resulting amides ( 2b and 3b , respectively) were assayed for activity on the vanilloid receptor TRPV1. Despite a structural difference limited to the location of a methyl group in their conformationally rigid pentacyclic core, oleanoloyl vanillamide dramatically outperformed ursoloyl vanillamide in terms of potency (EC 50 = 35 ± 2 nM for 2b and 5.4 ± 2.3 μM for 3b ). Using molecular docking and dynamics, this difference was translated into distinct accommodation modes at the TRPV1 vanillyl ligand pocket, suggesting a critical role of a C-H π phenyl interaction between the triterpenoid C-29 methyl and Phe591 of TRPV1. Because the molecular mechanisms underlying the activation process of transient receptor channels (TRPs) remain to be fully elucidated, the observation of spatially restricted structure-activity information is of significant relevance to identify the molecular detail of TRPV1 ligand gating.