Anandamide and analogous endocannabinoids: a lipid self-assembly study

Abstract

Anandamide, the endogenous agonist of the cannabinoid receptors, has been widely studied for its interesting biological and medicinal properties and is recognized as a highly significant lipid signaling molecule within the nervous system. Few studies have, however, examined the effect of the physical conformation of anandamide on its function. The study presented herein has focused on characterizing the self-assembly behaviour of anandamide and four other endocannabinoid analogues of anandamide, viz., 2-arachidonyl glycerol, arachidonyl dopamine, 2-arachidonyl glycerol ether (noladin ether), and o-arachidonyl ethanolamide (virodhamine). Molecular modeling of the five endocannabinoid lipids indicates that the highly unsaturated arachidonyl chain has a preference for a U or J shaped conformation. Thermal phase studies of the neat amphiphiles showed that a glass transition was observed for all of the endocannabinoids at ∼ −110 °C with the exception of anandamide, with a second glass transition occurring for 2-arachidonyl glycerol, 2-arachidonyl glycerol ether, and virodhamine (−86 °C, −95 °C, −46 °C respectively). Both anandamide and arachidonyl dopamine displayed a crystal-isotropic melting point (−4.8 and −20.4 °C respectively), while a liquid crystal-isotropic melting transition was seen for 2-arachidonyl glycerol (−40.7 °C) and 2-arachidonyl glycerol ether (−71.2 °C). No additional transitions were observed for virodhamine. Small angle X-ray scattering and cross polarized optical microscopy studies as a function of temperature indicated that in the presence of excess water, both 2-arachidonyl glycerol and anandamide form co-existing Q_II^G (gyroid) and Q_II^D (diamond) bicontinuous cubic phases from 0 °C to ∼20 °C, which are kinetically stable over a period of weeks but may not represent true thermodynamic equilibrium. Similarly, 2-arachidonyl glycerol ether acquired an inverse hexagonal (H_II) phase in excess water from 0 °C to 40 °C, while virodhamine and arachidonyl dopamine exist as an isotropic L₂ phase, even at very low temperatures. Due to their preferential conformation and lipid self-assembly behaviour, all five endocannabinoids constitute high curvature lipids that can impart membrane stress within a cell membrane which has been linked to a number of membrane and membrane protein associated processes.