Exploration of Biologically Relevant Conformations of Anandamide, 2-Arachidonylglycerol, and Their Analogues Using Conformational Memories

The endogenous cannabinoid anandamide (N-arachidonoylethanolamide) has been shown to possess higher affinity for the cannabinoid CB1 receptor than for the CB2 receptor. Carrier-mediated transport has been proposed to be essential for the termination of the biological effects of anandamide, while hydrolysis of anandamide is performed by a membrane-bound amidohydrolase, fatty acid amidohydrolase (FAAH). As interaction of anandamide with each of these targets occurs in different environments, the conformations of anandamide for interaction with each target may differ. To ascertain what conformations of anandamide, a highly flexible molecule, are favored in polar and nonpolar environments, the new method of Conformational Memories (CM) was used. CM has been shown to achieve complete conformational sampling of highly flexible ligands, to converge in a very practical number of steps, and to be capable of overcoming energy barriers very efficiently (Guarnieri et al. J. Am. Chem. Soc. 1996, 118, 5580). The generalized Born/surface area (GB/SA) continuum solvation models for chloroform and for water were used in the CM calculations. As a means of validation, CM was first applied to arachidonic acid because both experimental and theoretical conformational studies of arachidonic acid have been reported. CM was also applied to sn-2-arachidonylglycerol (2-AG), another endogenous CB ligand; to a 1,1-dimethylheptyl derivative of anandamide, an analogue with higher CB1 affinity than anandamide; and to N-(2-hydroxyethyl)prostaglandin-B2-ethanolamide (PGB2-EA), a prostanoid ligand which does not bind to CB1. Consistent with the literature, arachidonic acid was found to exist in an extended, angle-iron shape and in back-folded conformations which were U, J, or helical in shape. The angle-iron and U-shapes were both highly populated conformations with the angle-iron preferred in CHCl3 and the U-shape preferred in H2O. Results for anandamide and 2-AG paralleled those for arachidonic acid with the exception that anandamide in water does not adopt a pure extended conformation but, rather, favors a hybrid-extended/U-shape. For the dimethyl-heptyl derivative of anandamide, the U-shape was found to be predominant in both environments (42% in CHCl3, 38% in H2O), but the population of the angle-iron shape was still significant (25% in CHCl3, 29% in H2O). For all of these ligands, J-shaped conformers constituted from 7% to 17% of the conformer population, while the helical shape was l