EXPLORATION OF BIOLOGICALLY RELEVANT CONFORMATIONS OF ANANDAMIDE, 2-ARACHIDONYLGLYCEROL, AND THEIR ANALOGS USING CONFORMATIONAL MEMORIES

The endogenous cannabinoid anandamide (N-arachidonoylethanolamide) has been shown to possess higher affinity for the cannabinoid CB1 recepto r than for the CB2 receptor. Carrier-mediated transport has been propo sed to be essential for the termination of the biological effects of a nandamide, while hydrolysis of anandamide is performed by a membrane-b ound amidohydrolase, fatty acid amidohydrolase (FAAH). As interaction of anandamide with each of these targets occurs in different environme nts, the conformations of anandamide for interaction with each target may differ. To ascertain what conformations of anandamide, a highly fl exible molecule, are favored in polar and nonpolar environments, the n ew 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 o f overcoming energy barriers very efficiently (Guamieri et al. J. Am. Chem. Sec. 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 conformati onal studies of arachidonic acid have been reported. CM was also appli ed to sn-2-arachidonylglycerol (2-AG), another endogenous CB ligand; t o a 1,1-dimethylheptyl derivative of anandamide, an analogue with high er CB 1 affinity than anandamide; and to N-(2-hydroxyethyl)prostagland in-B-2-ethanolamide (PGB(2)-EA), a prostanoid ligand which does not bi nd to CB1. Consistent with the literature, arachidonic acid was found to exist in an extended, angle-iron shape and in back-folded conformat ions which were U, J, or helical in shape. The angle-iron and U-shapes were both highly populated conformations with the angle-iron preferre d in CHCl3 and the U-shape preferred in H2O. Results for anandamide an d 2-AG paralleled those for arachidonic acid with the exception that a nandamide in water does not adopt a pure extended conformation but, ra ther, favors a hybrid-extended/U-shape. For the dimethyl-heptyl deriva tive of anandamide, the U-shape was found to be predominant in both en vironments (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 th e conformer population, while the helical shape was less than 5%. In b oth CHCl3 and H2O, the presence of the five-membered ring attenuates t he ability of PGB2-EA to adopt an extended conformation. PGB2-EA was f ound instead to exist predominantly in an L-shape (i.e., distorted U-s hape). The low probability of PGB2-EA adopting an extended conformatio n may be why PGB2-EA shows such low affinity for the CB1 receptor. The conformational information obtained here for anandamide and 2-AG may be useful in the design of rigid analogues which mimic the preferred m olecular conformations (shapes) of these ligands. Such rigid analogues may be useful in deducing the bioactive conformation of these endogen ous cannabinoids, not only at the CB receptors but also at the FAAH. e nzyme active site and possibly at the binding site(s) on the newly pro posed anandamide transporter.