Theoretical study of metiamide, a histamine H2 antagonist

The requirements for H-2-antagonist activity so far identified for most of the known antagonists of histamine are the presence of a heterocyclic ring containing a basic center linked via a methylene chain to a substituted guanidine or thiourea polar side chain. Metiamide is a potent H-2 antagonist (pA2 = 6.06). We have used the ab initio Hartree-Fock (HF) method in order to study the conformational properties of the N-3-H tautomers of metiamide molecule; and histamine monocation. Three basis set (the 3-21G*, 6-31G**, and 6-31 + G**) were used, the results compared, and the geometric parameters fully optimized. Our results indicate the preference of metiamide for a folded conformation with an intramolecular hydrogen bonding between the imidazole ring and one of the NH groups. The optimized geometrical parameters and charge distributions of both molecules, using the Mulliken, and natural bond order (NBO) analysis, are given and discussed. (C) 1998 John Wiley & Sons, Inc.