Theoretical ab initio study of ranitidine

The presence of a heterocyclic ring containing a basic center linked via a methylene chain to a substituted guanidine or thiourea polar side chain, such as found in the H2-antagonist metiamide, which has an imidazole heterocyclic ring, has often been identified as one of the requirements for H-2-antagonist activity. In ranitidine, on the other hand, the imidazole ring is substituted for a furan ring, yielding a more active biological H2 antagonist. hi this work, we have used the ab initio Hartree-Fock (HF) and second-order Moller-Plesset (MP2) methods in order to investigate the open and folded ranitidine conformations, of the type observed in metiamide. Five basis sets (3-21G, 3-21+G**, 6-31G, 6-31+G**, and 6-31+G**) were used in order to obtain fully optimized geometric parameters that indicated good agreement with the experimental crystallographic data, We have also investigated in this work the effects of solvents in both ranitidine and metiamide. Monocationic ranitidine was also investigated. All our results, indicate that, as in metiamide, the folded conformation is also preferred. We have investigated Mulliken and natural bond order (NBO) charge distributions, electrostatic and hydrogen bond effects on stabilizing the conformations and discussed the interactions of ranitidine with the biological receptor. (C) 2002 Wiley Periodicals, Inc.