A theoretical model for the histamine H2-receptor.

We describe an electronic and conformational study of histamine H2-receptor ligands of the imidazole series, in which the possibilities of configurational isomerism (the thiourea group) and N3H and N1H tautomerism (imidazole ring) were considered. The results suggest that the conformational flexibility of the molecules and the properties of the imidazole ring are of special importance in the display of H2-receptor activity. A theoretical model of histamine H2-receptor interactions is proposed on the basis of these and other results. A very important characteristic of our model is its ability to explain H2-receptor activation by compounds which differ structurally, and to explain antagonism at the same receptor. The stereospecificity of rigid analogues of cimetidine and tiotidine, and the importance of chain length in flexible histamine H2-antagonists are also accounted for.