EXPLORING THE POTENTIAL-ENERGY HYPERSURFACE OF HISTAMINE MONOCATION - TAUTOMERISM IN GAS-PHASE

The potential energy hypersuface of the histamine monocation is determined by ab initio methods at the STO-4G level using analytical gradient techniques. Three transition states and two minima have been found for the N-tau-H tautomer. One of the transition states connects the trans conformational region with a minimum gauche structure, where the proton of the ammonium group is approximately halfway between the N-tau of the imidazole group and the N of the ammonium group, but nearer to the N-pi. This minimum connects the potential energy surface of the N-tau-H tautomer with the imidazolium one. In the latter region, three transition states and two minima have been found. Critical points are discussed in relation with experimental data and histamine H-2 receptor models.