The tautomerism of the histamine molecule was explored based on quantu
m chemical calculations both in the gaseous state and in aqueous mediu
m. Structural optimizations were performed at the ab initio Hartree-Fo
ck (HF) and the second order Moller-Plesset (MP2) perturbation level.
The most stable tautomer in aqueous medium appeared to be the N(3)-H m
onocation; with strong intramolecular hydrogen bonding. Results from c
alculations prove that the previous hypothesis on interaction with the
hypothetical H-2 receptor remains valid, even though this hypothesis
involved a slightly different assumption regarding the structure of th
e most stable form. The most stable neutral form in aqueous medium tur
ned out to be the opened N(3)-H gauche structure.