Theoretical studies on substitution isomerism and tautomerism in iodo-histamine molecules

Stabilization energies Delta E (kcal/mol) of various substitution isomers o f the two tautomeric iodo-histamine forms have been calculated at the RHF/3 -21G**, MP2/3-21G** and HF/Sadlej-PTZ levels. The most stable forms are pro duced by iodo-substitution of histamine in the side chain, ca. 95% of the 5 -(2'-amino-2'-iodoethyl) 3H-imidazole and ca. 5% of the 5-(2'-amino-2'-iodo ethyl)-1H-imidazole isomer. If iodo-substituted is the imidazole ring only, then the 4-iodo-histamine and 2-iodo-histamine N(1)-H tautomers are predic ted to coexist is a mole ratio of ca. 95.5. The only stable form of the dep rotonated histamine anion is that with both the imidazole nitrogen atoms de protonated. (C) 2000 Elsevier Science B.V. All rights reserved.