TETRAHEDRON LETTERS Pergamon Tetr~edronLetters40(1999) 2425-2428 1H NMR Study on Putative Intramolecular Hydrogen Bonding for Histamine H3-Receptor Agonists Jari T. Kovalainen, a'* Johannes A.M. Christiaans,~ Antti Poso, a Jouko VepsN~iinen,b Reino Laatikainen b and Jukka Gynthe¢ "Department of Pharmaceutical Chemistry, ~'Department of Chemistry, UniversiO, of Kuopio, P.OBox 1627, FIN- 70211 Kuopio, Finland Received 30 November 1998; accepted 11 January 1999 Abstract: Conformationalstabilizationby intramolecularhydrogen bonding of two histamine H 3- receptor agonists is studied by ~H NMR. Stabilization of each individual conformation of the compounds by intramolecular hydrogenbonding is not strong at physiologicalpH and temperature. However, 52% - 61% of the moleculesexist in conformations where an intramolecular hydrogenbond is possible. © 1999ElsevierScienceLtd. All rights reserved. Histamine is a biologically active amine acting on three distinct histamine receptors. The histamine H 3- receptor is a presynaptically located autoreceptor modulating the release and synthesis of histamine from histaminergic neurones. The H3-receptor also controls the release of some other neurotransmitters at non- histaminergic neurones. Many therapeutic targets for H3-receptor ligands have been suggested i.e. asthma, migraine, learning and memory degenerative disorders like Alzheimer's disease.~'2 Molecular modelling predicts intramolecular hydrogen bond formation for many histamine H3-receptor agonists.34 Intramolecular hydrogen bonding may be involved in the mechanism of activation of the H 3- receptor via a proton transfer process. A proton relay process between the ligand and the receptor protein is proposed as a mechanism of activation for both the serotonin 5-HT receptor5and histamine H2-receptor) The prediction of conformational behaviour of the system in solution is difficult by computational methods. The aim of this work was to study the intramolecular hydrogen bond formation for two histamine H3-receptor agonists in solution, namely R-(c0-methylhistamine 7 (1) and 2-S-amino-3-(1H-imidazol-4(5)- yl)propyl cyclohexylmethyl ether8 (2) by temperature dependence of ~H,~H couplings. 9'~°The approach based on iterative fitting of data enables the characterization of the individual rotamers of chiral molecules and their thermodynamics. Three rapidly equilibrating rotamers of compounds I and 2 are illustrated in Fig. 1. MATERIALS AND METHODS The pH and pD of 0.03-0.04 mM H20 and D20 solutions of 1 and 2 were adjusted to 7.4 using 0.1 M HC1/DCI and NaOH/NaOD in order to get the monocationic state of the compounds. H The CD3OD samples were converted to the monocationic form by adding first 0.5 equivalent of NaOH. After careful evaporation of the solvent, the residues were dissolved in CD3OD and filtrated. ~H NMR spectra in H20, D20 and CD3OD *E-mail:Jari.Kovalainen@uku.fi 0040-4039/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S0040-4039(99)00120-3