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1 2009

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Structural Studies of Position 2 Modified Endomorphin-2 Analogs by NMR Spectroscopy and Molecular Modeling

Janecka A., Perlikowska R., Gach K., Fichna J., Mazur A., Kruszyński R., Janecki T., Jankowski S.

Polish Journal of Chemistry

2009

Vol. 83, nr 7

1293-1307

Endomorphin-2 (EM-2, Tyr-Pro-Phe-Phe-NH2) is an endogenous ligand for the mi-opioid receptor. To examine the importance of Pro2 in EM-2 structure, we synthesized a series of analogs in corporating piperidine-2-, 3- and 4-carboxylic acids (Pip, Nip and Inp, respectively) in position 2. Pip, Nip and Inp are six-membered mimics of Pro and can be considered as alfa-, beta- and gamma-amino acids, respectively. Receptor binding studies revealed that [(R)-Nip2]EM-2 had greatly in creased mi-opioid receptor affinity compared with the parent peptide, while two other analogs were in active. In order to determine which structural elements of [(R)-Nip2]EM-2 could be responsible for the out standing affinity of this analog, the solution conformations of EM-2 analogs in corporating Promimics were investigated by the combination of 2D 1H NMR measurements and molecular modeling calculations. Evaluating the ratios of cis/trans rotamers, aromatic inter actions and dihedral angles we have found that all three analogs exist as a mixture of cis/trans rotamers of the Tyr–Xaa peptide bond and have flexible, extended conformations, with no intramolecular hydrogen bonds or aromatic ring inter actions observed for EM-2. The obtained results do not allow to draw conclusions on the bioactive conformation of the most active analog. We can suggest that a well known preference of the substituents to occupy equatorial positions in six-membered rings, to gether with a greater distance between Tyr1 and Phe3 aromatic rings, in Nip, which is a beta-amino acid, as compared with EM-2, are the main differentiating factors which are responsible for the exceptional affinity of [(R)-Nip2]EM-2.