SPONTANEOUS DIKETOPIPERAZINE FORMATION VIA END-TO-END CYCLIZATION OF A NONACTIVATED LINEAR TRIPEPTIDE - AN UNUSUAL CHEMICAL-REACTION

The delta-opioid antagonist H-Tyr-Tic Psi[CH2-NH]Phe-OH (TIP[Psi], Tic = tetrahydroisoquinoline-3-carboxylic acid) was shown to undergo spon taneous formation of N-(Mti)-Phe-Tyr diketopiperazine (Mti = [(3'S)-te trahydroisoquinolinyl] methyl) in DMSO and MeOH but not in aqueous sol ution or DMSO:H2O solvent mixtures which were at least 20% water. The reaction mechanism proposed for the diketopiperazine formation involve s amide bond formation between the N and C terminal groups of the nona ctivated linear tripeptide, followed by transannular nucleophilic atta ck of the Tyr carbonyl group by the secondary amine. Molecular mechani cs calculations carried out on TIP[Psi] revealed low-energy structures available to the molecule in which the carboxyl and amino terminal fu nctions are within a distance suitable for nucleophilic attack (2.8 An gstrom), The rate of diketopiperazine formation was shown to be depend ent on both sample pH and nucleophilicity of the terminal peptide amin o group. Reaction intermediates or additional compounds in tautomeric equilibria with the diketopiperazine were not observed during the HPLC analyses or NMR investigations. The diketopiperazine obtained from TI P[Psi] was isolated and structurally characterized by H-1 NMR spectros copy in conjunction with molecular modeling. According to H-1 NMR coup ling constants, several orientations of the side chains around the dik etopiperazine ring are possible. Large chemical shift displacements fr om expected values observed for the Tyr beta and Mti methylene protons are explained in terms of ring current effects manifested by aromatic interactions which stabilize the low-energy structures. The calculate d diketopiperazine ring structure is slightly puckered out of plane by -5 degrees to -15 degrees.