The cyclic structures are characteristic for many classes of compounds, which exhibit pharmacological activity and are used as medicines (cyclosporin A, polyene antibiotics, etc.). A creation of cyclic structure may modified susceptibility of peptide compounds to bioavailability and enzymes’ action.

The study include quantum-chemical computations of stability of cyclic structures built from n glycine residues connected with peptide bonds or n glycine analogue moieties, in which nitrogen atom was replaced with phosphorus atom. The calculations were conducted with application of Hartree-Fock method and STO-3G and 6-31G* basis functions. All calculations were preformed with the GAUSSIAN 03W program package.

The cyclic structure of hexaglycine containing six identical intramolecular hydrogen bonds (Figure 1.) was found to be the global energetic minimum. By the analogy to this structure a series of cyclic peptides was built. The small cyclopeptides up to hexaglycine are circle like, whereas larger compounds lose this symmetry. Also their linear equivalents and phosphorus analogues were designed.

Computed energies per one created peptide or quasipeptide bond decrease with the increase of molecule’s size. For cyclic structures stand out two local energetic minima by compounds built of six or twelve units. For peptides they are even more energetically favourable than linear compounds. These findings suggest that such cyclic peptides should occur in nature and should be fairly stable.

Figure 1. Cyclohexaglycine

Bibliography:
[1] Humphrey J. M., Chamberlin A. R., Chem. Rev., 1997, 97, 2243-66.
[2] Böhm H. J., Brode S., J. Comput. Chem., 1995, 16(2), 146-53.
[3] Paul P. K. C.,Ramakrishnan C.,Int. J. Peptide Protein Res., 1987, 29, 433-54.
[4] Santos M. A., Brennan R. L., Drew M. G. B., J. Mol. Struct. (Theochem), 1993, 286, 109-23.

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