The cyclic hexadecapeptide containing human heat shock protein 70(29-42) fragment cyclized by the disulfide bridge between two L-cysteine residues introduced at the N- and C-termini was synthesized by the solid phase method. It was established that the cyclic analogue, contrary to its linear counterpart, had much lower ability to generate immune response in rabbits. Conformational studies of cyclic peptide performed using 1D and 2D 1H-NMR spectroscopy in conjunction with theoretical conformational analysis revealed that the cyclization constrained the 3D structure of this peptide, reflected by the observed rate of cis/trans isomerization of Arg9-Thr10 peptide bond and the presence of Gly7-Asn8 peptide bond in cis geometry.We, therefore, postulate that the conformational flexibility in the case of Human Heat Shock Protein fragments is a key element for their immunogenicity.
The immunotropic activity of peptides studied evaluated in the murine system indicated that they are as active as the linear precursor in the resistance to hydrocortisone, but did not show activity in the human system. As reported previously, similar immunotropic profiles were observed in the case of two cyclic analogues. Superposition of most representative conformations of all four peptides in the fragment mentioned above (RMSD of A carbons is0.86 A) leads to the conclusion that this hexapeptide segment might be considered as bioactive.
Their solution structures are characterized by families consisting of 7 and 8 conformers, respectively, with statistical weights higher than 2%. The 3D structure of Scyl is rather flexible, whereas the presence of a local constrait in position 7 significantly rigidifies the whole molecule.
These peptides were designed based on the immunoregulatory activity of linear peptides obtained after chymotrypsin digestion of PRP. Despite the fact that the structures of both analogues cannot be interpreted in terms of a single conformation, the superposition of the most populated conformations of the cyclic peptides studied revealed a similar geometry for the Tyr-Val-pro-Leu-Phe-Pro fragment (RMSD=1.6 A) in both peptides and therefore might be considered to be responsible for the biological activity.
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