NMR and circular dichroic studies of the solution structure of conformationally constrained antigenic peptides

Circular dichroic and nuclear magnetic resonance spectroscopies were used to evaluate the conformational properties in solution of a series of 20-amino-acid peptides derived from the primary structure of an antigen from Echinococcus granulosus. The linear peptide corresponding to the sequence 93-112 in the antigen was found to populate in a significant proportion the alpha-helix conformational state. In the presence of 2,2,2-trifluoroethanol, a cosolvent known to stabilize peptide secondary structure, the helical population, estimated from circular dichroic spectra, increases up to 60-70%. Two-dimensional nuclear magnetic resonance studies under these conditions showed that the segment K96-K108 meets all the criteria of an alpha-helix at 281 K and 298 K. Three different variants were synthesized with the same or similar primary structure but containing a lactam-bridged (>) side chain: D107 > K110, D97 > K100 and K94 > E98. Generally, the observed helical content in these variants was lower than in the parent molecule and the stability of the helical conformation decreased in the order D107, K110, K94, E98, D97, K100. Analysis of chemical shift and nuclear Overhauser enhancement data suggested that the lactam rings induce significant distortions of the local features of helix secondary structure. The possible factors of helix destabilization induced by lactam bridges, observed in the studied peptides are discussed in relation to the stabilizing effect of ion pairs in model compounds.