Conformational studies of peptides.

Abstract

The conformational and dynamic properties of peptides play critical roles in their biological functions. Various methods have been applied for studying peptide conformations and dynamics. On the basis of structure-function studies and NMR work on the conformation of CCK, we designed a series of conformationally constrained peptides to test the hypothesis of a C-terminal folded structure required for interaction with the CCK-B receptor. Various cyclic peptides were synthesized to explore the possible spatial arrangements of key amino acid side chain groups. Based on the NMR conformational studies and receptor binding assay data, it was suggested that a C-terminal folded conformation of CCK4 is a necessary but not a sufficient structural feature for strong CCK-B receptor binding. In a series of tyrosine analogues, slow rotations around Cβ-Cγ were studied by dynamic NMR. The free energies of activation (ΔG(^≠)) at their coalescence temperatures were estimated to be in the range of 14-20 Kcal/mol. This energy barrier is comparable to the one experienced by tyrosine aromatic rings in the interior of proteins, bioactive peptides or in peptide-protein complexes. This study demonstrates the ability of conformational constraints on restricting molecular motions. These tyrosine analogues with constrained side chains can be useful tools in studying the structure-function relationships of peptides and proteins. In peptides and proteins, amino acid residues are covalently linked together by either amide bonds or disulfide bonds. The molecules Me₂S₂, N-methylacetamide and other simple amides were used as models for disulfide or peptide bond linkages in our ab initio MO studies. An angular dependence of the ¹³C and ³³S NMR chemical shifts on the C-S-S-C dihedral angle was predicted based on our calculations. The well-known syn/anti dependence of ¹³C chemical shift around the peptide bond was found to have the same origin as the angular dependence of the γ-substituent effect. A systematic geometry optimization and chemical shift calculation on a model peptide, N-acetyl-N'-methyl glycine amide, enabled us to construct the first non-empirical energy surface and ¹³C NMR chemical shift surface for this molecule. The experimentally observed correlation between ¹³C chemical shifts of Cα in amino acid residues and protein secondary structures is attributed to the effect controlled by φ and ψ dihedral angles.