Effect of solvent on an NMR chemical shift difference between glycyl geminal α-protons as a probe of β-turn formation of short peptides

Abstract

Proton NMR spectra of short peptides with a glycyl (Gly) or N-methylglycyl (sarcosyl, Sar) residue were measured in various mixed solvents with a wide range of dielectric constants: 78.3–2.3. From analyses of the octet and quartet signals of the geminal α-protons of Gly and Sar residues, respectively, we have estimated chemical shift differences between the two α-protons, Δ δ α/α′. It is found that the Δ δ α/α′ values increase with decreasing solvent polarity and the increasing rates depend significantly on amino acid sequences. By referring to infrared spectra and chemical shift of the terminal NH protons, δ NH, of the peptides, the Δ δ α/α′ values were found to be a good probe of β-turn formation. From solvent-dependent change of Δ δ α/α′, we estimated the free energies for the β-turn formation and compared the results with those estimated from δ NH. Using the resulting free energies, we have discussed effects of solvent on the β-turn formation.