Conformational analysis of the tetrapeptide Pro-D-Phe-Pro-Gly in aqueous solution

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Abstract

Conformational investigations of the tetrapeptide Pro-D-Phe-Pro-Gly in water solution were carried out by 1H and 13C NMR spectroscopy. The internal proline residue allows for the possibility of cis/trans isomerization about the D-Phe-Pro peptide bond resulting in two conformational isomers. The major isomer was identified as the trans isomer. The pH-dependence of the cis/trans equilibrium supports an additional stabilisation of the trans isomer by an intramolecular ionic interaction between the amino- and carboxy-terminus in the zwitterionic state. Based on 13C spinlattice relaxation times(T1), different pyrrolidine ring conformations of Pro1 and Pro3 could be determined. By combination of several NMR data ( vicinal coupling constants 3JNa, temperature dependence of the NH chemical shifts, differences in the chemical shifts between the β and γ carbons of the proline residues) and energy minimization calculations, a type II' β-turn should contribute considerably to the overall structure of the trans isomer.

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