Secondary structures of Val–Phe and Val–Tyr(Me) peptide chains in the gas phase: effect of the nature of the protecting groups

Abstract

Recent experimental gas-phase studies of very similar peptide chain models (Ac–Val–Tyr(Me)–NHMe and Ac–Val–Phe–NH₂) have led to different assignments for the secondary structures adopted: β-strand and β-turn, respectively. We present a discussion of the possible causes for such different behaviour in the light of quantum chemistry calculations. The consistent set of data presently obtained (relative energies and IR calculated spectra) leads us to propose the same structural assignment for the experimentally observed Val–Tyr(Me) and Val–Phe peptide chains, i.e. a β-turn conformation. In addition, calculations also suggest that the nature of the chemical protection on the C-terminal (–NHMe vs. –NH₂) of the chain model does not affect its conformational preference, nor its structure or its energetics, which suggests the less simple, but more informative, –NH₂-protected models for the determination of the intrinsic structural properties of a peptide chain.

Keywords:

• Peptide chain
• Peptide folding
• Secondary structure
• Beta-turn
• Beta-strand
• IR spectroscopy
• Density functional theory

Acknowledgement

The authors wish to thank Dr Isabelle Compagnon for her helpful comments on the manuscript.