Single-conformation spectroscopy of cold, protonated DPG-containing peptides: switching β-turn types and formation of a sequential type II/II' double β-turn

Phys Chem Chem Phys. 2022 Jan 26;24(4):2095-2109. doi: 10.1039/d1cp04852j.


D-Proline (DPro, DP) is widely utilized to form β-hairpin loops in engineered peptides that would otherwise be unstructured, most often as part of a DPG sub-unit that forms a β-turn. To observe whether DPG facilitated this effect in short protonated peptides, conformation specific IR-UV double resonance photofragment spectra of the cold (∼10 K) protonated DP and LP diastereomers of the pentapeptide YAPGA was carried out in the hydride stretch (2800-3700 cm-1) and amide I/II (1400-1800 cm-1) regions. A model localized Hamiltonian was developed to better describe the 1600-1800 cm-1 region commonly associated with the amide I vibrations. The CO stretch fundamentals experience extensive mixing with the N-H bending fundamentals of the NH3+ group in these protonated peptides. The model Hamiltonian accounts for experiment in quantitative detail. In the DP diastereomer, all the population is funneled into a single conformer which presented as a type II β-turn with A and DP in the i + 1 and i + 2 positions, respectively. This structure was not the anticipated type II' β-turn across DPG that we had hypothesized based on solution-phase propensities. Analysis of the conformational energy landscape shows that both steric and charge-induced effects play a role in the preferred formation of the type II β-turn. In contrast, the LP isomer forms three conformations with very different structures, none of which were type II/II' β-turns, confirming that LPG is not a β-turn former. Finally, single-conformation spectroscopy was also carried out on the extended peptide [YAADPGAAA + H]+ to determine whether moving the protonated N-terminus further from DPG would lead to β-hairpin formation. Despite funneling its entire population into a single peptide backbone structure, the assigned structure is not a β-hairpin, but a concatenated type II/type II' double β-turn that displaces the peptide backbone laterally by about 7.5 Å, but leaves the backbone oriented in its original direction.

MeSH terms

  • Glycine / chemistry*
  • Oligopeptides / chemistry*
  • Proline / chemistry*
  • Protein Conformation
  • Spectrophotometry, Infrared
  • Spectrophotometry, Ultraviolet
  • Stereoisomerism


  • Oligopeptides
  • Proline
  • Glycine