D-Proline (Pro, P) is widely utilized to form β-hairpin loops in engineered peptides that would otherwise be unstructured, most often as part of a PG sub-unit that forms a β-turn. To observe whether PG facilitated this effect in short protonated peptides, conformation specific IR-UV double resonance photofragment spectra of the cold (∼10 K) protonated P and P diastereomers of the pentapeptide YAPGA was carried out in the hydride stretch (2800-3700 cm) and amide I/II (1400-1800 cm) regions. A model localized Hamiltonian was developed to better describe the 1600-1800 cm region commonly associated with the amide I vibrations. The CO stretch fundamentals experience extensive mixing with the N-H bending funda... More
D-Proline (Pro, P) is widely utilized to form β-hairpin loops in engineered peptides that would otherwise be unstructured, most often as part of a PG sub-unit that forms a β-turn. To observe whether PG facilitated this effect in short protonated peptides, conformation specific IR-UV double resonance photofragment spectra of the cold (∼10 K) protonated P and P diastereomers of the pentapeptide YAPGA was carried out in the hydride stretch (2800-3700 cm) and amide I/II (1400-1800 cm) regions. A model localized Hamiltonian was developed to better describe the 1600-1800 cm region commonly associated with the amide I vibrations. The CO stretch fundamentals experience extensive mixing with the N-H bending fundamentals of the NH group in these protonated peptides. The model Hamiltonian accounts for experiment in quantitative detail. In the P diastereomer, all the population is funneled into a single conformer which presented as a type II β-turn with A and P in the + 1 and + 2 positions, respectively. This structure was not the anticipated type II' β-turn across PG 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 P isomer forms three conformations with very different structures, none of which were type II/II' β-turns, confirming that PG is not a β-turn former. Finally, single-conformation spectroscopy was also carried out on the extended peptide [YAAPGAAA + H] to determine whether moving the protonated N-terminus further from PG 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.
