Conformation-Specific Infrared and Ultraviolet Spectroscopy of Cold [YAPAA+H]⁺ and [YGPAA+H]⁺ Ions: A Stereochemical “Twist” on the β‑Hairpin Turn

Incorporation of the unnatural d-proline (^DP) stereoisomer into a polypeptide sequence is a typical strategy to encourage formation of β-hairpin loops because natural sequences are often unstructured in solution. Using conformation-specific IR and UV spectroscopy of cold (≈10 K) gas-phase ions, we probe the inherent conformational preferences of the ^DP and ^LP diastereomers in the protonated peptide [YAPAA+H]⁺, where only intramolecular interactions are possible. Consistent with the solution-phase studies, one of the conformers of [YA^DPAA+H]⁺ is folded into a charge-stabilized β-hairpin turn. However, a second predominant conformer family containing two sequential γ-turns is also identified, with similar energetic stability. A single conformational isomer of the ^LP diastereomer, [YA^LPAA+H]⁺, is found and assigned to a structure that is not the anticipated “mirror image” β-turn. Instead, the ^LP stereocenter promotes a cis-alanine–proline amide bond. The assigned structures contain clues that the preference of the ^DP diastereomer to support a trans-amide bond and the proclivity of ^LP for a cis-amide bond is sterically driven and can be reversed by substituting glycine for alanine in position 2, forming [YG^LPAA+H]⁺. These results provide a basis for understanding the residue-specific and stereospecific alterations in the potential energy surface that underlie these changing preferences, providing insights to the origin of β-hairpin formation.