Gas-Phase Folding of a Prototypical Protonated Pentapeptide: Spectroscopic Evidence for Formation of a Charge-Stabilized β-Hairpin

Ultraviolet and infrared-ultraviolet (IR-UV) double-resonance photofragment spectroscopy has been carried out in a tandem mass spectrometer to determine the three-dimensional structure of cryogenically cooled protonated C-terminally methyl esterified leucine enkephalin [YGGFL-OMe+H]+. By comparing the experimental IR spectrum of the dominant conformer with the predictions of DFT M05-2X/6-31+G(d) calculations, a backbone structure was assigned that is analogous to that previously assigned by our group for the unmodified peptide [Burke, N.L.; et al. Int. J. Mass Spectrom. 2015, 378, 196], despite the loss of a C-terminal OH binding site that was thought to play an important role in its stabilization. Both structures are characterized by a type II′ β-turn around Gly3-Phe4 and a γ-turn around Gly2, providing spectroscopic evidence for the formation of a β-hairpin hydrogen bonding pattern. Rather than disrupting the peptide backbone structure, the protonated N-terminus serves to stabilize the β-hairpin by posi...