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' beta-turn around Gly(3)-Phe(4) and a gamma-turn around Gly(2), providing spectroscopic evidence for the formation of a beta-hairpin hydrogen bonding pattern. Rather than disrupting the peptide backbone structure, the protonated N-terminus serves to stabilize the beta-hairpin by positioning itself in a pocket above the turn where it can form H-bonds to the Gly(3) and C-terminus C=0 groups. This beta-hairpin type structure has been previously proposed as the biologically active conformation of leucine enkephalin and its methyl ester in the nonpolar cell membrane environment [Naito, A.; Nishimura, K. Curr. Top. Med. Chem. 2004, 4, 135-143].