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

Abstract

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 Gly(3)-Phe(4) and a γ-turn around Gly(2), 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 positioning itself in a pocket above the turn where it can form H-bonds to the Gly(3) and C-terminus C═O groups. This β-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 ].

Towards theoretical spectroscopy with error bars: systematic quantification of the structural sensitivity of calculated spectra

“…1 In many cases, structural information can only be extracted from experimental spectra by combining them with computations. 2 Examples include the elucidation of the gas-phase structure of polypeptides with vibrational spectroscopy, [3][4][5][6][7] the assignment of the absolute conguration of chiral molecules with chiroptical spectroscopic techniques, [8][9][10][11] and the identication of active species and catalytic intermediates with X-ray spectroscopy. [12][13][14][15] While in some cases, high-resolution spectroscopic experiments can resolve the individual spectroscopic transition, this is usually not the case for most common applications that aim at obtaining structural information from spectroscopic experiments, such as those mentioned above.…”

Mapping the conformational free energy of aspartic acid in the gas phase and in aqueous solution

“…Experimental information on low energy conformers can be obtained by spectroscopic methods (see, e.g., Refs. [3][4][5][6][7][8][9][10]. The most powerful technique for conformational identification in the gas phase is rotational spectroscopy, which has been successfully applied in combination with laser ablation 11,12 to determine the conformations of many natural amino acids.…”

CID fragmentation, H/D exchange and supermetallization of Barnase-Barstar complex

“…The effect was explained by partial unfolding of one of the monomers, followed by proton migration to the unfolded one. Overall, the problem of conformational changes and correlation between the solution phase structures and gas phase structures is not yet resolved 48 ; recently evidence was provided for the gas phase folding of peptides 49 .…”