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. 2017 Aug;52(8):550-556.
doi: 10.1002/jms.3959.

Tailoring the Volatility and Stability of Oligopeptides

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Free PMC article

Tailoring the Volatility and Stability of Oligopeptides

J Schätti et al. J Mass Spectrom. .
Free PMC article

Abstract

Amino acids are essential building blocks of life, and fluorinated derivatives have gained interest in chemistry and medicine. Modern mass spectrometry has enabled the study of oligo- and polypeptides as isolated entities in the gas phase, but predominantly as singly or even multiply charged species. While laser desorption of neutral peptides into adiabatically expanding supersonic noble gas jets is possible, UV-VIS spectroscopy, electric or magnetic deflectometry as well as quantum interferometry would profit from the possibility to prepare thermally slow molecular beams. This has typically been precluded by the fragility of the peptide bond and the fact that a peptide would rather 'fry', i.e. denature and fragment than 'fly'. Here, we explore how tailored perfluoroalkyl functionalization can reduce the intermolecular binding and thus increase the volatility of peptides and compare it to previously explored methylation, acylation and amidation of peptides. We show that this strategy is essential and enables the formation of thermal beams of intact neutral tripeptides, whereas only fragments were observed for an extensively fluoroalkyl-decorated nonapeptide. © 2017 The Authors. Journal of Mass Spectrometry Published by John Wiley & Sons Ltd.

Keywords: fluorination; molecular beams; peptides; thermal evaporation; vuv ionization.

Figures

Figure 1
Figure 1
Gallery of peptides 19 with increasing molecular weight employed in this study. Tripeptides 18 resulted from variation of the Ala‐Trp‐Ala motif: charges and hydrogen bond donors present in parent tripeptide 1 were removed by acylation, methylation and amidation in derivative 2; one perfluoroalkyl chain was introduced at the N‐terminus by acylation and the C‐terminus amidated in derivative 3; 4 was obtained by methylation of 3; fluorinated alkyl chains of different or equal length were introduced at the N‐ and C‐terminus by acylation and amidation, respectively, in derivative 5 and 6; 7 and 8 are sequence isomers of 6; high Trp and fluoroalkyl content realized by alternating Trp and Lys followed by acylation of the lysine side chains and the N‐terminus as exemplified in peptide 9.
Figure 2
Figure 2
Experimental scheme for the volatilization/ionization tests. The peptides were heated in a ceramic cell with an aperture of 3 × 0.05 mm2. The molecular beam reached the mass spectrometer through two differential pumping stages, separated by one skimmer and one slit of 3 mm as the relevant dimension. Under heat load, the pressure in the three chambers was 1 × 10−5, 3 × 10−6 and 1.5 × 10−7 mbar, respectively. Pulsed photoionization of the molecular beam was combined with time‐of‐flight mass spectrometry (panel (a)). Alternatively, continuous electron impact ionization (b) was combined with a quadrupole mass spectrometer. Because both spectrometers were optimized for high transmission, their mass resolution is limited to about 2% with a calibration uncertainty of 5% across the entire mass range.
Figure 3
Figure 3
Panel (a) shows the mass spectrum of the native tripeptide Ala‐Trp‐Ala 1 after evaporation at 595 K and VUV postionization with a pulse intensity of I ion = 2.9(3)MW/cm2. The native biomolecule (346 amu) falls apart under these conditions, and the following main fragments are observed: the indole cation (C8H6N+, 116 amu), the skatole cation (C9H8N+,130 amu) and a signal that is tentatively assigned to a cationic Ala‐Trp diketopiperazine fragment (C14H15N3O2 +, 257 amu). The spectrum was calibrated to the indole cation (C8H6N+, 116 amu). (b) Under similar experimental conditions, but at lower temperature (T = 525 K), the mass spectrum of the methylated tripeptide 2 displays the intact parent ion at 471 amu. (b). Fragments include the N‐methyl indole cation (C9H8N+,130 amu) and the N‐methylated skatole cation (C10H10N+, 144 amu) as well as several unidentified species. The spectrum was calibrated to the N‐methyl indole cation (C9H8N+,130 amu).
Figure 4
Figure 4
VUV‐TOF versus EI‐QMS. (a) VUV‐TOF mass spectrum of a thermal perfluoroalkyl functionalized peptide beam (3), recorded at T = 548 K. A strong parent peak (m = 669 amu) is observed and accompanied by the indole cation (C8H6N+, 116 amu) and skatole cation (C9H8N+, 130 amu). The spectrum was calibrated to the indole cation (C8H6N+, 116 amu). (b) In contrast, the EI‐QMS spectrum at 70‐eV electron energy yields a pronounced fragment spectrum under otherwise identical boundary conditions. The spectrum was calibrated to the skatole cation (C9H8N+, 130 amu). The green highlight indicates the fluoroalkyl‐tag. [Colour figure can be viewed at wileyonlinelibrary.com]
Figure 5
Figure 5
(a) VUV‐TOF mass spectrum of a thermal beam of perfluoroalkyl functionalized and methylated tripeptide 4 at T = 552 K and I ion = 2.9(3) MW/cm2. We observe the methylated skatole cation (C10H10N+, 144 amu) and the N‐methyl indole cation (C9H8N+, 130 amu) as well as unidentified fragments. The spectrum was calibrated to the N‐methyl indole cation (C9H8N+,130 amu). (b) A second perfluoroalkyl chain adds to the molecular mass but results in a relatively clean mass spectrum of the non N‐methylated peptide 5 (compare also Fig. 4(a)). The spectrum was calibrated to the indole cation (C8H6N+, 116 amu). [Colour figure can be viewed at wileyonlinelibrary.com]
Figure 6
Figure 6
(a–c) Sequence isomers of tripeptide derivative with perfluoroalkyl decorated termini. Mass spectra for 6, 7 and 8 are very similar, although a higher proportion of fragments is observed for 7. Spectra were calibrated to the indole cation (C8H6N+, 116 amu). The indole cation is not indicated in (a) and (c) for improved clarity. (d) No intact ion was detected for highly perfluoroalkyl decorated Trp‐Lys oligomer 9 after thermal evaporation and photoionization. (e) Variation of the oven temperature leads to an increase of both the parent signal and the fragments of compound 8. Significant thermal decomposition is observed at 615 K. [Colour figure can be viewed at wileyonlinelibrary.com]

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