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.
Highlights
Grodsky reported on the sublimation and decomposition of unmodified amino acids and certain dipeptides in 1955.[33,34,35,36,37,38] Methylation and acylation of peptides have already been investigated in the late 60s and early 70s in combination with electron impact mass spectrometry (EI-MS) as a means for increased volatility in sequence analysis of unknown proteins.[39,40,41]
We started by comparing the native tripeptide 1 with its methylated derivative 2 where internal charges were removed by acetylation and amidation of the termini (Fig. 3)
Earlier studies with stable organic molecules showed that their volatility and stability can be enhanced by functionalization with perfluoroalkyl chains.[65]
Summary
Since the early days of Otto Stern[1] and Immanuel Estermann,[2] neutral molecular beams have played a key role in fundamental studies of physics and physical chemistry.[3,4,5] Experiments with isolated molecules in the gas phase have laid the ground for highprecision spectroscopy,[6,7] molecule and cluster deflectometry[8,9,10] and for an improved understanding of chemical reactions with quantum state control.[11,12] Modern molecular beam experiments have allowed setting new bounds on the electric dipole moment of the electron[13,14] and enabled the observation of quantum interference with clusters and molecules,[15,16] even with masses exceeding 100000 amu.[17]. As reported in the literature, the removal of internal charges and hydrogen bond donors – through acylation of the N-terminus, amidation of the C-terminus and methylation of all nitrogen atoms – reduces the intermolecular binding and increases the volatility of the peptides.[39] evaporation of 2 permitted the observation of intact molecular ions (m = 471 amu), already at 525 K (see Fig. 3(b)).
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