Abstract
High field asymmetric waveform ion mobility spectrometry (FAIMS) is well-established as a tool for separating peptide isomers (sequence inversions and post-translationally modified localization variants). Here, we demonstrate the FAIMS is able to differentiate cis and trans isomers of polyproline. Polyproline assumes an all-cis conformation—the PPI helix—in 1-propanol, and an all-trans conformation—the PPII helix—in aqueous solutions. Differentiation of these conformers may be achieved both through use of a cylindrical FAIMS device and a miniaturized ultrahigh field planar FAIMS device. Graphical ᅟ Electronic supplementary materialThe online version of this article (doi:10.1007/s13361-016-1482-1) contains supplementary material, which is available to authorized users.
Highlights
Proline is the only naturally occurring amino acid that can exist in both cis and trans conformations, unlike other amino acid residues, which favour trans
Peaks corresponding to the singly and doubly charged peptide ions are observed in mass spectrum obtained from propanol, whereas only the doubly charged ion is observed from aqueous solution
This observation is similar to that seen by Clemmer and co-workers for the polyproline peptide Pro7 [12]
Summary
Proline is the only naturally occurring amino acid that can exist in both cis and trans conformations, unlike other amino acid residues, which favour trans. In a series of alanine substitution experiments, Pierson et al showed that the proline residues were responsible for the multiple peaks observed in drift tube ion mobility spectra of triply protonated bradykinin ions [3]. Multiple conformers of [M + 2H]2+ ions of bradykinin were observed by Brown et al in their evaluation of a miniaturised ultrahigh field FAIMS device [9]. Polyproline exists as two major conformers: the all-cis polyproline I (PPI) helix and the all-trans polyproline II (PPII) helix, see Figure 1 The former is favored in aliphatic alcohols, whereas the latter is favored in aqueous solutions. Clemmer and co-workers showed that by equilibrating solutions of polyproline-13 (Pro13) in 1-propanol and subsequent dilution in water, the conversion of PPI to PPII, via various cis/trans intermediates, could be followed by ion mobility spectrometry. The compensation field (CF) corresponding to the maximum peak intensity [i.e., Ec(max)], was calculated
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