Protonating polymer oligomers in the gas phase to change fragmentation pathways

Abstract

Ionization of polymers in mass spectrometry is usually achieved by forming metal ion adducts. The metal ion has been shown by Wesdemiotis to often play a spectator role in the collision-induced dissociation (CID) chemistry of these species, wherein they fragment according to a free-radical mechanism similar to that found in their pyrolysis. The result is a predominance of low-mass ions in the CID mass spectrum. We have changed this behavior by generating protonated oligomers in the gas phase by first forming proton-bound complexes of the oligomers with amino acids or peptides by electrospray ionization. These complexes dissociate first by loss of the amino acid/peptide to form protonated oligomers, which then undergo a unique fragmentation chemistry. In this article we discuss the results for poly(methyl methacrylate) (PMMA) and poly(butyl acrylate) (PBA). Initially, protonated PMMA and PBA lose methanol and butanol, respectively, from the side chains of the respective monomers. The resulting PMMA-derived ion then undergoes a series of neutral losses corresponding to 32 and 28 Da, methanol and carbon monoxide. This continues as collision energy increases until a final, carbon-rich backbone ion is formed, which then undergoes a classic hydrocarbon fragmentation pattern. The PBA-derived ions are proposed to fragment by the loss of butylether molecules to form anhydride rings along the oligomer chain. The number of ether molecules lost corresponded to half the number of available side chains in the oligomer. The resulting poly-anhydride ion dissociates by small molecule loss. A though the predominant mode of ionization for peptides and proteins in mass spectrometry is protonation in solution, the same cannot be said for synthetic polymers. Typically, synthetic polymers such as poly(methyl methacrylate) (PMMA) are not basic enough in solution to acquire a proton from an added acid. To overcome this limitation, salts incorporating metal ions are usually added to the polymer/ solvent solution before electrospray ionization (ESI) (or to the polymer/matrix solution before ionization by matrix-assisted laser desorption/ionization). This generates gas-phase polymer oligomers ionized by the metal ion. This has consequences for the analysis of these oligomers by collision-induced dissociation (CID) mass spectrometry. For example, Jackson and Scrivens studied poly(alkyl methacrylates) (PAMAs), i.e., PMMA and poly(n-butyl methacrylate) (PBMA) by different mass spectrometry techniques [1][2][3][4][5][6][7]. In each case the CID mass spectrum was dominated by low mass-to-charge ratio (m/z) fragment ions. In the case of PMMA the fragments were proposed to be generated mainly by homolytic cleavage and 1,5 hydrogen rearrangement reactions along the polymer backbone [2,7]. Low-intensity peaks at high m/z were assumed to be formed by neutral losses and fragmentation along the polymer backbone [5]. The end result was a variety of fragment ion progressions each differing by one monomer unit. Bowers et al. proposed a mechanism that rat...

Tandem Mass Spectrometry of Trimethylsilyl-Terminated Poly(Dimethylsiloxane) Ammonium Adducts Generated by Electrospray Ionization

“…Both cases were envisaged and modelization of transition states involved in these two pathways was performed for the smallest precursor ions (up to n=5). Results obtained for the calculated transition states clearly show that a concerted elimination of NH 3 and CH 4 would be more favorable than their sequential losses (Table 3). These data also reveal that the energy requested for the single-stage release of two neutrals from the oligomer ammonium adduct increases with the size of the precursor ion.…”

“…Product ions formed upon a concerted release of NH 3 and CH 4 would carry a positive charge located on an end-ofchain silicon atom, that is, the case described after ring opening of the ionic structure generated in step 2′ of Scheme 2. As a result, so-formed product ions would either adopt a more stable cyclic conformation via electron transfer from an oxygen lone pair or release a cyclic PDMS neutral as in step 3′ of Scheme 2, depending on their size.…”

“…In some cases, however, MS/MS experiments do not allow end-group analysis because polymer ion dissociation mainly proceeds via reactions involving pendant groups, such as for sodiated poly(methacrylic acid) [2], deprotonated poly(methacrylic acid) [3], or protonated poly(methyl methacrylate) [4]. The choice of the cation to be adducted to polymer molecules was also shown to have a great impact in the production of informative fragment ions under CID conditions.…”

“…The choice of the cation to be adducted to polymer molecules was also shown to have a great impact in the production of informative fragment ions under CID conditions. For example, while dissociation of protonated poly(methyl methacrylate) (PMMA) was shown to proceed via charge-induced eliminations of methanol from the pendant ester groups [4], alkali-cationized PMMA mainly dissociates to yield low-mass radical ions carrying one of the original chain ends, according to charge-remote mechanisms [5][6][7][8]. Within the same class of metals, the nature of the adducted cation can also influence MS/MS results, as shown in the case of poly(ethylene glycol) for which dissociation of lithiated molecules generates polymeric product ions after internal backbone cleavages or rearrangements, whereas a bare alkali cation was often the only reported product ion arising from dissociation of sodium or potassium adducts [9][10][11][12][13].…”

Multi-stage Mass Spectrometry of Poly(vinyl pyrrolidone) and Its Vinyl Succinimide Copolymer Formed upon Exposure to Sodium Hypochlorite

“…It has been reported that the use of protonation or ammonium adduction (indirect protonation, the protonated oligomer is formed upon the release of ammonia in MS/MS) opens up new fragmentation routes and/or allows a dramatic decrease of the activation energy (i.e., higher signal to noise ratio). [44][45][46] e ESI-MS spectrum of the pristine PVP diluted in a methanolic solution of NH 4 Cl is depicted in Fig. S7 + adducts are seen in the background (Fig.…”