Study of the gas-phase decomposition pathways of poly(ethylene glycol) by Electron Transfer Dissociation

Abstract

Tandem mass spectrometry involving electron transfer dissociation (ETD) could be very useful for polymer characterization. However, parameters influencing gas-phase decomposition pathways are poorly understood and this method is rarely applied. In this report, precursor ions [polyether + cation(s)]2+ generated from three poly(ethylene glycol) (PEG), which differ only in their chain ends (dimethoxy PEG, monomethoxy/monohydroxy PEG or dihydroxy PEG), were submitted to ETD conditions to study the influence of the cationizing agent, and that of the chain ends on the dissociation pathways. Experiments were carried out using various cationization agents: H+, alkali metals (Li+, Na+ and Cs+), an alkaline earth (Ba2+) and transition metals (Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+). The gas phase interaction of a fluoranthene anion with precursor ions [PEG24Me2+cation(II)]2+ or [PEG24Me2+2 cation(I)]2+ gives very variable results depending on the metal. Bond dissociation is not always observed, because competitive reactions can lead to the formation of sufficiently stable species (by addition of fluoranthene, simple reduction of the complex, elimination of the metal, or by addition of O2), to the detriment of fragmentation reactions (in particular with the monovalent cations Na+ and Cs+). In contrast, from [PEG24Me2+cation(II)]2+ all divalent cations lead to bond cleavages. These cations can be classified according to their ability to cause the highest fragmentation: Fe2+ > Ba2+ > Mn2+ ≈ Ni2+ > Co2+ > Cu2+ > Zn2+. Although the Fe2+ adduct presents the highest fragmentation rate, the spectrum is dominated by the loss of a small molecule (ethylene), the Ni2+ cation gives the richest fragmentation (the number of fragment series was the most important). Within the same family of cationization agents (alkaline or transition metals), the processes implemented under ETD conditions can sometimes be very different. Some fragment ions are common to a set of metals (e.g. the an ion series is generated from all adducts), while others are more specific (e.g. the loss of ethylene from Mn2+, Fe2+ and Co2+ adducts). The C–O bond is the most frequently broken bond. From monomethoxy/monohydroxy PEG and dihydroxy PEG, the presence of a hydroxyl function at one end of the chain can compromise bond dissociation along the carbon skeleton if H+ loss is favored, but the use of a transition metal such as Mn and Fe, induces nevertheless fragmentation. This work establishes that ETD of polyether backbone is possible if a suitable cation is chosen (Fe (II), Mn (II) for example), regardless of the chain end (-OCH3 or –OH).