Solvent-Free MALDI Investigation of the Cationization of Linear Polyethers with Alkali Metals

Abstract

The MALDI technique with solvent-free sample preparation has been applied to evaluate relative gas-phase affinities of polyether chain polymers with alkali metal cations. The study is performed on poly(ethylene glycol) and poly(propylene glycol) polymers of different lengths (PEG600, PEG1000, PPG425, PPG750) and the alkali metal cations Li(+), Na(+), K(+), and Cs(+). The experiments show that the lattice energy of the alkali metal salts employed as cation precursors can have a strong influence on the outcome of conventional MALDI measurements. With the solvent-free method, these crystal binding effects can be made negligible by combining in the same sample alkali metal salts with different counterions. The recorded MALDI spectra show that the polyether-cation aggregation efficiencies decrease systematically with growing cation size. This cation size selectivity is considerably enhanced for the polymers with the shorter chains, which can be attributed to the reduced ability of the polymer to build a coordination shell around the larger cations. The steric effects introduced by the side CH3 group of propylene glycol with respect to ethylene glycol also enhance the preference for cationization of the polymer by the smaller cations. These observations correct some qualitative trends derived from previous studies, which did not account for lattice energy effects of the cation precursors.

Binding Selectivity of Macrocycle Ionophores in Ionic Liquids versus Aqueous Solution and Solvent‐free Conditions

“…In these experiments, the difference in lattice energy of the alkali metal salts employed as cation precursors prevents the use of more than two cations in as ingle sample. [31,32] The solvent-free measurements corroborate the behavior found forc yclodextrins and polyethers in previous investigations; [31,32] in the absence of solvent, the stabilityo ft he complexes is highest for Li + ,a nd decreases rapidly with increasingc ation size. This is ac onsequence of the greater charge density of the smaller cations,w hich leads to at ighter coordination with the oxygen atoms of the host, involving shorter interaction distances.…”

“…Previous studies have demonstrated that the stabilityo f these inclusion complexes in the gas phasei sm aximal for Li + and decreases rapidly with the size of the cation. [31,32] This was specifically shown for cyclodextrins in solvent-free laser desorptione xperiments in which,f or instance, a1 0-foldg reater abundance was found for the aCD-Li + complex relative to the aCD-K + complex. [31] Similar solvent-free measurements performed here for the CI calixarene corroboratet hose findings (see Section 2.2).…”

“…This was achieved by combining pairs of alkali metal salts of similar lattice energy in each sample, such as LiI/NaBr,N aI/KCl, KBr/RbCl, KI/CsBr, and RbBr/CsCl (see Ref. [31,32] for details).…”

Theoretical Study on 15-Crown-5 Complex With Some Metal Cations

“…In this study, the interaction energies for the 15-crown-5 complexes fall in this order: K + < Na [7][8]. Qualitatively it has also been observed in experiments for non-cyclic polyether's [19].…”

Effect of Dihydroxybenzoic Acid Isomers on the Analysis of Polyethylene Glycols in MALDI-MS

“…8 The advent of solvent-free sample preparation has made MALDI mass spectrometry (MALDI-MS) applicable to polyether chain polymers 9 or polymers that are insoluble in common organic solvents. [10][11][12][13] The solvent-free method is known to have several advantages, including investigations of the MALDI mechanism, 9,14 the possibility of multi-sample preparations, 15 analyses of insoluble synthetic polymers, 11 and the removal of potential solvent-induced chemical modifications.…”