Observation of poly(ethylene glycol) clusters with the chlorine anion in the gas phase under electrospray conditions

Abstract

It is demonstrated herein that poly(ethylene glycol) (PEG) oligomers can form stable complexes with the chlorine anion in the gas phase as evidenced by results from electrospray ionization mass spectrometry (ESI-MS) and molecular dynamics simulation. While the formation of crown-ether-like structures by acyclic polyethers in their complexes with alkali metal cations coordinated by the ether oxygen atoms has been extensively studied, the possibility of forming 'inversed' quasi-cyclic structures able to bind a monoatomic anion has not been proved till now. We have observed the formation of stable gas-phase complexes of oligomers of PEG-400 with the Cl(-) anion experimentally by ESI-MS for the first time. It is suggested that a necessary precondition for obtaining the polyether-chlorine anion clusters is the prevention of the formation of neutral ion pairs. Molecular dynamics simulation has demonstrated the wrapping of the Cl(-) anion by the PEG chain, to stabilize the PEG(n)•Cl(-) clusters in the gas phase. The conformation of the polyether chain in such quasi-cyclic or quasi-helical complexes is 'inversed' compared with that in the complexes with cations: that is its hydrogen atoms are turned towards the central anion. Awareness of the possibility of the Cl(-) anion being trapped in quasi-cyclic PEG structures may be of practical importance when considering the intermolecular interactions of PEGs.

Interactions of oligomers of organic polyethers with histidine amino acid

“…In contrast, lower charge density on an organic histidine polyatomic cation decreases the efficiency of its binding with the Cl – anion, thus allowing clusters of polyethers with both cation and anion to be observed under electrospray conditions. Another explanation suggested in our previous paper, which dwells upon the electrochemical nature of the electrospray process, finds further confirmation in the current research. It is known that high electric potential applied to the electrospray capillary (needle) is necessary to create the excess of either positive or negative ions in the sprayed droplets via oxidation or reduction reactions respectively .…”

“…Thirdly, clusters of polyethers with the Cl – anion, observed in the negative ion mass spectra of the systems containing histidine hydrochloride and some other organic salts, were absent in the mass spectra of the systems containing inorganic chloride salts under similar electrospray conditions. A straightforward qualitative explanation suggests that the efficient formation of neutral ion pairs of the Cl – anion (or a chlorine‐oligomer complex) with stable complexes of polyethers with monoatomic metal cations prevents the observation of chlorine‐oligomer clusters for the systems with inorganic salts.…”

“…In the positive ion mode ( Fig. 2(a)) protonated histidine-oligomer associates PEG n •His•H + (n = 3-16, max at n = 8) were present, while the PEG n •Cl -(n = 4-15) set described in our earlier work [20] was recorded in the negative ion mode ( Fig. 2(b)).…”

“…Computer modelling has shown that in associates of metal ions with polyethers the metal cation coordinates ether oxygen atoms of the polyether chain which adopts quasi‐cyclic crown ether‐like or quasi‐helical conformations . We have reported recently on formation of stable quasi‐cyclic complexes of polyethers with the chlorine anion …”

“…[10][11][12][17][18][19] We have reported recently on formation of stable quasi-cyclic complexes of polyethers with the chlorine anion. [20] Noncovalent complexes of acyclic polyethers with organic and biomolecules are less studied by mass spectrometry. So far only a few works on the subject have appeared.…”