Alkali-cation affinities of polyoxyethylene dodecylethers and helical conformations of their cationized molecules studied by electrospray mass spectrometry

Abstract

Relative alkali-cation affinity of polyoxyethylene (POE) dodecylethers in gas phase was studied by electrospray ionization (ESI) mass spectrometry using dodecylether-poly-ethoxylate (C 12 EO:n, "n" denotes ethyleneoxide unit number) nonionic surfactants, and possible helical conformations of the cationized molecules were demonstrated. The alkali-cation affinity highly depended on the cation diameters. The mass spectra of C 12 EO:8 cationized by alkali-metal ions were dominated by potassiated molecules. The results indicated that the POE moiety could have specific affinity to K ϩ ions based on a host-guest interaction between POE helix and potassium ions. This is very similar to the relationships between 18-crown-6 and K ϩ . The ESI mass spectra exhibited the multiply cationized C 12 EO:n in addition to the singly cationized molecules. The critical EO unit numbers necessary for producing the multiply-charged cationized molecules also depended on the cation diameters. In addition, the POE surfactants highly preferred alkali cations to proton. The results were strongly supported by molecular mechanics/dynamics calculations. A helical conformation of the POE moiety of C 12 EO:15 including two K ϩ ions gave a potential minimum, while a lowest energy structure of the protonated molecule took irregular conformations due to the formation of local hydrogen bonds. (J Am Soc Mass

Conformational Study for Ionic Aggregates of Poly(oxyethylene) Dodecyl Ethers with Ferric-Thiocyanate Anion through the Intermediary of Potassium Ion

“…The mass-spectrometric procedure was based on our previous work, 23 although the ion-trap MS was used this time in place of a quadrupole MS used in earlier work.…”

“…The initial structure of the polyethoxylate with the helical conformation used in this study was constructed in the same way as described in our previous paper. 23 The equations of motion were integrated in the NPT ensemble with a temperature of 300 K and a pressure of 1 atm using the Verlet integrator scheme with a step size of 1 fs. Non-bonded interactions were truncated at 14 Å on a neutral group-by-group basis after being made to go smoothly to zero at between 12.0 and 13.5 Å using switching functions.…”

“…This can successfully indicate the decrease in stability of the ionic aggregate, leading to the decrease in colorimetric sensitivity. Since the stability in conformation of such a supramolecule depends on both the EO unit number and the cation size for inclusion, 23 a less-stable helical conformation can only be established by POENS oligomers having EO unit numbers less than 7.…”

“…23 The mass spectra have exhibited intense peaks of potassiated molecules of the POENSs significantly stable rather than the other alkali-cationized molecules. The mass spectral results have been well-defined by the estimation that such a non-cyclic POE chain can include K + inside, leading to a stable helical conformation, probably due to the size-fitting effect and electrostatic interaction between K + and the electron-rich EO moiety.…”

“…24,25 In addition, such a mass spectral interpretation has been supported theoretically by molecular mechanics/dynamics simulations, which can provide energy-minimum conformations in the gas phase. 23 This paper describes the formation mechanism and possible conformation of the ionic aggregate between a poly(oxyethylene) nonionic surfactant and a ferric-thiocyanate complex through an intermediary of the potassium ion. The stoichiometry of ferric thiocyanate taking part in the ionic aggregate has been made clear, and some convincing explanations or energy-minimized A possible formation mechanism of the ionic aggregate of poly(oxyethylene) dodecyl ether nonionic surfactants with a ferric-thiocyanate chromophore is presented, and the dependence of their colorimetric sensitivity on the degree of ethoxylation can be clearly explained through both experimental and theoretical approaches.…”