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

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(+) 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(+). 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(+) 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.