A combined FTIR and DSC study on the bilayer-stabilising effect of electrostatic interactions in ion paired lipids

Maximilian Schmid, Christian Wölk, Julia Giselbrecht, K.L. Andrew Chan, Richard D. Harvey

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14 Citations (Scopus)
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Abstract

Investigating lipid ion pair formation is important for understanding the mechanisms of lipid-mediated drug resistance in bacteria. In this study we have used the charged amphiphiles dipalmitoylphosphatidylglycerol (DPPG) and dihexadecyldimethylammonium bromide (DHDAB), as a model to evaluate the formation of ion pairs by a combined Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) analysis. FTIR was employed to study the environment of the DPPC headgroup phosphate and lipid/surfactant alkane chains, in vesicles formed by the two amphiphiles mixed in various molar ratios. An increase of the absorbance ratio of 1221 to 1201 cm−1 in the asymmetric phosphate stretching mode was found to follow a sigmoidal relationship with the proportion of DHDAB, increasing to a plateau above a DPPG/DHDAB 1:1 molar ratio of, providing evidence that the PG headgroup phosphate is involved in ion pairing. A consistent red shift was measured for the position of the symmetric CH2 stretch band for the lipid/surfactant 1:1 molar ratio mixture, which is indicative of an increased ordering of the hydrophobic chains. The DSC experiments yielded information about the thermotropic and the mixing behaviour of the lipid/surfactant systems. DPPG and DHDAB seem to form an ion pair with cluster compound characteristics at the equimolar ratio. Most interestingly, the DPPG/DHDAB 2:1 molar ratio mixture is characterized by strong intermolecular interactions, which result in a pronounced stabilization of the gel phase, possibly through the formation of a closely-associated ion triplet configuration in which the charges are delocalised across the headgroups.
Original languageEnglish
JournalColloids And Surfaces B-Biointerfaces
DOIs
Publication statusPublished - 18 May 2018

Keywords

  • Model bacterial membranes
  • differential scanning calorimetry
  • Fourier transform infrared spectroscopy
  • ion pair
  • zeta potential
  • liposome

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