Alcohols dehydrate lipid membranes: an infrared study on hydrogen bonding

Abstract

The effects of alcohols (methanol, ethanol, and n-butanol) on the hydrogen bonding of dipalmitoylphosphatidylcholine (DPPC) were studied by Fourier-transform infrared spectroscopy (FTIR) in water-in-oil (carbon tetrachloride) reversed micelles. The bound O-H stretching mode of water, bonded to DPPC, appeared as a broad band at around 3400 cm-1. The O-H bending mode of this complex appeared as a weak broad band at 1644 cm-1. No free O-H signal was observed. When alcohols were added, a part of DPPC-bound water was replaced by the alcohols. The released 'free' water appeared at 3680 cm-1. This free O-H stretching band represents water-alcohol complex. A new broad band of O-H stretching appeared at 3235 cm-1, which represents the alcohol molecules bound to the phosphate moiety of DPPC. When the alcohol concentration was increased, the intensities of the free O-H stretching and bending bands increased. The P = O- antisymmetric stretching band at 1238 cm-1 became broader and shifted to lower frequencies. This means that alcohols interacted with the phosphate moiety and replaced the bound water. In the deconvoluted spectra of the C = O stretching mode, the ratio between the free sn-2 and the hydrogen-bonded sn-2 bands increased; a part of the bound water at the sn-2 carbon in the glycerol skeleton is also released and the free sn-2 signal increased. From the change in the intensity of the P = O- stretching band, the partition coefficients of alcohols between the phosphate region of DPPC and water were estimated: methanol 7.8, ethanol 16.7 at 22.0 degrees C in mole fraction bases. In molality, these values translates into methanol 0.21 and ethanol 0.45. These results indicate that short-chain alcohols interact with lipid membranes at the phosphate moiety at the hydrophilic head, weaken the membrane-water interaction, and destabilize membranes.