Interfacial dehydration by alcohols: hydrogen bonding of alcohols to phospholipids.
The interaction between alcohols (ethanol and n-butanol) and dipalmitoylphosphatidylcholine (DPPC) in carbon tetrachloride was studied by Fourier transform infrared spectroscopy (FTIR). Upon addition of the alcohols, the P = O stretching band of DPPC at 1260 cm-1 shifted to lower frequency (red-shift). The red-shift indicates that the P = O vibration became slower possibly because the heavier alcohol molecules replaced the water molecules hydrogen bonded to the PO2 moiety. The formation constants between the PO2 group and ethanol or n-butanol (n-butanol data in parenthesis) were 19.0 M-1 (7.1 M-1) when estimated from the spectral change. A new absorbance peak appeared at 3265 cm-1 (3275 cm-1) representing the DPPC-alcohol complex. The formation constant of this complex was also 19.0 M-1 (7.1 M-1). The identical formation constant suggests that the DPPC-alcohol complex was formed at the PO2 moiety of DPPC with hydrogen bonding to the alcohol OH. At higher alcohol concentrations, the absorbance peak of DPPC-alcohol complex shifted to 3225 cm-1 (3235 cm-1). Apparently, the lower frequency shift at higher alcohol concentration occurred by the formation of alcohol multimers (dimer, trimer, and tetramer) interacting with DPPC.