Molecular mechanisms of phosphatidylcholine monolayer solidification due to hydroxyl radicals

Abstract

The reaction of L-α-1,2-dipalmitoylphosphatidylcholine (DPPC) with hydroxyl radicals (HO˙ from Fenton solutions) was investigated using monolayer techniques: isotherms, infrared reflection absorption spectroscopy (IRRAS), grazing incidence X-ray diffraction, X-ray reflection and fluorescence microscopy. The DPPC monolayer is attacked with different HO˙ concentrations. The decrease in the lateral pressure was used as a measure of the efficiency of the HO˙ attack. With increasing HO˙ concentration, the plateau region in the isotherm was shifted to a lower surface pressure; eventually it disappeared. Fluorescence microscopy during the HO˙ attack showed that new domains in the condensed phase nucleate immediately. With isotherms and X-ray diffraction we found that the monolayer can be compressed to smaller molecular areas, with a reduced tilt angle of the alkyl chains. IRRAS experiments indicated a partial cleavage of the head group leading to a reduced head group size.