The phase transition behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) model membrane influenced by 2,4-dichlorophenol -: an FT-Raman Spectroscopy Study

The effect of 2,4-dichlorophenol (DCP) on the structures and phase transitions of fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes was studied using FT-Raman spectroscopy. Whereas the Raman frequency shifts of the most frequently investigated bands of C-C and C-H stretching regions only indicate the main phase transition (PP-L.) of the pure DPPC/water system, the Raman shift of C-H scissoring vibration at 1440 cm(-1) was found to be able to reveal the pretransition (L-beta'-P-beta') as well. Analyzing the spectral parameters of the trans band at 1128 cm(-1), which does not overlap with DCP vibrational modes, a continuous decrease of trans conformations was found with increasing DCP concentration at 26 degrees C accompanying the phase transitions L-beta'-P-beta' and P-beta'-L-alpha. The intensity ratio of the symmetrical and asymmetrical methylene stretching bands (at 2850 cm(-1) and 2880 cm(-1)), defined as the disorder parameter by Levin [Levin, I.W., 1985. Two types of hydrocarbon chain interdigitation in sphingomielin bilayers. Biochemistry 24, 6282-6286], indicated that in the interdigitated phase (L-1) the order is markedly high and comparable with that of L-beta. Both the phase transition P-beta'-L-alpha in the DCP/DPPC molar ratio range of 10/100-50/100 and the phase transition L-1-L-alpha led to a significant increase of disordered chains and the presence of DCP molecules induced a more disordered chain region than that observed in the L-alpha phase of DPPC. Nevertheless, it was found that the L-alpha phase with DCP contains approximately the same amount of trans conformers than that without DCP. (C) 2005 Elsevier Ireland Ltd. All rights reserved.