Structural Parameters of Dipalmitoyl Phosphatidylcholine Lamellar Phases and Bilayer Phase Transitions

Abstract

Thicknesses d_l of the lipid bilayer and d_w of the water layer were determined separately with X-rays for dipalmitoyl phosphatidylcholine lamellar phase. In both cases of the presence and the absence of excess water, d_l does not change appreciably at the pretransition (35°C) but drops down by about 5 Å at the main transition (42°C) with increasing temperature. In the presence of excess water, d_w jumps up by 4.0 Å at the pretransition and by 2.0 Å at the main transition. These results prove that there is practically no change of the tilting angle of hydrocarbon chains at the pretransition and that the remarkable increase of the repeat distance of the lamellar phase at the pretransition in the presence of excess water is caused by the increase of the thickness of the water layer.