The hydrophobic-hydrophilic interface of phospholipid membranes studied by lamellar X-ray diffraction

We measured the lamellar X-ray diffraction of Dilauroyl Phosphatidylcholine (DLPE) in the form of oriented multilayers, with their water content carefully controlled by vapor pressure. Consistent diffraction patterns were recorded along the 35.0 degreesC isotherm, with the relative humidity varying from similar to5% to similar to98%, covering both the gel phase and the fluid phase. In the gel phase, each side of the electron density profile showed a main peak and a secondary peak. The separation I-h between the two peaks was the same for both lipids and constant throughout the gel phase. These two peaks also appeared after the phase transition to the fluid phase, with the same peak separation, I-h = 4.85 Angstrom. A comparison with crystallographic data allowed us to identify the secondary peak as the location of the hydrophobic-hydrophilic interface. Our study verifies that the molecular structure of the phospholipid headgroup region is the same in the gel phase and the fluid phase. It also provides a formula, which is practically identical to the currently accepted one proposed by other investigators, which gives the membrane's hydrophobic thickness.