X-Ray diffraction measurements of dipalmitoylphosphatidylcholine as a function of pressure

Results of the first X-ray diffraction measurements of the lamellar periodicity of dipalmitoylphosphatidylcholine as a function of pressure are presented. Below the gel transition the periodicity decreases with increasing pressure. Above the transition, the periodicity increases with increasing pressure at a rate ∼10 times that below the gel transition. This discovery is interpreted in terms of the relative compressibility within and perpendicular to the bilayer plane. We find the gel transition temperature, T_m, is elevated with pressure. We determine dP_m dT_m ≅ 48 atm./o_C and is constant with pressure. In addition, we find coexistance of gel and liquid crystalline phases near the transition as a function of pressure. We believe these measurements to represent the first observation of the gel transition as a function of pressure using a structural technique.