An analysis of the X-ray interchain peak profile in dipalmitoylglycerophosphocholine

The primary X-ray peak profile characterizing the interchain structure in the dipalmitoylglycerophosphocholine membrane has been measured as a function of temperature. The scattering between 23 and 34.6 degrees C is characterized by an asymmetric crystalline reflection accounting for 85% of the total intensity, the remaining 15% being liquid-like in character. At a pre-transition temperature of 34.6 degrees C, the reflection profile becomes (nearly) symmetrical, indicating a change in tilt angle of the chains with respect to the membrane surface. This change is accompanied by an increase of 20% in the amount of liquid-like scattering, indicating that the pre-transition mechanism includes a partial melting of the chains. At the melting point, 41.5 degrees C, the crystalline reflection disappears, and the liquid component of the scattering increases to a point where it includes all the scattered intensity. The relative values of the integrated intensities at each temperature are tabulated, and the significance of the peak widths and shapes are discussed.