Thermotropic phase behavior of the platelet-activating factor: an infrared spectroscopic study.

Thermotropic phase characteristics of the platelet-activating factor 1-O-octadecyl-2-acetoyl-sn-glycero-3-phosphocholine in excess water were studied by Fourier transform infrared spectroscopic techniques. Infrared bands characteristic of the interface and alkyl chain regions were monitored between -4 and 40 degrees C. Spectral parameters derived from these infrared bands demonstrate that in an aqueous environment this highly bioreactive lipid can assume three different structures: a micellar form at temperatures above 21 degrees C, a crystalline lamellar coagel form at temperatures below 12 degrees C, and a lamellar gel form that exists at temperatures between 12 and 21 degrees C. The three structures are separated by two phase transitions: a lower temperature (T1) phase transition at 12 degrees C, that involves a structural rearrangement in the region of the interface, and an upper temperature (Tu) phase transition at 21 degrees C, that involves the "conformational melting" of the alkyl chains.