The kinetics of formation and structure of the low-temperature phase of 1-stearoyl-lysophosphatidylcholine.

A combination of differential scanning calorimetry (DSC) and X-ray diffraction have been used to study the kinetics of formation and the structure of the low-temperature phase of 1-stearoyl-lysophosphatidylcholine (18:0-lysoPC). For water contents greater than 40 weight %, DSC shows a sharp endothermic transition at 27 degrees C (delta H = 6.75 kcal/mol) corresponding to a low-temperature phase----micelle transition. This sharp transition is not reversible, but is regenerated in a time and temperature-dependent manner. For example, with incubation at 0 degrees C the maximum transition enthalpy (delta H = 6.75 kcal/mol) is generated in about 45 min after an initial slow nucleation process of approx. 20 min. The kinetics of formation of the low-temperature phase is accelerated at lower temperatures and may be related to the disruption of 18:0-lysoPC micelles by ice crystal formation. X-ray diffraction patterns of 18:0-lysoPC recorded at 10 degrees C over the hydration range 20-80% are characteristic of a lamellar gel phase with tilted hydrocarbon chains with the bilayer repeat distance increasing from 47.6 A at 20% hydration to a maximum of 59.4 A at 39% hydration. At this maximum hydration, approx. 19 molecules of water are bound per molecule of 18:0-lysoPC. Electron density profiles show a phosphate-phosphate distance of 30 A, indicating an interdigitated lamellar gel phase for 18:0-lysoPC at all hydration values. The angle of chain tilt is calculated to be between 20 and 30 degrees. For water contents greater than 40%, this interdigitated lamellar phase converts to the micellar phase at 27 degrees C in a kinetically fast process, while the reverse (micelle----interdigitated bilayer) transition is a kinetically slower process (see also Wu, W. and Huang, C. (1983) Biochemistry 22, 5068-5073).