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

Abstract

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°C ( ΔH = 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°C the maximum transition enthalpy ( ΔH = 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°C over the hydration range 20–80% are characteristics of a lamellar gel phase with tilted hydrocarbon chains with the bilayer repeat distance increasing from 47.6 Å at 20% hydration to a maximum of 59.4 Å 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 Å, 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°. For water contents greater than 40%, this interdigitated lamellar phase converts to the micellar phase at 27°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).