Temperature-dependence of the solubilization of dipalmitoylphosphatidylcholine (DPPC) by the non-ionic surfactant Triton X-100, kinetic and structural aspects

Most of the studies on the solubilization of model membranes conducted thus far involved model membranes made of liquid-crystalline phospholipids. Relatively little is known on the influence of temperature and of the phase of the lipid bilayers on their solubilization by detergents. The aim of the present study was to gain knowledge about the temperature and phase dependence of the solubilization of phospholipid bilayers by the non-ionic detergent Triton X-100 (TR). Detailed investigation of the kinetics of the solubilization of dipalmitoylphosphatidylcholine (DPPC), as well as of palmitoyloleoylphosphatidylcholine (POPC) by TR at different temperatures reveals that: (i) solubilization of DPPC is a relatively slow process, especially below T_m. This means that in order to prevent misleading conclusions it is important to monitor the solubilization after a steady state is established. (ii) Both the steady state structure and size of DPPC/TR aggregates and the kinetics of solubilization depend on temperature. (iii) The TR concentration required for solubilization of POPC bilayers is an increasing function of temperature, although no phase change of bilayers occurs in the studied temperature range. (iv) Detailed studies of the temperature-induced changes of the aggregates present in DPPC/TR or POPC/TR mixtures suggest that the state of aggregation at any temperature above 23°C represents equilibrium. By contrast, for DPPC/TR mixtures at 4°C all the processes are very slow, which complicates the interpretation of results obtained through the common practice of studying "rafts" by investigating detergent-resistant membranes.