Composition fluctuations in Phospholipid- Sterol Vesicles - a Small-angle Neutron Scattering Study

摘要

Sterols regulate biological processes and sustain the domain structure of cellular membranes. While cholesterol is the major sterol of vertebrates, ergosterol plays a key role in fungal membranes. In this study, small-angle neutron scattering in combination with the H/D contrast variation technique was employed to probe the lateral compositional organization in dipalmi-toyl-phosphatidylcholine/ergosterol (DPPC/erg) model membranes over the temperature range from 26 to 78 °C. In addition, small-angle X-ray scattering and calorimetric measurements were carried out to characterize the phase behavior of this system in the temperature interval covered. The SANS measurements clearly demonstrate the absence of critical-point-like composition fluctuations at the temperature where the liquid-ordered/liquid-disordered (l_o/l_d) two-phase region closes and gives way to the all-fluid phase. Furthermore, no scattering due to large-scale composition fluctuations has been found in the whole l_o+l_d two-phase region of the system. It is concluded that this binary phospholipid-sterol mixture in the l_o+l_d coexistence region exhibits a homogeneous membrane with fluctuating nano-scale domains rather than a microscopically separated two-phase coexistence region as observed for ternary phospholipid-sterol mixtures. Such small domains are expected to have particular properties like an increased line energy, a spontaneous curvature and limited lifetimes, which will probably also prevail for the small raftlike domains in cellular membranes.