NMR Studies of Liquid Disordered and Liquid Ordered Phase Coexistence in Model Membranes
Biological membranes separate the cell from the environment. A fundamental understanding of multi-component model membrane systems may provide insight into the phase behaviour and the relationship between the lipids, proteins, water and temperature in biological membranes containing phase separated domains or ‘rafts’. Model membranes composed of saturated and unsaturated phospholipids with cholesterol can exhibit coexisting liquid disordered (ld) and liquid ordered (lo) phases. Peptides can modify the phase behaviour of these model membranes and may also partition preferentially into either fluid phase. Several model membranes were investigated using 2H NMR to determine the phase behaviour of lipid bilayers with and without peptides. Critical fluctuations have been observed in systems exhibiting ld-lo coexistence. Multilamellar dispersions of DOPC/DPPC/cholesterol exhibit critical phase behaviour that agrees with systems belonging to the 3D Ising universality class. Coexisting ld-lo phases were observed in magnetically aligned DMPC/cholesterol/DCPC bicelles with cholesterol mole fractions ≥0.13. DPoPC/DMPC/cholesterol/DCPC or DOPC/DPPC/cholesterol/DHPC bicelles also showed two fluid phase coexistence. 2D exchange experiments were used to study the exchange of lipids between the ld and lo phase domains. DPoPC/DMPC-d54/cholesterol multilamellar dispersions showed broadening due to lipids exchanging between the two phases. Corresponding bicelles however did not show exchange. The presence of the short chain lipid may move the sample away from a critical composition eliminating the exchange phenomena. Multilamellar dispersions of DOPC/DPPC/cholesterol with gramicidin A resulted in very broad and weak 2H signals and did not provide evidence of preferential partitioning into either of the two fluid phases. Two short peptides, Conolysin-Mt1 and ALGA, were studied in aligned lipid bilayer samples. The lipid chain order in DMPC/DCPC bicelles was affected slightly by Conolysin-Mt1. DPoPC/DMPC/cholesterol with ALGA on glass slides showed that some of this peptide is undergoing rapid axial reorientation while the rest is in a powder environment. Preferential partitioning of ALGA into either the ld or lo phase was not observed in these experiments.