NEAREST-NEIGHBOR RECOGNITION IN PHOSPHOLIPID-MEMBRANES - A MOLECULAR-LEVEL APPROACH TO THE STUDY OF MEMBRANE SUPRASTRUCTURE

A molecular-level approach has been devised for probing the lateral organization of phospholipid bilayers. This method is based on the equilibration of disulfide-based phospholipid dimers via thiolate-disulfide interchange reactions. Analysis of resulting product mixtures defines the tendency of one phospholipid monomer to become a covalently attached nearest neighbor of another. Investigation of equilibrium mixtures derived from dimeric analogs of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) has yielded conclusive evidence that the lipid monomers are randomly distributed throughout the membrane in the fluid phase and also in the gel-fluid coexistence region. Additional support for random organization in the gel-fluid region has been obtained by use of differential scanning calorimetry (DSC). When the difference in alkyl chain length between the equilibrating monomers is increased from two to four methylene groups (i.e., dimeric analogs of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) are used instead of DPPC), the membrane suprastructure remains random in the fluid phase. In the gel-fluid coexistence region, however, a significant deviation from randomness is observed. This deviation implies the presence of lipid domains and is consistent with the appearance of phase separation, as indicated by DSC analysis. Examination of the temperature dependence of this nearest-neighbor recognition supports the hypothesis that the packing forces that govern such recognition can be very similar to those that govern domain formation. Previous conclusions that have been drawn from quick-freeze DSC experiments with DMPC/DPPC bilayers are critically examined in light of these findings.