This research program focuses on the development of a new approach to the study of phospholipid organization that is based on the lipid's tendency to form homodimers. This technique has been termed, nearest- neighbor recognition (NNR). Because this method provides unambiguous and quantitative information within fluid bilayers, it represents a unique opportunity for addressing questions that have not previously been possible. The immediate objectives of this research are to synthesize disulfide-based phospholipid dimers that bear exchangeable monomer units that mimic phosphoglycerols, phosphocholines, phosphoethanolamines, and sphingomyelins. These lipids will then be used in NNR experiments to test the following hypotheses: (1) The immiscibility of ester and ether phospholipids will be enhanced when anionic head groups are replaced by ones that are zwitterionic. (2) Differences in head group charge can provide a driving force for lipid segregation. (3) Hydrogen bonding within the head group region, and also the glycerol backbone, can promote lateral heterogeneity. (4) Hydrophobic mismatch between phospholipids and integral peptides can induce lipid clustering. (5) The presence of cationic peptides can promote the lateral separation of anionic from zwitterionic phospholipids. In addition, efforts will be made to extend the NNR method to more complex and more biologically-relevant membranes such as reconstituted erythrocyte ghosts. The long-term objective of this program is to develop a fundamental understanding of the two-dimensional organization of phospholipid membranes in the physiologically-relevant fluid phase. In principle, such an understanding should help to bring exploitable targets (e.g., the plasma membrane of cancer cells, bacterial cells, and fungal cells) into sharper focus, which could assist the rational design of novel classes of therapeutic agents.
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