Phospholipid Flip-Flop and Molecular Transport Across Biomembranes

This dissertation describes the ability of four classes of synthetic small molecules to promote the transport of anions across biomembranes without disturbing membrane integrity. The anions are either glycerophospholipids or anionic small dyes. The first motivation for exploring these transport molecules was to better understand the minimal supramolecular requirements to achieve membrane transport. The second motivation of this research was to find biological applications for these synthetic small molecules. Bolaamphiphile molecules, which have two hydrophilic ends connected by a hydrophobic core, in a bilayer membrane have been shown to facilitate transport of fluorescently labeled phospholipids in vesicles. While bolaamphiphiles are generally thought of as membrane rigidifying agents, these two classes of bolaamphiphiles facilitate phospholipid translocation and perturb membrane integrity in vesicle systems. Two classes of hydrogen bonding transporters were studied, the cholates and tris(2-aminoethyl)amine) (TREN) compounds. They appear to work by associating with the phosphate portion of the phospholipid head group; thus, masking the polarity and raising the probability for the complex to enter the hydrophobic membrane core. The steroid-derived cholates have a bis-urea anion-binding pocket. In general, it seems that the apparent binding constant does not have a direct correlation to the translocation half-life of a cholate:phosphatidylcholine complex. A cationic cholate series was shown to facilitate the exposure of phosphatidylserine on the surface of erythrocytes. The synthetic scramblase class based on the structure of TREN also transports phospholipids across vesicle and nucleated cell's plasma membranes. The decrease in translocation of phosphatidylserine as compared to phosphatidylcholine may be due to the residence time of the TREN localized at the plasma membrane. The last class of small molecule sensors and scramblases are the zinc coordinated dipicolylamine compounds that coordinate to the head group of anionic phospholipids and facilitate the transport of anions across the membrane. This research has lead to the development of compounds that can alter or sense the phospholipid asymmetry and composition of plasma membranes for use as biological sensors.