Abstract
This dissertation describes the use of several classes of small molecules that perturb or mimic membrane functions to explore specific membrane-related processes. For example, it was demonstrated that cationic triple-chain amphiphiles facilitate membrane fusion, as compared to double- and single-chained analogues. These triple-chain compounds were observed to promote negative membrane curvatures and most likely act through stabilization of membrane fusion intermediates that contain such inverse curvatures. However, our results also provide support for the extended conformation hypothesis of membrane fusion initiation.
In addition, the properties of two symmetrical, non-natural phospholipids, a conformationally restricted, cyclopropyl-containing compound and more-flexible control analogue, were explored and compared to the properties of natural phospholipids. Both analogues assemble in bilayers, however remarkable differences in the leakage of aqueous contents of vesicles formed by the two compounds were observed. It was found that restriction of the structural flexibility in the interfacial region of these molecules diminishes the permeabilty of their bilayer assemblies through closer packing of individual molecules. In a separate study we explored the extraordinary ability of a series of synthetic compounds containing a steroid backbone and two phenylurea moieties to facilitate transport of chloride anions across phospholipid and cellular membranes. It was established that these anionophores act via a carrier-based anion antiport mechanism. Also, after a structure-activity study of a large number of analogues, it was demonstrated that the activity of these compounds depends on the subtle balance between lipophilicity and anion binding affinity. A different example of ion transport across phospholipid membranes presented in this dissertation demonstrates symport of chloride-sodium/potassium ion pairs. In this case, a ditopic salt-binding macrocyclic compound was used as a transport vehicle. Finally, we describe two generations of synthetic fluorescent compounds that mimic the ability of the protein annexin V to preferentialy bind membrane surfaces that contain negatively charged phospholipids. More importantly, it was demonstrated that these compounds can detect phosphatidylserine at the outer surface of mammalian cells, after its externalization during apoptosis. These annexin mimics, therefore, can be used as fluorescent sensors for detection of apoptotic cells. All synthetic molecules described in this dissertation have potential utility as tools in membrane research and possibly in therapeutical and diagnostic practice.
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