Molecular self-assembly is of key importance for the rational design of advanced materials. To investigate the causal relation between molecular structure and the consequent self-assembled microstructure, self-assembled tubules of diacetylenic lipids were studied. Circular-dichroism studies give experimental evidence that the formation of tubules is driven by chiral molecular packing, in agreement with recent theories of tubules. On the basis of these results, a molecular mechanism for the formation of tubules is proposed.

We report on spectroscopic studies of the chiral structure in phospholipid tubules formed in mixtures of alcohol and water. Synthetic phospholipids containing diacetylenic moieties in the acyl chains self-assemble into hollow, cylindrical tubules in appropriate conditions. Circular dichroism provides a direct measure of chirality of the molecular structure. We find that the CD spectra of tubules formed in mixtures of alcohol and water depends strongly on the alcohol used and the lipid concentration. The relative spectral intensity of different circular dichroism bands correlates with the number of bilayers observed using microscopy. The results provide experimental evidence that tubule formation is based on chiral packing of the lipid molecules and that interbilayer interactions are important to the tubule structure.The self-assembly of biologically based lipids into unusual microstructures has been the subject of intense study in recent years, both for basic research and for potential applications in areas ranging from controlled release to electroactive composites (1, 2). Most long-chain phospholipids self-assemble into spherical bilayer aggregates, known as liposomes (3). However, certain synthetic phospholipids, with modified head groups or acyl chains, self-assemble into novel microstructures (4). One class of synthetic phospholipids, with photopolymerizable diacetylenic moieties in the acyl chains, was originally developed as an approach for increasing the durability of lipid bilayer aggregates (5-7). These lipids have been observed to self-assemble into hollow, cylindrical structures, known as tubules (8,9). Similar cylindrical tubules have also been observed in other synthetic surfactants (10) and in bile (11). Such tubules appear to have potential for long-term release applications such as marine antifouling (12).To explain the formation of tubules, several investigators have developed theories based on molecular chirality (11,(13)(14)(15)(16)(17)(18). Although the details of these theories differ, they are all based on the principle that chiral interactions cause the molecules to pack at a nonzero angle with respect to their nearest neighbors. This chiral packing induces a twist in the bilayer, which results in the formation of a cylindrical structure. Electron microscopy of tubules decorated with metallic particles show helical markings that suggest chiral order in the bilayers (19,20).In an earlier study, we used circular dichroism to test the theoretical concept that the formation of tubules is driven by chiral molecular packing (20). CD, the difference in the absorption of right and left circularly polarized light, arises from the chirality of a molecular architecture. This chirality can arise from either the structure of individual molecules or from the chiral packing of molecules into larger aggregates (21,22). Our experiments showed that diacetylenic lipid tubules have a very strong CD signal. By contrast, spherical liposomes of the same diacetylenic lipids have only a very weak CD signa...

Pyranine entrapped soylipid liposomes have been used as a model system to study the proton transport across membrane in the presence of A23187, a carboxylic ionophore specific for electroneutral exchange of divalent cations. An apparent rate constant (k app ) for transport of protons has been determined from the rate of change of fluorescence intensity of pyranine by stopped flow rapid kinetics in the presence of proton gradient The variation of k app has been studied as a function of ionophore concentration and the results have been compared with gramicidin-a well known channel former under the similar experimental conditions. The rates thus obtained showed that A23187 is not only a simple carrier but also shows channel behaviour at high concentration of ionophore.

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