|Ph.D Student||Kolik Shmuel Lubov|
|Subject||Mechanistic Aspects in the 1D Self-Assembly of lipids into|
Nano- and Microtubes
|Department||Department of Biotechnology and Food Engineering||Supervisor||Professor Dganit Danino|
In nature, 1D supramolecular structures are expressed on various length scales. They prevail from the formation of nanotubes by lipids, steroids and their mixtures, to templating of the chiral property onto the inorganic phase organization at the organism level as in sea shells and insect exoskeletons. Fibrillation is also associated with many human amyloid diseases, including Alzheimer, type II diabetes and multiple sclerosis, thus motivating research from various bio-related fields. Apparent advantages of 1D molecular assemblies (e.g., structural strength, mechanical rigidity, stability), together with their structural diversity, foster the application of natural building blocks and their mimics (e.g. nanowires, biosensors, drug delivery systems).
Here we focus on the principles of 1D self-assembly of chiral/ achiral amphiphiles into ribbons and nanotubes. Interestingly, we find several distinct self-assembly pathways, all leading eventually to identical morphologies.
A specific class of synthetic lipids comprising diacetylene moieties in their tails, diacetylenic phospholipids, were shown to present unique self-assembly, and form multilamellar nanotubes in alcohol-water solutions. The first tube-forming amphiphilic lipid - 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine, designated DC8,9PC, has highly ordered and kinked alkyl chains which were found to induce chiral self-assembly, regardless whether the constituent molecules are chiral. Previous work focused on the properties and possible applications of the tubes, yet fundamental questions regarding their formation mechanisms were not addressed.
By applying cryo-TEM, a powerful method enabling the direct visualization of complex nanostructures while preserving their natural state, we provided the first direct evidence to the route by which multilamellar lipid nanotubes are formed, showing sequential events during the transformation of uni- and multilamellar lipid vesicles to corresponding uni- and multilamellar nanotubes, though membrane attachment. Here, using 3D cryo-TEM analysis by cryo-electron tomography (cryo-ET), an emerging powerful method for 3-dimensional reconstruction of structures in solution, we show that the multilamellar structure of the tubes uniquely emerges by the attachment of membrane patches. Cryo-ET enabled to resolve overlapping morphologies and to understand complex membrane-tubular intermediate assemblies identified through the self-assembly pathway.
Aiming towards an even further understanding the self-assembly mechanism, we investigated the influence of charge and the addition of a short spacer lipid to the diacetylenic phospholipid, DC8,9PC. Interestingly, the mechanism of membrane attachment was affected only when disturbing the structure of the lipid hydrocarbon chain area, causing the system to rearrange through a different pathway.
Further, the combination of direct-imaging cryo-TEM with a thermodynamic technique, differential scanning calorimetry (DSC), has proven as extremely efficient and valuable in the study of lipid polymorphism of our model membranes. The thermodynamic profiles strengthen the effect of solvent quality, headgroup charge and lipid composition on the unique structural transformations leading to nanotubes. Melting temperatures, enthalpy and transition cooperativity enabled us to emphasize the role of ethanol in the formation of multilamellar structures and the dramatic effect on system’s self-assembly pathway upon adding a short lipid component.