Bilayer formations, molecular self-assembly

Surfactants have been used for over 1000 years in everyday applications, for example as emulsifiers in cleaning and in foods. They occur widely in nature, where as a bilayer they constitute a vital structural unit of biological membranes. Their functionality derives from the molecular structure, with a polar (hydrophilic) head-group, which conveys water-solubility, being attached to a non-polar (hydrophobic) tail, which drives the formation of self-assembled aggregates (micelles). Other chapters in this volume detail the wide variety of chemical structures that can form the polar groups (ionic, nonionic, zwitterionic, etc.) and tail structures. 

Another type of self-assembly mode is based on looser molecular interactions, where one of the main binding forces comes from hydrophobic interactions in aqueous media. Amphiphihc molecules (amphiphiles) that have a hydrophihc part and a hydrophobic part form various assembhes in water and on water. The simplest example of this kind of assembly is a micelle, where amphiphiles seh-assemble in order to expose their hydrophilic part to water and shield the other part from water due to hydrophobic interactions. A similar mechanism also leads to the formation of other assembhes, such as hpid bilayers. These molecules form spherical assembhes and/or two-dimensional membranes that are composed of countless numbers of molecules. These assembhes are usually very flexible. When external signals are applied to them, they respond flexibly while maintaining their fundamental organization and shape. This research held was initiated by the work of Bangham in 1964. It was found that dispersions of hpid molecules extracted from cells in water spontaneously form cell-like assembhes (liposomes). In 1977, Kunitake and Okahata demonstrated the formation of similar assembhes from various arti-flcial amphiphiles. The latter finding showed that natural lipids and artificial amphiphiles are not fundamentally different. 

The systems described above formed linearly extended, fibrous self-assemblies, although the morphological structures depended on amphiphiles. The molecular arrangement in fibers were maintained by the formation of concentric multilamellar bilayer, besides hydrophobic interaction and hydrogen bonding, except C Asp fibers with chiral character. The assemblies underwent temperature-dependent fibril-vesicle transition. The fibril-vesicle transition resulted from the rearrangement of bilayers. Generally, multilamellar layers were diminished in ves-... 

Abstract Amphiphilic polymers have the ability to self-assemble into supramolec-ular structures of great complexity and utility. Nowadays, molecular dynamics simulations can be employed to investigate the self-assembly of modestly sized natural and synthetic macromolecules into structures, such as micelles, worms (cylindrical micelles), or vesicles composed of membrane bilayers organized as single or multilamellar structures. This article presents a perspective on the use of large-scale computer simulation studies that have been used to xmderstand the formation of such structures and their interaction with nanoscale solutes. Advances in this domain of research have been possible due to relentless progress in computer power plus the development of so-called coarse-grained intermolecular interaction models that encode the basic architecture of the amphiphUic macromolecules of interest. 

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