Lipid supramolecular structure

Attempts have recently been made to link the RNA world with the lipid world. Two groups involved in RNA and ribozyme research joined up with an expert on membrane biophysics (Szostak et al., 2001). They developed a model for the formation of the first protocells which takes into account both the most recent experimental results on replication systems and the self-organisation processes of amphiphilic substances to give supramolecular structures. 

The close packing of the acyl groups associated with the inclination of the lipid A backbone with respect to the fatty acid orientation seems to constitute a common and characteristic feature of the lipid A conformation. This specific (endotoxic) conformation is very likely to influence greatly the tendency of the amphiphilic lipid A to adopt peculiar supramolecular structures. 

In general, it may be said that the variety of architecture built by surfactants and lipids in particular is extremely rich, that small variations in chemical structure of the surfactant may bring about significant changes in the supramolecular structure of the... 

Ewert K, Ahmad A, Evans H, Safinya CR (2005) Cationic lipid-DNA complexes for non-viral gene therapy relating supramolecular structures to cellular pathways. Expert Opin Biol Ther 5 33-53... 

Fig. 5.8 Correlation between supramolecular structure/conformation and biological activity for various forms of lipid A at 37 °C.

Although proteins are large molecules they are small compared with a cell and even with supramolecular structures which may be part of a cell, such as plasma and organelle membranes, ribosomes, chromosomes, filaments, enzyme complexes and viruses (Chap. 1). Supramolecular structures are also prominent outside cells and are, for example, essential components of connective tissues such as tendon, ligament, cartilage and bone. Supramolecular structures can consist of a variety of different types of molecule from the small (such as membrane lipids) to macromolecules (such as proteins, DNA and RNA). 

There has been a number of examples of cylindrical (tubular) supramolecular structures reported over recent years. These encompass all-carbon structures as well as other inorganic and organic tubes (and columns) and include lipid-based systems. However, the majority of these tubes represent extended arrays (on a multi-nanometre or higher scale) and hence fall outside the scope of the present discussion - in any case, many were not obtained directly by self-assembly processes. Indeed, only a limited number of self-assembled, discrete cylindrical systems have been described. 

Water behaves differently in different environments. Properties of water in heterogenous systems such as living cells or food remain a field of debate. Water molecules may interact with macromolecular components and supramolecular structures of biological systems through hydrogen bonds and electrostatic interactions. Solvation of biomolecules such as lipids, proteins, nucleic acids, or saccharides resulting from these interactions determines their molecular structure and function. 

Several comprehensive reviews on polymerizable lipids and supramolecular structures derived from them appeared between 1985 and 2002 [3,25-31], Consequently, this review focuses on developments in this field during 2000-2008. These include synthesis of new types of polymerizable lipids, creation and characterization of novel poly(lipid) membrane systems, and applications of polymerized vesicles and membranes in chemical sensing, separations science, drug delivery, materials biocompatibility, and other fields. 

This article is organized primarily on the geometry of the supramolecular structure (e.g., vesicle, planar supported film, etc.). Functionalization of poly(lipid) structures and their technological applications are presented in a separate section as these have expanded greatly as the field has matured. The analytical techniques available for characterization of substrate-supported, thin organic films have advanced considerably since polymerized lipid films were first reported in the early 1980s, and examples of the use of these techniques to study poly(lipid) membranes are presented throughout this review. 

The molecules formed in biosynthetic reactions perform several functions. They can be assembled into supramolecular structures (e.g., the proteins and lipids that constitute membranes), serve as informational molecules (e.g., DNA and RNA), or catalyze chemical reactions (i.e., the enzymes). 

Water is the ideal biological solvent. It easily dissolves a wide variety of the constituents of living organisms. Examples include ions (e.g., Na+, K+, and CF), sugars, and many of the amino acids. Its inability to dissolve other substances, such as lipids and certain other amino acids, makes supramolecular structures (e.g., membranes) and numerous biochemical processes (e.g., protein folding) possible. In this section the behavior of hydrophilic and hydrophobic substances in water is described. This discussion is followed by a brief review of osmotic pressure, one of the colligative properties of water. Colligative properties are physical properties that are affected not by the specific structure of dissolved solutes, but rather by their numbers. 

Pressures used to investigate biochemical systems range from 0.1 MPa to about 1 GPa (0.1 MPa = 1 bar, 1 GPa = 10 kbar). Such pressures only change intermolecular distances and affect conformations, but do not change covalent bond distances or bond angles. In fact, pressures in excess of 30 kbar are required to change the electronic structure of a molecule. The covalent structure of low molecular mass biomolecules (peptides, lipids, saccharides), as well as the primary structure of macromolecules (proteins, nucleic acids and polysaccharides), is not perturbed by pressures up to about 20 kbar. Pressure acts predominantly on the conformation and supramolecular structures of biomolecular systems. 

For decades, colloid and surface scientists have known that amphiphilic molecules such as phospholipids can self-assemble or self-organize themselves into supramolecular structures of bilayer lipid membranes (planar BLMs and spherical liposomes), emulsions, and micelles [2-4]. As a matter of fact, our current understanding of the structure and function of biomembranes can be traced to the studies of these experimental systems such as soap films and Langmuir monolayers, which have evolved as a direct consequence of applications of classical principles of colloid and interfacial chemistry. As already mentioned in Section I, the seminal work on the self-assembly of planar lipid bilayers and bilayer or black lipid membranes was carried out in 1959-1963. The idea started while one of the authors was reading a paperback edition of Soap Bubbles by C. 

Two important aspects of Uposome formation must be emphasized here. First, in some cases we observe the formation of ordered supramolecular structures starting from a chaotic disordered mixture of surfactants (as in the ethanol injection method . As noticed before, this increase of order is attended by a simultaneous increase of water entropy and a decrease of overall free energy (Upids and solvent). Secondly, every time a liposome forms, there is the anergence of a division, with an inside world that is different from the external environment, even if the two worlds actually interact with each other. The discrimination between inside and outside, appUcable to lipid vesicles, is the first structural pre-requisite for the living cell. It is therefore clear that lipid or fatty acid vesicles may be considered relevant experimental model of simplified cells, and their role on... 

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