Amphiphilic biomolecules

Studies of membranes, hpids (including phospholipids), and other such amphiphilic biomolecules on electrodes are rather hmited due to the redox inactivity of these materials. Nevertheless, a few reports exist of studies of bilayer membranes and hpid amphiphiles on SERS surfaces and are mentioned here as exemplar studies due to the unique insight they offer. 

An alternative efficient approach to disperse CNTs relies on the use of synthetic peptides. Peptides were designed to coat and solubilise the CNTs by exploiting a noncovalent interaction between the hydrophobic face of amphiphilic helical peptides and the graphitic surface of CNTs (Dieckmann et al., 2003 Zoibas et al., 2004 Dalton et al., 2004 Arnold et al., 2005). Peptides showed also selective affinity for CNTs and therefore may provide them with specifically labelled chemical handles (Wang et al., 2003). Other biomolecules, such as Gum Arabic (GA) (Bandyopadhyaya et al., 2002), salmon sperm DNA, chondroitin sulphate sodium salt and chitosan (Zhang et al., 2004 Moulton et al., 2005), were selected as surfactants to disperse CNTs (Scheme 2.1). 

Such hydrophobic interactions are important characteristics of many biological systems. Many biomolecules (e.g., proteins) possess both hydrophilic and hydro-phobic features in the same molecule and are therefore called amphiphilic. Hydro-phobic amino acids of proteins tend to be buried in the interior of the protein. Hydrophilic amino acids are at the surface of the protein, exposed to solvent water. 

Reverse micelles are self-organized aggregates of amphiphilic molecules that provide a hydrophilic nano-scale droplet in apolar solvents. This polar core accommodates some hydrophilic biomolecules stabilized by a surfactant shell layer. Furthermore, reverse micellar solutions can extract proteins from aqueous bulk solutions through a water-oil interface. Such a liquid-liquid extraction technique is easy to scale up without a loss in resolution capability, complex equipment design, economic limitations and the impossibility of a continuous mode of operation. Therefore, reverse micellar protein extraction has great potential in facilitating large-scale protein recovery processes from fermentation broths for effective protein production. 

Phospholipids are an important class of biomolecules. Phospholipids are the fundamental building blocks of cellular membranes and are the major part of surfactant, the film that occupies the air/liquid interfaces in the limg. These molecules consist of a polar or charged head group and a pair of nonpolar fatty acid tails, connected via a glycerol linkage. This combination of polar and nonpolar segments is termed amphiphilic, and the word describes the tendency of these molecules to assemble at interfaces between polar and nonpolar phases. 

Most biomolecules are amphiphilic, meaning they have both hydrophilic and hydrophobic groups. In an aqueous environment, these molecules self-assemble where the apolar parts of the molecules are grouped together. Some examples of structures formed as a result of this self-assembly are illustrated in Figure 1.2. The lipid bilayer is in fact the backbone of all biological membranes that makes compartmentalization possible in all cells. The assembly of these molecules would not be possible without the presence of water around them. 

Liposomes are aggregates of amphiphilic block copolymers or surfactant molecules that self-assemble into spherical nanostructures in aqueous solution. Typically, liposomes consist of a bilayer in which hydrophilic blocks of the polymer form the outer and inner shell of the bilayer while the hydrophobic blocks lie between the inner and outer shell. This configuration shields the hydrophobic blocks from the external aqueous solution and the aqueous internal core of the liposome. Liposomes can be functionalized with various biomolecules and loaded during the self-assembly process with reporters facilitating use of liposomes as effective labels for DNA detection. 

We can also mention the use of bio-sourced building blocks based on cellulose or dextran. Kadla et al. described value-added materials from naturally abundant polymers for system that may serve as a platform for the design and development of biosensors [197]. A hierarchically strucmred honeycomb film from dextran-ft-PS amphiphilic linear diblock copolymers has also been described by Chen et al. leading to ordered porous bio-hybrid films. [198] Honeycomb patterned surfaces functionalized with biomolecules for specific recognition of proteins or bacteria have been also achieved either by self-assembly of amphiphilic copolymers based on galactose moieties [155] or by post-modification with peptide sequences [199]. 

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