Amphiphiles, giant

While most vesicles are formed from double-tail amphiphiles such as lipids, they can also be made from some single chain fatty acids [73], surfactant-cosurfactant mixtures [71], and bola (two-headed) amphiphiles [74]. In addition to the more common spherical shells, tubular vesicles have been observed in DMPC-alcohol mixtures [70]. Polymerizable lipids allow photo- or chemical polymerization that can sometimes stabilize the vesicle [65] however, the structural change in the bilayer on polymerization can cause giant vesicles to bud into smaller shells [76]. Multivesicular liposomes are collections of hundreds of bilayer enclosed water-filled compartments that are suitable for localized drug delivery [77]. The structures of these water-in-water vesicles resemble those of foams (see Section XIV-7) with the polyhedral structure persisting down to molecular dimensions as shown in Fig. XV-11. 

A microemulsion (p.E) is a thermodynamically stable, transparent (in the visible) droplet type dispersion of water (W) and oil (O a saturated or unsaturated hydrocarbon) stabilized by a surfactant (S) and a cosurfactant (CoS a short amphiphile compound such as an alcohol or an amine) [67]. Sometimes the oil is a water-insoluble organic compound which is also a reactant and the water may contain mineral acids or salts. Because of the small dispersion size, a large amount of surfactant is required to stabilize microemulsions. The droplets are very small (about 100-1000 A [68]), about 100 times smaller than those of a typical emulsion. The existence of giant microemulsions (dispersion size about 6000 A) has been demonstrated [58]. 

The same research group recently carried out the synthesis of other hybrid copolymers in which hydrophobic PS chains had been covalently linked to natural polypeptides such as a lipase enzyme [264], The resulting biohybrid has been referred to as a giant amphiphile and forms catalytic micellar rods in water, as shown in Fig. 17. 

Velonia K, Rowan AE, Nolle RJM. Lipase polystyrene giant amphiphiles. J Am Chem Soc 2002 124 4224-4225. 

In the last Section 6.4 new supramolecular approaches to construct synthetic biohybrid catalysts are described. So-called giant amphiphiles composed of a (hydrophilic) enzyme headgroup and a synthetic apolar tail have been prepared. These biohybrid amphiphilic compounds self-assemble in water to yield enzyme fibers and enzyme reaction vessels, which have been studied with respect to their catalytic properties. As part of this project, catalytic studies on single enzyme molecules have also been carried out, providing information on how enzymes really work. These latter studies have the potential to allow us to investigate in precise detail how slight modifications ofthe enzyme, e.g., by attaching a polymer tail, or a specific mutation, actually infiuence the catalytic activity. 

While Nature uses primarily small amphiphilic building blocks, chemists have access to a more vast array of amphiphiles ranging from the natural analogues through to block copolymer super amphiphiles and more recently the biohybrids and giant amphiphiles . The application of this range of amphiphiles in the construction of nanosized assemblies, some with unique properties, is clearly going to be at the forefront of the future developments in the nanosciences. 

Very recently however, a new exciting class of biohybrid amphiphiles, the giant amphiphiles, has been developed. These giant amphiphiles consist of a natural biomacromolec-ular head group, such as an enzyme or protein and a polymeric tail. They possess the same hydrophilic/hydrophobic character as their phospholipid molecular counterparts but have dimensions many times larger (Section 4.3). 

The Biotin-Streptavidin Approach The synthesis of giant amphiphiles was first reported by the group of Nolte, using the well-established biotin-streptavidin approach (Figure 7.38).270... 

FIGURE 7.42. Computer generated models of the molecular, super and giant amphiphiles and their respective dimensions. 

FIGURE 7.44. Transmission electron micrographs of the micellar assemblies formed by the aggregation of a lipase-polystyrene giant amphiphile in water. Expansion reveals a single micellar fiber with a diameter of 20-30 nm. Schematic representation of the micellar rod which possesses a polystyrene core. 

The metal-to-ligand coordination coupling chosen for the construction of a family of giant amphiphiles was that of a metal-terpyridine complex, which is known to form stable bis-complexes with a variety of transition-metal ions.275 The versatility of this metal-to-ligand system has been recently demonstrated in the construction of a series of coordination polymers and block copolymers.276... 

FIGURE 7.45. Schematic representation of the reconstituted polystyrene-horseradish peroxidase giant amphiphile (left) and the vesicular architectures it forms in solution (middle TEM image, right CryoSEM image). 

The successful synthesis of the biohybrid members of the family of amphiphiles, the giant amphiphiles, has also attracted the spotlight of fundamental sciences. The incorporation of biological molecules (proteins and enzymes) in to the amphiphilic structure mimics the self-assembly seen in Nature by amphiphilic proteins. Though enzymes and proteins have been extensively funtionalized in the past, the realization that they can express assembling properties similar to that of their molecular and polymeric counterparts offers new methods for the construction of functional biomimetic assemblages. The future for amphiphiles appears to be unlimited. 

Jaeger et al. described [46] some novel double-chain vesicle-forming cleav-able amphipiles which were synthesized via ion-pairing of two oppositely charged head groups. These amphiphiles formed giant vesicles which cleave at the labile carbon-nitrogen double bond around pH 3. 

N0rgaard K, Weygand MJ, Kjaer K, Brust M, Bjqmholm T (2004) Adaptive chemistry of bifunctional gold nanoparticles at the air/water interface. A synchrotron X-ray study of giant amphiphiles. Faraday Discuss 125 221-233... 

Su W, Han K, Luo Y et al. (2007) Pormation and photoresponsive properties of giant microvesicles assembled from azobenzene-containing amphiphilic diblock copolymers. Mac-romol Chem Phys 208 955-963... 

The synthesis of giant amphiphiles was recently reported by Meijer et al, who linked a linear hydrophobic poly(styrene) block to a dendritic poly(propyleneimine) as a polar head group. The association structure was found to vary with the dendrimer generation changing Irom inverse micelles for poly(styrene-JenJr-(NH2)4) through vesicles and rod-like micelles to spherical aggregates for poly-(styrene-fife fi r-(NH2)32 [210]. 

Inaoka, Y. and Yamazaki, M. (2007) Vesicle fission of giant unilamellar vesicles of liquid-ordered-phase membranes induced by amphiphiles with a single long hydrocarbon chain. Langmuir, 23 (2), 720-728. 

Fig. 18 a Computer-generated model of a HRP-polystyrene-based giant amphiphile b SEM micrograph of an aqueous solution of aggregates formed by the giant amphiphile. Reprinted with permission from [81]. Copyright 2002 WUey... 

Complex fibrillar assembhes, among others, were also observed for sodium N-(4-dodecyloxybenzoyl)-L-valinate (Dey et al. [30]) and a lipase polystyrene giant amphiphile (Nolte et al. [31]) in water. 

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