Self-assembly cylindrical structure

The fiber-like structure of polypeptide block copolymers is essential in organogels that are formed by the side-by-side packing of polypeptide rods. Similarly, such side-by-side packing of polypeptide rods can also be found in cylindrical micelles self-assembled from polypeptide block copolymers. In both the fiber-like gels and cylindrical micelles, polypeptide rods assemble into ordered structures, while the flexible chains are spread out into the surroundings to stabilize the structures. 

Amphiphilic molecules (surfactants) are composed of two different parts hydrophobic tail and hydrophilic head [1 ]. Due to their chemical structure they self-assemble into internal surfaces in water solutions or in mixtures of oil and water, where the tails are separated from the water solvent. These surfaces can form closed spherical or cylindrical micelles or bicontinuous phases [3,5]. In the latter case a single surface extends over the volume of the system and divides it into separated and mutually interwoven subvolumes. 

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

The experiments discussed in this chapter have shown that a variety of chiral molecules self-assemble into cylindrical tubules and helical ribbons. These are indeed surprising structures because of their high curvature. One would normally expect the lowest energy state of a bilayer membrane to be flat or to have the minimum curvature needed to close off the edges of the membrane. By contrast, these structures have a high curvature, with a characteristic radius that depends on the material but is always fairly small compared with vesicles or other membrane structures. Thus, the key issue in understanding the formation of tubules and helical ribbons is how to explain the morphology with a characteristic radius. 

In this chapter, we have surveyed a wide range of chiral molecules that self-assemble into helical structures. The molecules include aldonamides, cere-brosides, amino acid amphiphiles, peptides, phospholipids, gemini surfactants, and biological and synthetic biles. In all of these systems, researchers observe helical ribbons and tubules, often with helical markings. In certain cases, researchers also observe twisted ribbons, which are variations on helical ribbons with Gaussian rather than cylindrical curvature. These structures have a large-scale helicity which manifests the chirality of the constituent molecules. 

A certain type of lipid (or lipid-like) molecules are found that when dispersed in water tend to make self-assembly structures (Figure 4.13). Detergents were shown to aggregate to spherical or large cylindrical-shaped micelles. It is known that if egg phosphatidylethanolamine (egg lecithin) is dispersed in water at 25°C, it forms a self-assembly structure, which is called liposome or vesicle. 

FIGURE 7.15. Molecular structure of a peptide amphiphile that self-assembles into a cylindrical aggregate. The five key structural elements of die peptide are highlighted. The cylindrical aggregate assembles into fibers that template the mineralization of die hydroxapatite crystal. The c axis of the crystal is aligned with the long axis of... 

The enthalpy effect of rod packing influences the microstructures at the air- water interface. The copolymer with the long rods, forms a cylindrical structure in the monolayer due to the bigger enthalpy decrease. Copolymer, with the middle-sized rods, will form micellar structure in the monolayer by self - assembly. Copolymer... 

The utilization of peptide self-assembly additionally allows control of structural parameters and the rational control of functionalities, which are displayed at the nanofiber surface. This makes the presentation of biological signals and thus the introduction of bioactivity feasible. Stupp and coworkers investigated the self-assembly of peptide-amphiphiles (Fig. 4) [87, 115], The resulting worm-like, cylindrical nanostructures consist of a hydrophobic core that is formed by the alkyl... 

The solution conditions enter the description through the pH, AB, and AD. Expression (19) provides an explanation for why self-assembled protein structures become more stable with increasing concentration of salt, which seems to be true for a wide variety of systems, including spherical and cylindrical viruses (Kegel and van der Schoot, 2004, 2006). 

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