Higher order architectures

The higher-order architectures of DNA/protein complexes are achieved on the balance of the physical properties of DNA and the interactive forces of DNA-binding proteins (Hizume et al., 2002 Yoshimura et al, 2000). The superhelical strain of the DNA has been suggested to play a critical role in a variety of genome events such as transcriptional regulation and DNA replication (Opel et al, 2001). 

Higher order architectures have been successfully used to build converters for digital audio applications [Adams, 1986][Hauser, 1991], However, the complexity of modulator and demodulator increases with the order. Higher order architectures also exhibit stability problems [van de Plassche, 1994], Dithering is a remedy to many of the problems of low level signals (see the previous section) and has been used in sigma delta converters as part of the noise shaping loop [Vanderkooy and Lipshitz, 1989]. 

Other than accelerating the formation of spherical nanoparticles of silica from metastable silicic acid solutions, the silaffins appear to have no structure-directing activity. If they are responsible for silica formation in the living diatom, as seems quite likely, control of the higher order architecture of the resulting silica apparently must be determined by the pre-formed shape of the silica deposition vesicle (the envelope within which the silica grows) serving as a complex three-dimensional mold. 

Polytopic nitrogen ligands such as tripyridyl-triazine, non-covalently finked multi-pyridyl ligands via bipy-metal interactions or buUt up through dendritic cores, were used for the formation of higher-ordered architectures. Several examples shall be presented here to demonstrate the utility of these poly topic ligands for the assembly of metalloporphyrins. 

When the coordination bonds that hold these assemblies together are both stable and inert, their formation from the components occurs under kinetic rather than thermodynamic control and it is thus disputable if they can be truly defined as supramolecular systems. On the other hand, owing to their kinetic stability, some of these species can be further exploited as building blocks in the construction of higher order architectures through a hierarchical self-assembly approach (see for example the molecular sandwiches 13-15). Through this modular approach, multichromophore systems become easily accessible on demand, with a limited synthetic effort. 

In nanotechnology, self-assembly underlies various types of molecular structures (e.g., Langmuir-Blodgett films (Whang et al., 2003), self-assembled monolayers (Love et al., 2005 Bain et al., 1989 Witt et al., 2004), amphiphilic fibers (Hartgerin et al., 2001, 2002), as well as higher-order architectures built... 

Seemann, J., Jokitalo, E., Pypaert, M., and Warren, G. (2000). Matrix proteins can generate the higher order architecture of the Golgi apparatus. Nature 407, 1022-1026. 

Sheet peptides mostly self-assemble into fibrous structures, which often aggregate further into higher order architectures. Systematic studies have been carried out by several research groups to disclose the rules associated with forming well-defined higher order -sheet assemblies. In this section, some examples of -sheet forming PAs, composed of amino acids only, are discussed. 

The essential distinction between the approaches used to formulate and evaluate proteins, compared with conventional low molecular weight drugs, lies in the need to maintain several levels of protein structure and the unique chemical and physical properties that these higher-order structures convey. Proteins are condensation polymers of amino acids, joined by peptide bonds. The levels of protein architecture are typically described in terms of the four orders of structure [23,24] depicted in Fig. 2. The primary structure refers to the sequence of amino acids and the location of any disulfide bonds. Secondary structure is derived from the steric relations of amino acid residues that are close to one another. The alpha-helix and beta-pleated sheet are examples of periodic secondary structure. Tertiary... 

Organoclay materials with higher-order organization can also be prepared by template-directed methods involving self-assembled supramolecular structures. In this approach, preformed organic architectures in the form of tubes, fibers, hollow shells, gyroids, helicoids, and so on are transferred into hybrid materials exhibiting structural hierarchy, complex form and ordered mesoporosity [47-55]. For example,... 

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