Chain structure nanotubes

Porphyrin-based complexes pass their redox sensitivity on to the corresponding polymers. Advanced conjugated main-chain structures with attached dendrons have been introduced [220]. Grafting of porphyrin-zinc main-chain polymers onto carbon nanotubes was reported [221]. An interesting example dealing with a nonelectrochemically switched polymer was recently given. Here, polymers with porphyrin-iron end groups could be switched between linear and cyclic polymers by a dimerization procedure (Fig. 14) [222, 223],... 

Maehashi et al. (2007) used pyrene adsorption to make carbon nanotubes labeled with DNA aptamers and incorporated them into a field effect transistor constructed to produce a label-free biosensor. The biosensor could measure the concentration of IgE in samples down to 250 pM, as the antibody molecules bound to the aptamers on the nanotubes. Felekis and Tagmatarchis (2005) used a positively charged pyrene compound to prepare water-soluble SWNTs and then electrostatically adsorb porphyrin rings to study electron transfer interactions. Pyrene derivatives also have been used successfully to add a chromophore to carbon nanotubes using covalent coupling to an oxidized SWNT (Alvaro et al., 2004). In this case, the pyrene ring structure was not used to adsorb directly to the nanotube surface, but a side-chain functional group was used to link it covalently to modified SWNTs. 

Protein structures are so diverse that it is sometimes difficult to assign them unambiguously to particular structural classes. Such borderline cases are, in fact, useful in that they mandate precise definition of the structural classes. In the present context, several proteins have been called //-helical although, in a strict sense, they do not fit the definitions of //-helices or //-solenoids. For example, Perutz et al. (2002) proposed a water-filled nanotube model for amyloid fibrils formed as polymers of the Asp2Glni5Lys2 peptide. This model has been called //-helical (Kishimoto et al., 2004 Merlino et al., 2006), but it differs from known //-helices in that (i) it has circular coils formed by uniform deformation of the peptide //-conformation with no turns or linear //-strands, as are usually observed in //-solenoids and (ii) it envisages a tubular structure with a water-filled axial lumen instead of the water-excluding core with tightly packed side chains that is characteristic of //-solenoids. 

It has been suggested that the Sup35p filament may be a bundle of four cylindrical //-sheets or nanotubes (Kishimoto et al., 2004). The nanotube is a hypothetical structure (Perutz et al, 2002) whose winding of the polypeptide chain is topologically similar to that of a //-helix but it is round in cross section and water filled whereas //-helices have cross sections with triangular or other shapes and water is largely excluded from their interiors (Kajava and Steven, 2006). The model of Kishimoto et al. envisaged six coils... 

As mentioned in Section IV, BC2N nanotubes have two isomers with distinctly different structure and properties. One of them, with alternating carbon and B-N chains, was predicted to be a semiconductor. In the armchair (n, n) configuration,... 

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