Noncovalent Attachment

In a different approach towards the preparation of polymer/NTs composites, polymers have been used in the formation of supramolecular complexes with NTs. Besides possibly improving the mechanical and electrical properties of polymers. 

In particular, the rate of scientific publications on conducting polymers and NTs has steadily increased since their discoveries in 1977 [59] and 1991 [1], respectively, to around 120 articles per week, indicating that interest in polymers, NTs and their composites continues to grow. (The number of pubUcations was obtained from the Web of Science database for the term carbon nanotubes or conducting polymers www.isiwebofknowledge.com.) 

Homogeneous nanocomposites of poly(phenyleneethynylene) (PEE)-SWNTs/ polystyrene (PS) and PPE-SWNTs/polycarbonate were noncovalently functionalized and revealed dramatic improvements in the electric conductivity with very low percolation thresholds (i.e., 0.05-0. lwt% SWNTs loading) [65]. 

Tang and Xu [68] have prepared soluble MWNT-containing photoconductive poly(phenylacetylenes) (NTs/PPAs) by in situ polymerizations of phenylacetylene catalyzed by WClis-Ph4Sn and [Rh(nbd)Cl]2 (nbd = 2,5-norbornardiene) in the presence of the NTs. They demonstrated that the NTs in the composite solutions can be easily aligned by mechanical shear, and an efficient optical limiting property was observed in these nanocomposite materials. 

Although poly(9,9-dialkylfluorenes) exhibit the extended conjugation required for JT-stacking to the NTs surface, they have thus far attracted hmited attention as 

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The process of combining cyanine and squaraine dyes by encapsulation, or covalent or noncovalent attachment with macrocyclic hosts, macromolecules, and micro- or nano-particles is a promising way to design novel probes and labels with substantially improved properties and for the development of advanced fluorescence-based assays. Nevertheless, the physicochemical properties of these dye-compositions are strongly dependent on the dye structure as well as the nature of the host macrocycle, macromolecule, or particle. Finally, development of new methods to synthesize these tracers can also be considered a challenging task. 

Fig. 3.20 (a) Schematic representation of noncovalent attachment (i.e., n-stacking) of benzyl alcohol molecules for subsequent coordination with Titanium resulting in the coating of the tube, (b) SEM image of pristine CNTs prior to derivatization. (c) SEM image of CNTs after derivatization with benzyl alcohol and subsequent coordination with Titanium resulting in the coating of the tubes. Adapted with permission from [102], 2008 Wiiey-VCH. 

Noncovalent interactions such as van der Waals, hydrogen bonding, n-n stacking and electrostatic interactions have been widely used to hybridize pristine nanocarbons via ex situ approaches. The major advantage of this route is that the nanocarbons do not require modification prior to hybridization and their structure remains undisturbed, an important factor in many electronic applications. The strength of hybridization is weaker compared to covalent interactions but the synthetic process is generally simpler. Noncovalent attachment of small molecules to nanocarbons is often used to change the surface chemistry for subsequent ex situ or in situ hybridization. 

It was postulated that the differences in enzyme activity observed primarily result from interactions between enzyme-bound water and solvent, rather than enzyme and solvent. As enzyme-associated water is noncovalently attached, with some molecules more tightly bound than others, enzyme hydration is a dynamic process for which there will be competition between enzyme and solvent. Solvents of greater hydrophihcity will strip more water from the enzyme, decreasing enzyme mobility and ultimately resulting in reversible enzyme deactivation. Each enzyme, having a unique sequence (and in some cases covalently or noncovalently attached cofactors and/or carbohydrates), will also have different affinities for water, so that in the case of PPL the enzyme is sufficiently hydrophilic to retain water in all but the most hydrophilic solvents. 

Noncovalent attachment is a popular method of immobilization, and numerous different support materials have been employed, ranging from organic supports, like cellulose. 

Entrapment in polymeric matrices is a variation of noncovalent attachment where the support is instead generated in the presence of the enzyme. A particularly popular entrapment technique is sol-gel encapsulation, where the enzyme is trapped within an Si02 matrix formed by acid- or base-catalysed hydrolysis of tetraalkoxysilanes in the presence of enzyme. ° The technique can be tuned to provide the appropriate microenvironment for each enzyme in much the same way as can be done with other immobihzation methods. ... 

In summary, enzyme immobilization is extremely important in the scale-up of many biocatalytic processes. The preferred method for pharmaceutical production involves covalent binding through cross-linking or attachment to a support. Noncovalent attachment is less attractive, but it is heavily utihzed owing to the commercial availabihty of industrial quantities of some enzymes immobilized using this technique. 

Call et al. (2001) of the Pacific Northwest National Laboratory in Richland, WA, also studied the immobilization of unmodified oligonucleotides. Amine-modified and unmodified oligonucleotides could be attached to epoxysilane slides (covalent attachment) or acid-washed slides (noncovalent attachment) under the same conditions by printing in an alkaline-sodium... 

Mather BD, Baker MB, Beyer FL, Green MD, Berg MAG, Long TE. Multiple hydrogen bonding for the noncovalent attachment of ionic functionality in block copolymers. Macromolecules 2007b 40 4396-4398. 

Supramolecular chemistry also provides new tools for catalyst anchoring. We have shown that catalysts can be noncovalently attached to various soluble and insoluble supports, affording recyclable catalysts. Interestingly, the reversible nature of the noncovalent bond gives rise to new opportunities. In the first instance, we foresee an important role for supramolecular bidentate ligands in combinatorial catalysis - but as a consequence of the entirely new properties many new applications are envisioned. We look forward to new developments and results in this exciting emerging area of supramolecular catalysis. 

The resulting noncovalently immobilized complexes have been used as ligand systems for both the Pd-catalyzed allylic amination reaction and the Rh-catalyzed hydroformylation. A glycine-urea functionalized PPh3 ligand, 4(S), was noncovalently attached to the immobilized dendritic support, and the application of this system in the Pd-catalyzed allylic amination attains similar yields and product distributions as the homogeneous analogue for the... 

For the majority of fibril systems, simple deposition results in layers of fibrils that are largely unordered. The thickness of the fibril layer can also vary. This section explores a variety of methods that can be used to induce fibrillar alignment on a surface or create surface patterns. It also details methods that can be used to induce covalent and noncovalent attachment of fibrils to the surface. 

Magnetism is yet another successful method for the alignment of individual peptide fibers. Magnetite (Fe304) particles approximately lOnm in size were noncovalently attached to diphenylanalanine nanotubes during fiber... 

Also known as Polyhype-based composites, this is a composite of 10- 50% cross-linked polystyrene containing a very high pore volume (-90%) and containing polyacrylamide inside the cavities. Both covalently attached polystyrene-polyacrylamide and noncovalently attached polystyrene-polyacrylamide composites are available with substitutions in the range of 0.1-2.0 mmol amine/g resin [20,21],... 

Sun, J., Gao, L., and Iwasa, M., Noncovalent attachment of oxide nanoparticles onto carbon nanotubes using water-in-oil microemulsions, Chem. Commun., 832-833, 2004. 

Absorption. The noncovalent attachment of one substance to the surface of another. See Liquid-solid chromatography. 

Sorption. The noncovalent attachment of one substance to the surface of a packing. The term is generally used to indicate any or all types of attractive modes of LC that may be involved, for example, ion exchange, liquid-solid, and liquid-liquid partition. 

An example of a noncovalent attachment of a metal-phosphine complex to a solid support is presented in Figure 31, as reported by Bianchini et al. (120). The complex is attached via a sulfonated variant of the "triphos" ligand, which is known for its successful application in several catalytic reactions. The ligand is attached to the silica by an ionic bond, which is stable in the absence of water. The catalyst was used for the hydroformylation of styrene and of hex-1-ene in batch mode and showed moderate activity. The triply coordinated rhodium atom is strongly boimd although the conditions were rather harsh (120 °C, 30 bar) the concentration of leached metal measured by atomic emission spectroscopy was at most at the parts per million level. However, for commercial applications, for example, in a process such as hydroformylation of bulk products, these concentrations should be less than 10 ppb 111,121). 

FIGURE 32 Noncovalent attachment of metal ligand complex to a dendrimer (725). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

The b cytochromes contain noncovalently-attached protoheme in which the cysteine bridges are replaced by vinyl groups. The iron atom in most b cytochromes is ligated by two histidine nitrogen atoms, and has a redox potential of about 0 V. The 604 nm absorption band in reduced mitochondria is due to two a-type cytochromes, cyt a and cyt a, which are components of cytochrome oxidase (CcO) see Cytochrome Oxidase). A number of other classes of cytochromes have been discovered, including cytochrome peroxidases (CcP) (see Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases), cytochrome P-450, and cytochrome o see Iron Heme Proteins, Mono- Dioxygenases). 

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