Tethering

Mo and P sources, respectively [59]. This gives rise to a supported acidic species that can act as solid add catalyst, much more environmentally friendly than 

Molecular clusters have also heen hudt inside zeolite cavities [42,46,60-63]. 

Tethered chains can he placed at the surface of another polymer to change its surface characteristics. The placement can be through covalent bonding or by a physical process such as adsorption or embedding. It is also possible to tether quaternary ammonium salts to PDMS to enhance antifouling and antimicrobial characteristics. The embedding approach is likely to be more robust. 

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Tension leg platforms (TLP) are used mainly in deep water where rigid platforms would be both vulnerable to bending stresses and very expensive to construct. A TLP is rather like a semi-submersible rig tethered to the sea bed by jointed legs kept in tension. Tension is maintained by pulling the floating platform down into the sea below its normal displacement level. The legs are secured to a template or anchor points installed on the seabed. 

Klein and co-workers have documented the remarkable lubricating attributes of polymer brushes tethered to surfaces by one end only [56], Studying zwitterionic polystyrene-X attached to mica by the zwitterion end group in a surface forces apparatus, they found /i < 0.001 for loads of 100 and speeds of 15-450 nm/sec. They attributed the low friction to strong repulsions existing between such polymer layers. At higher compression, stick-slip motion was observed. In a related study, they compared the friction between polymer brushes in toluene (ji < 0.005) to that of mica in pure toluene /t = 0.7 [57]. 

Direct measurement of the interaction potential between tethered ligand (biotin) and receptor (streptavidin) have been reported by Wong et al [16] and demonstrate the possibility of controlling range and dynamics of specific biologic interactions via a flexible PEG-tether. 

Wong J Y ef a/1997 Direot measurement of a tethered ligand-reoeptor interaotion potential Science 275 820-2... 

Seitz M ef al 1998 Formation of tethered supported bilayers via membrane-inserting reactive lipids Thin Solid Films 327-9 767-71... 

Szleifer I and Carignano M A 1996 Tethered polymer layers Adv. Chem. Phys. 94 165... 

An equally important challenge for nanocrystal assembly is the fonnation of specific nanocrystal arrangements in solution. By using complementary DNA strands as tethers, Mirkin et al [102, 103] fonned aggregates of gold nanocrystals with specific sizes Alivisatos et al also used DNA to stmcture semiconductor nanocrystal molecules, though in this case the molecules contained only a few nanocrystals placed controlled distances from each other [104, 105 and 106]. The potential applications of biomolecular teclmiques to this area of nanoscience are immense, and the opportunities have been reviewed in several recent publications [107, 108, 109 and 110]. 

A restrain t (not to be confused with a Model Builder constraint) is a nser-specified one-atom tether, two-atom stretch, three-atom bend, or four-atom torsional interaction to add to the list ol molec-11 lar mechanics m teraction s computed for a molecule. These added iiueraciious are treated no differently IVoin any other stretch, bend, or torsion, except that they employ a quadratic functional form. They replace no in teraction, on ly add to the computed in teraction s. 

For simplicity, take the specific case where ki = k2 = k. Write the matrix of force constants analogous to matrix (5-29). Diagonalize this matrix. What are the roots Discuss the motion of the double pendulum in contrast to two coupled, tethered masses (Fig. 5-1). 

Fig. 7. A tethered cyclodextrin and the structure of P-cyclodextrin, the most common cyclodextrin used as a bonded ligand in Hquid chromatography.

Fig. 4. Atom manipulation by the scanning tunneling microscope (STM). Once the STM tip has located the adsorbate atom, the tip is lowered such that the attractive interaction between the tip and the adsorbate is sufficient to keep the adsorbate "tethered" to the tip. The tip is then moved to the desired location on the surface and withdrawn, leaving the adsorbate atom bound to the surface at a new location. The figure schematically depicts the use of this process in the formation of a "quantum corral" of 48 Fe atoms arranged in a circle of about 14.3 nm diameter on a Cu(lll) surface at 4 K.

Modified oligonucleotides can be used to cross-link DNA sequences via a reactive group tethered to an oligonucleotide. When irradiated with uv light, psoralens (31) reacts with thymine bases, and the reaction yields a cross-link if the thymine residues are adjacent to each other on opposite strands. Psoralen linked to oligonucleotides have been shown to induce site-specific cross-links in vitro (51). 

Optical Fibers. Pure and doped fused siHca fibers have replaced copper lines in the telecommunication area. Eused siHca fibers are used in laser surgery, optical sensor appHcation, and laser welding (see Sensors). Optical-fiber-tethered weapons such as fiber-optics-guided (EOG) missiles are another potential appHcation for fused siHca (249,250) (see Eiberoptics). 

AppHcations have been found for these batteries in emergency power appHcations for telecommunications systems in tethered balloons. Unfortunately, the system is expensive because of the high cost of the silver electrode. AppHcations are, therefore, generally sought where recovery and reclamation of the raw materials can be made. 

Enzymatic Reactors Adding free enzyme to a batch reactor is practical only when the value of the enzyme is relatively low. With expensive enzymes, reuse by retaining the enzyme with some type of support makes great economic sense. As some activity is usually lost in tethering the enzyme and the additional operations cost money, stabihty is very important. However, many enzymes are stabilized by immobilization thus, many reuses may be possible. 

Screen-printed gold electrodes were, firstly, modified with a mixed monolayer of a 25-mer thiol-tethered DNA probe and a spacer thiol, 6-mercapto-1 -hexanol (MCH). The DNA probe sequence was internal to the sequence of the 35S promoter, which sequence is inserted in the genome of GMOs regulating tire transgene expression. 

Mira Josowicz, John Hartung, Janusz Kowalik, Christine Kranz, Jiri Janata, Ashwini Sinha, Kendra McCoy. J. Electrochem.Soc., (submitted), Electropolymerization of Bilayer with Phosphonic Acid Tethers for Immobilization of Biomolecules ... 

There is a large range of resins available for SPOS. These resins are derivatised polymer supports with a range of linkers. The roles of linkers are (i) to provide point(s) of attachment for the tethered molecule, akin to a solid supported protecting group(s), (ii) to provide distance from the polymeric backbone in order to minimise interactions with the backbone, (iii) to enable cleavage of product molecules under conditions compatible with the stability of the molecules and the reaction conditions employed for chemical transformations. Hence in order to... 

Klemm, J.D., et al. Crystal structure of the Oct-1 POU domain bound to an octamer site DNA recognition with tethered DNA-binding modules. Cell 77 21-32, 1994. 

It is striking that in the English-speaking world, where academic departments are normal, no departments of either solid-state physics or of solid-state chemistry are to be found. These vast fields have been kept securely tethered to their respective... 

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. 

Tirrell et al. [42,43] studied the role of interfacial chains in a more detailed fashion. Tirrell et al. [42,43] used a crosslinked PDMS cap in contact with a silicon wafer on to which a,o)-hydroxyl terminated PDMS chains are tethered by adsorption from a solution. The molecular weight of the narrow disperse PDMS samples was in the range of 20,000-700,000. The surface chain density was given by27 yj g e 0 is the volume fraction of PDMS in solution. 

When compared to the early models that describe the adhesion of elastomers in the presence of interfacial chains, the central findings of this work, namely, observations b and d above, were quite surprising. In his recent work, de Gennes [102] pointed out that when a surface tethered chain diffuses into an elastomeric... 

Fig. 10. The variation of 6, with E for three different molecular weight polyisoprene tethered chains in a cross-linked polyisoprene lens [23].

A. Halperin, M. Tirrell, T. P. Lodge. Tethered chains in polymer microstructures. Adv Polym Sci 100 21-11, 1992. 

G. S. Grest, M. Murat. Computer simulations of tethered chains. In K. Binder, ed. Monte Carlo and Molecular Dynamics Simulations in Polymer Science. New York Oxford University Press, 1995, pp. 476-578. 

Bead-spring models without explicit solvent have also been used to simulate bilayers [40,145,146] and Langmuir monolayers [148-152]. The amphi-philes are then forced into sheets by tethering the head groups to two-dimensional surfaces, either via a harmonic potential or via a rigid constraint. 

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