Electrostatic tether

Tethers. Orbital momentum can be exchanged using a tether between two spacecraft. This principle has been proposed to efficiently transfer payloads from Earth orbit to lunar or Martian orbits and even to exchange payloads with the lunar surfece. An extreme version is a stationary tether linking a point on Earth s equator to a craft in geostationary Earth orbit, the tether running far beyond to a countermass. The electrostatic tether concept uses variations in the electric potential with orbital height to induce... 

A spacecraft can harvest electricity from the potential variation in the Earth s electromagnetic field by extending an electrostatic tether down from its orbit. A current-carrying electrodynamic tether between two spacecraft can be used to gain propulsive thrust because of the Faraday force exerted by the magnetic field on the tether. 

Irradiation with UV light isomerized the azobenzene units from the trans to the cis form, while the reverse isomerization occurred thermally in the dark. The cis to trans conversion is catalyzed by both protons and hydroxyl ions. Hence, the catalyzed dark process for tethered azobenzene is greatly modified in comparison with that for free azobenzene. For the tethered azobenzene, beginning at pH 6, the cis to trans return rate sharply decreased with increasing pH up to 10, whereas the rate for free azobenzene rapidly increased in the same pH range owing to OH- catalysis. These observations can be explained by the electrostatic repulsion which lowers the local OH concentration on the polyion surface below that in the bulk aqueous phase. 

The linearity of L with N is maintained at the theta point. Relative to Eq. 5, the chains have shrunk by a factor of (a/d),/3 but the linear variation indicates that the chains are still distorted at the theta point and characteristic dimensions do not shrink through a series of decreasing power laws as do free chains [29-31]. Experimentally, Auroy [25] has produced evidence for this linearity even in poor solvents. Pincus [32] has recently applied this type of analysis to tethered polyelectrolyte chains, where the electrostatic interactions can produce even stronger stretching effects than those that have been discussed for good solvents. Tethered polyelectrolytes have also been studied by others [33-35],... 

Smith and co-workers (194) have used this chemistry to prepare carboxyl-modified Si(lll) surfaces at which polylysine-tethered DNA is electrostatically adsorbed (Fig. 60). An alternative approach involved covalent attachment of a pre-synthesized oligonucleotide bearing a terminal carboxyl group to an amine-modified Si(001) surface (195). 

UCW = capped water, TW = tethered water (see text), k = force constant for restraining potential (kcal/mol/A2). b Radius (A) of solvation sphere. 1 Numbers of dynamical water molecules within solvation sphere. d Mean and standard error for the forward (i.e. 8-methyl-N5-deazapterin —> 8-methylpterin) and reverse mutation of the electrostatic force field Cutoff for protein-ligand and solvent-ligand interaction all other interactions are subject to a 9 A cutoff. 

Another method for the analysis of aptamer-protein complexes involved the use of a positively charged ferrocene-tethered polythiophene, (19), as redox label reporting unit (Fig. 12.19). The antithrombin aptamer was immobilized on an electrode surface, and the electrostatic binding of the redox polymer (19) to the aptamer monolayer resulted in a supramolecular complex that revealed electrical contact between the polymer and the electrode.74 The formation of the aptamer-thrombin complex removed the polymer from the surface and blocked the electrical contact between the polymer label and the electrode. As a result, higher concentrations of thrombin increased the surface coverage of the aptamer-thrombin complex on the electrode, and this decreased the amperometric responses of the sensing device. 

Individual macromolecules as well as cell surfaces usually carry a net negative charge at neutral pH. This causes the surfaces to repel each other. However, at a certain distance of separation the van der Waals attractive forces will balance tire electrostatic repulsion.21 Protruding hydrophobic groups may then interact and may "tether" bacteria or other particles at a fixed distance, often 5 nm, from a cell surface.28... 

The method of RNA catalyst capture really depends on the properties of the tethered product. Another method for selective partitioning is by chromatographic (HPLC) methods based on the properties of products that do not contain an affinity tag such as biotin. If the overall electrostatic or steric properties of the RNA product are significantly changed so that they are much different than those of the unreactecl RNA, chromatographic methods can be used to separate and recover the products and obtain their sequence information [14]. 

With the application of protein microarrays in mind, Spencer and coworkers immobilized poly(lysine) with grafted PEG side chains on various metal or semiconductor oxide surfaces via electrostatic adsorption [200], Part of the PEG side chain was functionalized with biotin at the distal end. Streptavidin was bound to the surface-tethered biotin in a subsequent step, and the remaining unoccupied binding pockets of streptavidin were then used to immobilize biotinylated capture antibodies. As an example of an immunoassay, biotinylated goat anti-rabbit IgG was immobilized, which then specifically bound rabbit IgG. 

Wade RC, Gabdoulline RR, Ludemann SK, et al. Electrostatic steering and ionic tethering in enzyme-hgand binding insights from simulations. Proc Natl Acad Sci USA 1998 95 5942-5949. 

The next selectivity issue, exo/endo preferences, can be predicted for both the ortho and meta modes of cycloaddition on the basis of secondly orbital interactions (FMO treatment) and by electrostatic considerations involving polarized species (54) and (27). In general, intermolecular reactions with simple al-kenes proceed with endo selectivity. Heteroatom-substituted or polarized alkenes (equation 11) give exolendo mixtures, whose composition can be explained by electrostatic considerations. Intramolecular cycloadditions of simple alkenes and arenes joined by a three-atom tether generally proceed with high exo selectivity due in part to orbital alignment effects. In all cases, alkene geometry is preserved, except for sterically encumbered alkenes, in which case excitation transfer from the arene to the alkene can occur. 

---