Molecular tethering

Direct interachon between enzyme and electrode is a particular challenge for many redox enzymes as the redox cofactor is buried deep within the protein structure. The cofactor of GOx, for example, is buried deep within the protein stmcture. One intereshng soluhon to this hmitahon is to anchor the cofactor flavin adenine dinucleohde (FAD) directly to the electrode surface. When the apoenzyme (enzyme without cofactor) is subsequently added, the protein reforms around the anchored FAD and tethers the enzyme in close 

ENZYME IMMOBILIZATION FOR BIOLOGICAL FUEL CELL APPLICATIONS 

Similarly, in 2006, Ivnitski et al. demonstrated the anchoring of GOx to CNTs and observed DET between the active site of the enzyme and MWCNTs that were grown directly on a Toray carbon paper electrode [55], This technique works for other cofactors such as pyrroloquinoUne quinone (PQQ), which when covalently bound to a gold electrode and subsequently modified with nicotinamide adenine dinucleotide (NAD ) can be used to reconstitote enzymes such as PQQ-dependent lactate dehydrogenase [56,57], 

The introduction of CNT as a conductive material has provided a means to develop new conductive architectures, and a range of buckypaper (CNT paper), buckygel (CNT gels), and carbon nanofiber electrodes have since been reported that demonstrate a signifieant enhancement in electron transfer characteristics for both anodic and cathodic catalysts [58-61], The advantage of CNT as an electrode material is the high electrical conductivity, mechanical stability, and, to some extent, adaptability in respect to the types of materials that can be modified with CNTs. Cathodic oxygen reduction catalysts can be immobilized to buckypaper by simple physical 

FIGURE 11.4 Electrical wiring of redox enzymes, (a) Optimal configuration for the electrical contacting of a redox enzyme with the electrode, (b) Reconstitution of an apoenzyme on a relay-cofactor monolayer for the alignment and electrical wiring of a redox enzyme. The structures of the redox relay molecule PQQ and cofactor amino-FAD are shown in the inset. (Reproduced with permission from Ref. [53]. Copyright 2006, Elsevier.) 

---

Using HPW as a molecular tether to form highly stable crystal materials, Sarkar et al.220 prepared the palladium complex [Pd(pyca)(PPh3)(OTs)] (pyca = 2-picolinate) immobilised on zeolite NaY. From the 31P SS NMR data, it was concluded that the Pd complex is attached to the surface of zeolite by the HPW via some ionic interactions. Moreover, neither the support nor the molecular tether (HPW) undergoes any form of degradation during synthesis. This kind of material can be explored as a catalyst for acid-free carbonylation reaction. 

Most viruses initiate infection of susceptible cells by first attaching to specific cell surface receptors. Receptors have been identified for many animal, insect, and bacterial viruses, and only plant viruses seem to pose an exception. In animal viruses, the receptor determines, in part, host range and tissue tropism. Viruses are able to subvert a wide variety of cell-surface molecules as receptors. Some viruses are able to recognize very specific proteins, whereas others use sugar moieties or common chemical groups for attachment. Many viral receptors act simply as molecular tethers that concentrate virus particles on the cell surface... 

Figure 2.20 X-Ray structure of PDK1 showing the covalent binding of the molecular tether.

A fundamental reason for measuring rates of electron transfer (ET) is to identify the physical-chemical factors that control those rates and to learn how to control those factors. For the past several years we have focused our experimental efforts on the measurement of the heterogeneous electron-transfer rate constant, k° (units s ), for systems where the redox species are covalently attached to the electrode by any of a variety of molecular tethers [1-3]. The indirect laser-induced temperature method, which we developed [4-6], has the capability of measuring k° values as large as 10 s and has proven to be ideally suited for these types of studies. We will show some recent development that we anticipate will allow the measurement of k° values greater than 10 s . ... 

Previously, two salient issues have beoi cited as important when dialing in int rin recogniticm activity on the textile sur ce 1) covalent and nmi-coval attachment through a molecular tether and 2) sterically allowed OTientations of the integrin recogniticm sequence on the sur ce of the fib. Massia and Hubbell rqxnted the... 

To exploit the 2 term in Equation 9.13, Mao et al. introduced an osmium redox polymer in 2003 in which the redox center was a pendant on the polymer backbone at the end of a 13-atom molecular tether [59]. The increased range of motion for the redox site led to an increase in D pp (see panel G of Table 9.1). A version with a higher redox potential, more appropriate to mediate a cathode enzyme, was also produced [74]. 

Arestraint (not to be confused with a Model Builder constraint) is a user-specified one-atom tether, two-atom stretch, three-atom bend, or four-atom torsional interaction to add to the list of molecular mechanics interactions computed for a molecule. These added interactions are treated no differently from any other stretch, bend, or torsion, except that they employ a quadratic functional form. They replace no interaction, only add to the computed interactions. 

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. 

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

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. 

Another method has also been suggested for tethering [23]. This would require all the molecules designated as the membrane molecules to be tethered to some or all of their neighbors, that are also part of the membrane. Fig. 1 shows the typical structure of a semi-permeable membrane while Fig. 2 shows a typical MD simulation system for osmosis with each membrane one molecular layer thick. In addition, as can be seen from Fig. 2, it is not necessary for the simulation system to be a cube. In fact it is desirable for... 

In contrast, if photoactive chromophores are covalently tethered to the polyelectrolyte main chain, the chromophore reactivity in the photoinduced ET is much more greatly changed because the reaction sites are totally constrained to the polyelectrolyte molecular surface. 

The importance of polydispersity is an interesting clue that it may be possible to tailor the weak interactions between polymer brushes by controlled polydispersity, that is, designed mixtures of molecular weight. A mixture of two chain lengths in a flat tethered layer can be analyzed via the Alexander model since the extra chain length in the longer chains, like free chains, will not penetrate the denser, shorter brush. This is one aspect of the vertical segregation phenomenon discussed in the next section. 

In one of several conceivable combinations of molecular weight and surface density, the binary brush forms a mixed, shorter layer and the extra length in and number of the longer chains is sufficient that these extra segments crowd one another in the outer layer and form an outer, stretched tethered layer. The Alexander analysis applied to the two-layer situation gives for the outer layer thickness of the mixture [132] ... 

The properties and yield of the polymer product were correlated to the NHC identity, providing clear evidence that the NHC ligand was bound and influenced the reaction. Smaller R groups (Me, Et) on 39-R provided low molecular weights, yields, and detectable amounts of impurity. Sugiyama only examined the influence of sterics on the formation of PC, but the initial success inspired Tanaka and coworkers to extend this application by tethering NHC ligands to styrene beads [48]. 

---