Complex polymers mediators

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

Several approaches have been undertaken to construct redox active polymermodified electrodes containing such rhodium complexes as mediators. Beley [70] and Cosnier [71] used the electropolymerization of pyrrole-linked rhodium complexes for their fixation at the electrode surface. An effective system for the formation of 1,4-NADH from NAD+ applied a poly-Rh(terpy-py)2 + (terpy = terpyridine py = pyrrole) modified reticulated vitreous carbon electrode [70]. In the presence of liver alcohol dehydrogenase as production enzyme, cyclohexanone was transformed to cyclohexanol with a turnover number of 113 in 31 h. However, the current efficiency was rather small. The films which are obtained by electropolymerization of the pyrrole-linked rhodium complexes do not swell. Therefore, the reaction between the substrate, for example NAD+, and the reduced redox catalyst mostly takes place at the film/solution interface. To obtain a water-swellable film, which allows the easy penetration of the substrate into the film and thus renders the reaction layer larger, we used a different approach. Water-soluble copolymers of substituted vinylbipyridine rhodium complexes with N-vinylpyrrolidone, like 11 and 12, were synthesized chemically and then fixed to the surface of a graphite electrode by /-irradiation. The polymer films obtained swell very well in aqueous... 

The complexities of the polymer-mediated forces evident from the above discussions make it difficult to formulate theories of coagulation and phase separation for such interactions. Nevertheless, it is instructive to consider in detail an example of how the effects of polymer chains are incorporated in quantitative prediction of dispersion stability. In the following section we discuss such an example, although we restrict ourselves to a discussion of a thermodynamic analysis of stability. 

Imparting new functional properties or an esthetic look to complex polymers is a fast growing area where peroxidases are actively being pursued. Basically the polymer is either oxidized by the enzyme alone or in the presence of mediators followed by grafting as shown in the general scheme in Fig. 7.3. This results in increased binding either between polymers or with introduced functional molecules. 

The same system was employed in the reduction of 4-phenyl-2-butanone to (S)-4-phenyl-2-butanol using HLADH as well as 5-ADH from Rhodococcus sp. with high enantioselectivity [113]. With pentamethylcyclopentadienyl-4-ethoxy-methyl-2,2 -bipyridinechloro-rhodium(III) as mediator and HLADH as catalyst, after 5 h 70% of 4-phenyl-2-butanone was reduced to (S)-4-phenyl-2-butanol with 65% ee. Using 5-ADH. 76% of the ketone was converted to the (S)-alcohol after 5 h with 77% ee. Furthermore, this system has been applied in an electrochemical EMR with a polymer bound rhodium complex as mediator. 

Dhal PK, Arnold FH (1991) Template-mediated synthesis of metal-complexing polymers for molecular recognition. J Am Chem Soc 113 7417... 

The reactions of transition-metal complexes with polynucleotides generally fall into two categories (i) those involving a redox reaction of the metal complex that mediates oxidation of the nucleic acid and (ii) those involving coordination of the metal center to the sugar-phosphate backbone so as to mediate hydrolysis of the polymer. Both redox and hydrolytic reactions of metal complexes with nucleic acids have been exploited with much success in the development of tools for molecular biology. 

The principle of "mediated" electron transfer, whereby electrons are passed from the reduced form of a relatively negative redox couple to the oxidized form of a relatively positive couple, has been demonstrated to occur between two polymer layers of slightly different Ru (bpy)3 complex polymers by Murray and coworkers (18), This kind of stepwise, unidirectional electron transfer may be very significant in future polymer coated PEC cells which seek to separate charge, and of additional Interest, Ru (bpy)3 complexes are frequently used as cyclic PEC catalysts in water splitting experiments. Some details of this experiment are thus Informative. 

Szoka and Haensler investigated the application of PAMAM dendrimers as non-viral vectors for in vitro gene transfer. Their study demonstrated that complexes consisting of G5 PAMAM dendrimers and DNA expression plasmids enhanced transfection efficiency over naked plasmid DNA in many cells, particularly cell lines derived from monkey and human neoplasms. PAMAM cascade polymers mediate efficient transfection of cells in culture. The efficiency of intact dendrimers as synthetic vectors for the delivery of genetic material into cells has also been demonstrated. 

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