Template-guided synthesis

Employing a polyelectrolyte to bind to and preferentially align the aniline monomers before polymerization (e.g., by S2082-) has shown promise in facilitating the desired head-to-tail coupling of the aniline substrates. During polymerization, the anionic polyelectrolytes such as poly(styrenesulfonate) and poly(acrylate)86-88 also provide the required counterions for charge compensation in the doped PAn products. This can lead to water-soluble or water-dispersed ES products. 

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A template-guided synthesis of water-soluble chiral-conducting PAn in the presence of (S)-(—)- and (R)-(+)-2-pyrrolidone-5-carboxylic acid [(S)-PCA and (R)-PCA] has been reported to produce nanotubes.228 The structures prepared have outer diameters of 80-220 nm with an inner tube diameter of 50-130 nm. It was proposed that the tubular structures form as a result of the hydrophobic aniline being templated by the hydrophilic carboxylic acid groups of the PCA in aqueous media during chiral tube formation. The resultant tubes were shown to be optically active, suggesting that the PAn chains possess a preferred helical screw. 

McCarthy and coworkers126 229 reported a template-guided synthesis of water-soluble chiral PAn nanocomposites. The nanoparticles were prepared by the physical adsorption of aniline monomer onto a templating poly(acrylic acid) in the presence of (+)- or (-)-CSA, followed by chemical oxidation. Using this approach, optically active nanocomposites of approximately 100 nm diameter were formed. Earlier work by Sun and Yang230 using polyelectrolytes produced similar nonchiral dispersions in which the PAn chain is interwound with a water-soluble polymer by electrostatic forces.231 Similar work by Samuelson and coworkers utilized DNA as a chiral template for PAn.232... 

An elaboration of this technique, incorporating the concept of template-guided synthesis, is the use of nanoporous matrices such as zeolites and polycarbonates as a template within whole pores to perform the chemical polymerization of aniline monomers. For example, Wu and Bein254 have prepared nanofilaments of conducting PAn in the 3-nm-wide channels (pores) of the aluminosilicate host, MCM-41, through initial adsorption of aniline vapor into the dehydrated host followed by oxidation with (NH)2S208. 

Template-guided synthesis 2,4) is currently being used increasingly to synthesize water soluble polyaniline. Apofyadd is used as a tenq>late to bind the aniline monomers to form the corresponding anilinium salt. Aniline monomers polymerize to form a conq)lex with the template. The hydrophobic and hydrophilic ends of the tenq)late he in sohibifizing the hydrophobic polyaniline chains. Some of the pofyadds used (25) in the synthesis of polyaniline are polystyrenesulfonic add, poly(aci dic add) and poly(methacrylic acid). 

Template-guided synthesis Comparison of chemically and electrochemically prepared PAn films In situ chemical polymerization Deposition on glass/plastics... 

The inter-relationship between colloid and polymer chemistries is completed by colloidal polymer particles. The formation of 50-nm-diameter, 100- to 200-nm-long polyaniline fibrils in a poly(acrylic acid)-template-guided polymerization, similar in many ways to those produced from polymerized SUVs (see above), provides a recent example of polymer colloids [449], The use of poly(styenesulfonic acid) as a template yielded globular polyaniline particles which were found to be quite different morphologically from those observed in the regular chemical synthesis of polyaniline [449]. 

Figure 7.1 Schematic representation of target-guided synthesis to generate a ligand (guest) templated by a target biomolecule (host) using the lock and key descriptors of Emil Fischer.

There are some exceptions to the norm of DNA-dependent DNA or RNA synthesis, mostly in lower eukaryotes or viruses (Fig. 5). One example is RNA-dependent RNA synthesis in plant, animal, or bacterial viruses. In these cases, a single-stranded RNA template rather than double-stranded DNA guides synthesis of the complementary RNA strand, based on conventional base pairing. The polarity of RNA adds a level of complexity during synthesis. Thus, the RNA genome of a virus that can be directly read and thus provides the mRNA function is called the positive strand, as in polio vims. In this case, the viral genome RNA functions as the mRNA and encodes the RNA polymerase, which is synthesized like other viral proteins in the infected cell. This RNA polymerase subsequently synthesizes the complementary... 

The RNA world hypothesis requires a nucleotide polymer to reproduce itself. Can a ribozyme bring about its own synthesis in a template-directed manner The self-splicing rRNA intron of Tetrahymena (Fig. 26-26) catalyzes the reversible attack of a guanosine residue on the 5 splice junction (Fig. 26-37). If the 5 splice site and the internal guide sequence are removed from the intron, the rest of the intron can bind RNA strands paired with short oligonucleotides. Part of the remaining intact intron effectively acts as a template for the... 

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