Addition template-directed

The heparin and poly-L-glutamate titrations show a markedly different behavior than do the DNA titrations. As polyanion is added, the fluorescence of the an-thrylpolyamine solution decreases until a well-defined minimum is reached. A new emission at 510 nm, which we assign to the anthracene excimer of 14, increases and decreases coincidently with the titrated fluorescence minimum. Likewise, the UV spectrum of 10 fiM 14 with added heparin shows hypochromism that occurs and disappears coincidently with the fluorescence minimum and a 2-nm red shift. We have proposed template-directed excimer formation as the physical basis for these observations. In the absence of heparin, fluorescence of the unassociated probe is observed. As heparin is added, the fluorescence decreases as a result of heparin-directed interaction between probe molecules. Additional heparin permits the fluorophore population to diffuse over the length of the poly anion, thus avoiding excimer formation and yielding a net CHEF. 

Fig. 16-9 Template-directed addition of a nucleotide unit to a growing DNA chain by DNA polymerase I. The bold letters indicate newly synthesized DNA. The rows of dots denote hydrogen bonds.

In 1992, Goodwin and Lynn reported the first example of template-directed synthesis of DNA analogs via formation of a reversible imino linkage [21]. Five years later, Hue and Lehn described the first use of the reaction of imine condensation for the selection, by DCC, of carbonic anhydrase inhibitors from a library of amines and aldehydes [8], Upon addition of the enzyme, the formation of one library component was strongly amplified when compared to a similar reaction carried out in the absence of template. Since then, imine exchange reaction has been applied... 

The synthesis of macrocycles with the aid of the dilution principle still remains a dominating cyclization method although there have been many attempts to avoid the standard procedure, i.e., the simultaneous addition of reactants to the reaction medium (see Sect. 4). The dilution principle can be avoided in some cases [90,92] by application of template-directed reactions and/or by use of the rigid group principle . 

DNA polymerases catalyze the formation of polynucleotide chains through the addition of successive nucleotides derived from deoxynucleoside triphosphates. The polymerase reaction takes place only in the presence of an appropriate DNA template. Each incoming nucleoside triphosphate first forms an appropriate base pair with a base in this template. Only then does the DNA polymerase link the incoming base with the predecessor in the chain. Thus, DNA polymerases are template-directed enzymes. 

Figure 2.37 Example of a regioselective, template-directed second addition to a fullerene.

DNA polymerases contain two active sites, a polymerase site that catalyzes the reversible, template-directed elongation of DNA, and the exonuclease site, which catalyzes the hydrolysis of the 3 -terminal base. During processive synthesis, DNA replication occurs by the consecutive addition of bases without the intervening dissociation of the DNA, and the DNA can migrate between the polymerase site and the exonuclease site without dissociating from the enzyme... 

Non-enzymatic template-directed synthesis is a positive aspect of the RNA world scenario (57). One problem with this is that the only effective non-enzymatic template-directed synthesis is that of 2-methylImpG on a poly(C) template. Additional catalysts may be needed to provide RNAs that contain all the nucleotides required for the establishment of the genetic code. 

Hirsch and co-workers subsequently discovered that the yield of hexakis-adducts such as 8 or 9 can be greatly increased and their separation from side products substantially facilitated, if 9,10-dimethylanthracene (DMA) is used as a template (Scheme 7-2) [14, 15]. Anthracene derivatives such as DMA are well known to undergo reversible Diels-Alder additions with fullerenes at ambient temperamre [16-18]. After addition to the fullerene, the template directs diethyl malonate addends in the Bingel addition tegio-selectively into e positions, ultimately yielding the hexakis-adducts with a pseudo-octahedral, all-e addition pattern. The templated activation of e 6-6 bonds (bonds between two six-membered rings) is also efficient starting from C o mono-adducts, and several examples are shown in Scheme 7-2. Thus, bis(alkynyl)methanofullerene 10 [19] reacted with diethyl 2-bromomalonate/DBU (8 equiv) and DMA (12 equiv) to provide hexakis-... 

In another approach Subramanian et al. [32] infiltrated the polymeric opal template directly with ultrafine particles instead of employing metal alkoxides or salts. In this approach infiltration with a nanocrystalhne material of known crystal phase is possible, and therefore materials with a predetermined crystal structure of the walls can be obtained even at mild processing conditions. For instance, Ti02 frameworks with a rutile phase of the wall material could be obtained without the necessity to resort to sintering at high temperatures. Additionally, shrinkage, which commonly occurs in the case of condensation of metal alkoxides or conversion of metal salts, is largely reduced. Finally, this route opens a pathway to obtain porous metal oxide materials, which are barely accessible by wet chemistry approaches. 

---