Templating mechanisms

Kresge C T, Leonowicz M E, Roth W J, Vartuli J C and Beck J S 1992 Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism Nature 359 710-12... 

Kokufuta, E Jinbo, E, A Hydrogel Capable of Facilitating Polymer Diffusion through the Gel Porosity and Its Application in Enzyme Immobilization, Macromolecules 25, 3549, 1992. Kresge, CT Leonowicz, ME Roth, WJ Vartuli, JC Beck, JS, Ordered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Template Mechanism, Nature 359, 710, 1992. 

The kinetics and mechanisms of template reactions are rarely studied in depth - they are often merely assumed to be genuine template reactions rather than a metal-activated reaction at a coordinated ligand. The best evidence for a true template mechanism is the isolation and characterization of intermediates with both reactants and with the macrocyclic product complexed with the template cation. 

In vitro, fibril formation by several proteins displays an initial lag phase, followed by a rapid increase in aggregation (reviewed in Rochet and Lansbury, 2000). Introduction of fibrillar seeds eliminates the lag phase. These cooperative aggregation kinetics suggest that fibril formation begins with the formation of a nucleus and proceeds by fibril extension. The structure of the nucleus must therefore act as a template for the protein s conformation in the fibril. As the structural requirements for templating are unclear, it is difficult to assess the consistency of the model classes with this feature of fibril formation. We have described one possible templating mechanism for the cross-/ spine of GNNQQNY (Nelson et al., 2005). 

Figure 1. Scheme for the liquid crystalline templating mechanism proposed by Kresge et al 1 for synthesis of mesoporous silica MCM-41. Formation of a hexagonal array of cylindrical micelles possibly mediated by silicate anions followed by condensation of the silicate anions from the silicate source (tetraethylorthosilicate) leads to templated framework structure. Calcination or extraction of the template produces hexagonally ordered mesoporous silica. 

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

Adachi M, Harada T, Harada M (1999) Formation of huge length silica nanotubes by a templating mechanism in the laurylamine/tetraethoxysilane System. Langamuir 15 7097-7100... 

The autoaccelerated character of acrylic acid polymerization is strictly correlated with such a form of monomer organization. The fast zip-up propagation takes place along oriented double bonds. Template mechanism of polymerization in these systems was also confirmed by examination of the tacticity of the polymer obtained. 

A template mechanism was also proposed for spontaneous polymerization of propargyl chloride in the presence of poly(4-vinylpyridine). The reaction consists of partial (6-8%) alkylation of P4VPy by propargyl chloride which leads to stable complex P4VPy with monomer and then to polymerization. The product obtained contains a conjugated bond system formed by opening triple bonds in the monomer according to reaction ... 

More than 90% double bonds react according to the template mechanism. 

Using labeled initiator and radiometric analysis, the number of initiator fragments from the number of end-groups, R, can be evaluated and the number of crosslinking points can be calculated. The number of unreacted double bonds can be found by the bromometric method. On the basis of such analysis, we can find the percentage of units in the multimonomer which had reacted according to template mechanism. The results of the calculations for two examined systems are presented in Tables 5.3 and 5.4... 

Above the critical concentration of the template, the monomer-template contacts are replaced in part by mutual contacts between template units, but description of this case seems to be more complicated. The considerations mentioned above do not pre-judge that polymerization proceeds inside the coils of the template according to template mechanism. It shows only that local concentration of the monomer in the reaction place is different from the overall concentration. 

Polymerization was carried out in dioxane solution at 75°C. For kinetics examination multimonomer with molecular weight M = 46,500 was selected. It was found that polymerization proceeds according pure template mechanism - without changes in molecular weight and molecular weight distribution up to more than 60% conversion. More experimental data is needed in order to determine the order of reaction with respect to monomer and initiator, as well as energy of activation. 

Fig. 77. Schematic drawing of the liquid-crystal templating mechanism. Hexagonal arrays of cylindrical micelles form (possibly mediated by the presence of silicate ions), with the polar groups of the surfactants (light grey) to the outside. Silicate species (dark grey) then occupy the spaces between the cylinders. The final calcination step burns off the original organic material, leaving hollow cylinders of inorganic material [473]...

The pore sizes of sol-PILB-Cn samples are larger than that of the normal sol-PILB (given on the vertical axis) and increases with Cn. The variation trend of the pore dimension with Cn is very similar to that for MCM41 samples. This supports the template mechanism suggested above. 

While peptide antibiotics are synthesized according to enzyme-controlled polymerization patterns, both proteins and nucleic acids are made by template mechanisms. Tire sequence of their monomer emits is determined by genetically encoded information. A key reaction in the formation of proteins is the transfer of activated aminoacyl groups to molecules of tRNA (Eq. 17-36). Tire tRNAs act as carriers or adapters as explained in detail in Chapter 29. Each aminoacyl-tRNA synthetase must recognize the correct tRNA and attach the correct amino acid to it. The tRNA then carries the activated amino acid to a ribosome, where it is placed, at the correct moment, in the active site. Peptidyltransferase, using a transacylation reaction, in an insertion mechanism transfers the C terminus of the growing peptide chain onto the amino group of... 

The genetic message carried by the sequence of bases in the DNA is replicated by a template mechanism. 

Single stranded RNA molecules are synthesized by a template mechanism from select regions of the DNA genome. 

Scheme 13 Tentative template mechanism leading to the formation of a trefoil knot. After...

Another approach to test a template mechanism is to use appropriate control compounds that can be expected to fail to yield the rotaxanes or catenanes, if the tentative mechanism is indeed operative. In order to gain further support for the template mechanism of amide rotaxane formation (Scheme 10), it is desirable to have additional evidence for the... 

So far. we have presented certain concepts on carbonate deposition using a template mechanism which is based on solid state principles. One should, however, bear in... 

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