Numbering substituted assemblies

A unique feature of such DNA-directed self-assemblies is their site-selective immobilization, which makes it possible to construct well-defined nanostructures. On the other hand, the possibility of the introduction of a vast number of substitutes (like peptidic sequences, nucleoproteins, of hydrophobic hydrocarbon chains) to an adamantane core (adamantyl) makes such a process capable of designing steric colloidal and supramolecular conformations by setting hydrophobic/hydrophilic and other interactions. In addition, the rigidity of the adamantane structure can provide strength and rigidity to such self-assemblies [150]. 

The structure of a single example of a chloro-substituted biguanide [Cu(clbg)2]Cl2 2H20 (clbg=2-chlorobiguanide),has been reported [34]. Chlorine substitution leads to a reduction in the number of N-H donors from seven to six. Five are involved in the construction of a 2-D architecture (Fig. 15). Two pairs of N-H donors form R2(4) assemblies with separate chlorine atoms the fifth forms a N-H 0 contact to the water molecule which also uses its O-H donors to link two chloride anions (Fig. 15). The resultant 2-D sheet architecture lies parallel to the (2 -1 1) plane. The sixth forms a N-H -Cl contact to a chloride anion in an adjacent sheet. 

Our original approach to polysaccharide C-13 n.m.r. spectral analysis consisted of making a minimum number of hypotheses about expected structure-to-spectra relationships (8). By then comparing spectra to known structure for a series of D-glucans, we attempted to establish the validity of these hypotheses and to establish how diverse a structural difference could be accommodated The hypotheses were as follows. Firstly, that each polymer could be considered as an assembly of independent saccharide monomers. Secondly, that these hypothetical saccharide monomers would be 0 alkylated (0 -methylated) in the same positions as the actual saccharide linked residues (it had previously been established that 0-methylation of any a-D-glucopyranosyl carbon atom position resulted in a down-field displacement of vlO p.p.m. for the associated resonance). Thirdly, that each differently substituted residue would have a completely different set of chemical shift values for each carbon atom position (different from the unsubstituted saccharide) but that only the carbon atom positions involved in inter-saccharide linkages would have A6 greater that 1 p.p.m. And, fourthly, that the hypothetical 0-alkylated residues would contribute resonances to the total spectrum proportional to their mole ratio in the polymers. 

In a recent report, 19- or 20-residue thiolated oligonucleotides were used in an exchange reaction with glutathione monolayer-protected gold clusters. After thiol substitution, the resulting DNA-nanoparticle conjugates can be separated by gel electrophoresis on the basis of the number of bound oligonucleotides and assembled with one another by DNA-DNA hybridization [151]. 

One type of oligoamide that can readily be prepared on supports without the need for any partially protected monomers (which are often tedious and expensive to synthesize) are N-substituted oligoglycines (Figure 16.21). These compounds are prepared by a sequence of acylation of a support-bound amine with bromoacetic acid, displacement of the bromide with a primary aliphatic or aromatic amine, and repeated acylation with bromoacetic acid. Because primary amines are cheap and available in large number, this approach enables the cost-efficient production of large, diverse compound libraries. Alternatively, protected N-substituted glycines can also be prepared in solution and then assembled on insoluble supports (Entry 5, Table 16.2). 

Intramolecular versions of the Heck reaction are very useful for the construction of ring systems. The entropic advantage of having both coupling partners present in the same molecule increases the efficiency of the insertion reaction and leads to efficient reactions. Moreover the intramolecular version can be carried out on hindered substituted alkenes, whereas the intermolecular Heck reaction is largely restricted to monosubstituted alkenes. These reactions illustrate the syn stereochemistry of both the insertion reaction and the elimination. A number of multicyclic natural products have been synthesized using intramolecular Heck reactions to assemble the skeletons, and this has become a powerful synthetic tool for such compounds. 

Lacunary spedes can act as structural motifs with numerous metal cations, leading to mono-, di- and trinuclear transition-metal-substituted POMs according to the number of vacant sites. The metal incorporation involves the reaction of aquated first-row and occasionally second-row d-block metal ions, M(H20)6z +, with a lacunary POM derived from the Keggin, Wells-Dawson and other POM families. In addition, lacunary spedes assemble into large POM structures, either directly or with incorporation of metal ion linkers. 

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