Self-complementary motifs

The simplest self-complementary motif involves two-point donor-acceptor (DA) H-bonding interactions that form cyclic dimers. Carboxylic acids, for example, are ubiquitous and have been investigated for decades. Fourier transform infrared (FTIR) spectroscopy of poly(acrylic acid)s and ethylene-methacrylic acid copolymers show that dimers persist well above the glass transition temperature (Tg) of the materials [12, 13]. Furthermore, carboxylic acids can interact within polymer melts. Instead of forming dimers, they organize into clusters, providing the basis for a supramolecular network. Lillya and colleagues showed that telechelic... 

Fig. 31. Supramolecular (hydrogen-bonded) motifs of self-complementary molecules (196).

Fig. 2.10 Possible self-complementary hydrogen bonding motifs for replication...

Applications of cyclised oligonucleotides are varied. They have been used to produce artificial human telomeres by rolling circle DNA synthesis/as inhibitors of viral replication in influenza virus and as structural motifs for quadruplex formation.A further form of cyclic oligonucleotide figures in a recently described method in which a self-complementary oligonucleotide, e.g., a hairpin structure, is denatured and allowed to re-anneal in the presence of circular DNA such as a plasmid (7). The effect is that the short oligonucleotide traps the plasmid in what has been termed a padlock. Such structures have been successfully used to inhibit transcription elongation reactions based on triple helix formation of the padlock structure. 

Carboxylic acids, oximes, diones and amino-pyridines can all form homomeric 0-D motifs through self-complementary and, frequently, symmetry-related hydrogen-bond interactions,... 

Amides [33], pyrimidines [34], dicarboxylic acids [35], dioximes [36, 37] and dipyr-idones [38] can all form homomeric 1-D motifs through self-complementary interactions, Scheme 2.5.3. 

Amides frequently combine two unique structural motifs, dimers and catemers, into a well-known 1-D ribbon-like structure. The dimer is constructed from self-complementary N-H O hydrogen bonds, while the catemeric N-H O hydrogen bonds result from the anti-proton of the amine and the bifurcated carbonyl oxygen atom. 

As well as the examples given in the previous section, in which complementarity of functional groups is used to form supramolecules, there are others in which selfcomplementarity generates interesting homodimeric complexes. A case in point is aminomethylbenzo[18]crown-6 which forms dimers when the amine sidearm is protonated [1]. The ammonium-[18]crown-6 motif is well known and, in this molecule, both host and guest are joined. More complex examples, such as those in Figure 5.8, include the self-complementary tennis balls [2] and other cavitands [3-5] designed by Rebek who has shown how the relative positions and orientations of solvent encapsulated within dimeric assemblies can be determined, in the absence of X-ray data, by NMR. A concise treatment of the many approaches to supramolecular self-assembly are outlined in a 1995 review [6]. 

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