Self-recognition, helicates

The diaminopyridodipyrimidinedione 73 has been synthesised as a cytosine-like Tecton designed for self assembly into a helical superstructure. Self recognition occurs in the solid state <96MI02 96CA(125)221771 >. 

Kramer, R. Lehn, J.-M. Marquis-Rigault, A. Self-recognition in heUcate self-assembly Spontaneous formation of helical metal complexes from mixtures of ligands and metal ions. Proc. Natl. Acad. Sci. USA 1993, 90, 5394-5398. 

Scheme 4.2 Self-recognition within mixtnres of bipyridine-based Cn+ donble helicates.

A particular point of interest included in these helical complexes concerns the chirality. The lielicates obtained from the achiral strands are a racemic mixture of left- and right-handed double helices. This special mode or recognition where homochiral supramolecular entities, as a consequence of homochiral. self-recognition, result from racemic components is known as optical self-resolution. It appears in certain cases from racemic solutions or melts (spontaneous resolution) and is often cited as one of the possible sources of optical resolution in the biological world. 

Fig. 48. Self-recognition m the self-assembly of the double helicates 132-135 (X = H) from a mixture of the ohgobipyndine strands 128-131 (X = CONEt2) and Cu(l) ions BF4- or PF - anions omitted [9 173]...

Fig. 49. Self-recognition in the self-asssembly of the double helicate 133 and the triple heli-cate 149 from a mixture of the oligobipyridine strands 129 and 148 and of Cu(l) and Ni(ll) ions (CIO4- anions omitted) [9.173].

In both experiments, the desired helicates are generated from a mixture of starting compounds by self-assembly with self-recognition it involves the spontaneous selection and preferential binding of like metal ions by like ligand strands in a mixture to selectively assemble into the corresponding helicates. 

As observed for the molecular clips reported above, the chiral scaffold is pivotal in promoting homo- or heterochiral self-discrimination. Amide hydrogen bonds were implemented on helicene chiral scaffolds as well, but in this case dimerization of the monomers was characterized by homochiral enantioselective self-recognition, that is self-association between molecules with the same helicity (Fig. 17B) [44], These species dimerized in solution with association constants of 207 M 1 by means of four non-covalent bonding interactions and, in combination with the peculiar helical shape of the monomers, forms only homochiral dimers. 

The spontaneous, highly selective process of helicate formation has been reported to exhibit self-recognition [120,136]. Amongst a variety of examples, Raymond and coworkers have reported that gallium(III) and iron(III) coordination by the bzs-bidentate catecholate ligands 3, 11 and 12 facilitate... 

Fig. 12 Self-recognition and formation of triple-stranded helicates 13, 4, and 14 from bis-catechol ligands 11, 3, and 12 and iron(III) or gallium(III) ions, respectively [120]...

Fig. 3a-d. A diagrammatic representation of double-stranded helicates (23, 34] containing (a) two, (b) three, (c) four, and (d) five 2,2 -bipyridine subunits [23], The Cu(I) ions function as a template around which the helicates can assemble. Furthermore, this assembly process exhibits (i) self-self recognition in the preferential pairing to the same ligand in the presence of others in the reaction mixture, and (ii) positive cooperativity in which the complexation of one metal ion facilitates the binding of the next... 

These complementarity rules owe their discovery to the chemical analysis of DNA by Chargaff and associates (3). The DNA from many different organisms shows the same patterns of base composition, namely A and T are always present in equal quantities, as are G and C. The immediate corollary of this observation, that a purine base (R) exists for every pyrimidine base (Y) and vice versa, led Watson and Crick to propose that two helical strands in DNA are held together by specific, intermolecular purine-pyrimidine (R Y) interactions (4). In turn, this unique chemical complementarity of the double-helical structure, proved to be a major breakthrough to understand the self-recognition and self-reproduction of DNA and forms the cornerstone of structmal biology as we know it today, more than half a century later. 

FIGURE 6 Self-recognition of (A) homopolymetallic double-stranded helicates [CUm(L/f)2]" = 4-6 Kramer et al., 1993), (B) multiple-stranded helicates [Cu3(Ll)2] ... 

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