Linking, catenane

Poly[2]catenanes Linked by Alternating Topological and Covalent Bonds... 

The -electron-deficient pyridinium ring has played an important role in template-directed synthesis in which the interaction of a -electron acceptor and a -electron donor facilitate self-assembly. This has led to the construction of a variety of supramolecular assemblies including catenanes (linked rings) and rotaxanes (wheel and axle) . 

For the functionalization of polyalkenes with cycle forming structural units, other more complicated ways of catenane linking of macromolecules can be devised and it is only a question of time which of them will be used in the crosslinking of polymers. 

Catenanes are formed when two or more closed-circular DNAs are linked together to form a chain. Catenanes were first isolated in human mitochondrial DNA and have since been identified in a number of biological systems. These stmctures often occur as intermediates during the repHcation of circular DNA molecules. 

Nonionic template synthesis of amide-linked catenanes and rotaxanes with macroheterocyclic fragments 97AG(E)930. 

These compounds contain two or more independent portions that are not bonded to each other by any valence forces but nevertheless must remain linked. Catenanes are made up of two or more rings held together as links in a chain, while in rotaxanes a... 

Artifical photosynthetic reaction centers have continued to peak the interest of scientists over the years and this year is no exception. One such catenane-type example contains a ruthenium /m(2,2 -bipyridine) center, as the sensitizer, which is linked with cycloW.v(paraquat-p-phcnylenc), as the acceptor, and covalently linked with a protoheme or Zn-protoporphyrin, as the donor, located in the myoglobin pocket <00JA241>. A related assembly and on a Au-particle has appeared which possesses a Ru(II)-rm(2,2 -bipyridine)-... 

R. Jager, F. Vogtle, A New Synthetic Strategy towards Molecules with Mechanical Bonds Nonionic Template Synthesis of Amide-Linked Catenanes and Rotaxanes , Angew. Chem Int. Ed. Engl. 1997,36,930-944. 

So, when either replication fork encounters a functional Tus-Ter complex, it halts the other fork halts when it meets the first (arrested) fork. The final few hundred base pairs of DNA between these large protein complexes are then replicated (by an as yet unknown mechanism), completing two topologically interlinked (catenated) circular chromosomes (Fig. 25-17b). DNA circles linked in this way are known as catenanes. Separation of the catenated circles in E. coli requires topoi-somerase IV (a type II topoisomerase). The separated chromosomes then segregate into daughter cells at cell... 

I was introduced to interlocked rings in 1956 by M.S. Newman, a seminar speaker at Harvard. In informal discussion after the talk he described the proposal of a graduate student at Ohio State, L. Friedman, for a many-step synthesis of a catenane. The final reaction was cleavage of the two bonds connecting the linked and chemically bound rings. 

We can use this same approach to prove that other molecular knots and links are topologically chiral. For example, consider the molecular link illustrated in Figure 18. This catenane was synthesized by Nierengarten et al. [12]. For this molecule the set T(G) consists of many unlinks together with many copies of the (4,2)-torus link, illustrated as L in Figure 12. However we saw earlier that this unoriented link is topologically chiral. Therefore, the molecular (4,2)-torus link is topologically chiral as well. 

Another fascinating approach to catenanes via self-assembly involves metal-coordination which templates or directs the assembly of catenane frameworks. After brief discussions on the recent examples of metal-containing catenanes and related topologically interesting molecules (Sections 4.2.1-4.2.3), the focus will be on the self-assembly of Pd(II)-linked catenanes (Sections 4.2.4 and 4.2.5). 

In this and the following sections self-assembling catenanes incorporating (en)Pd(II) units in their backbones are emphasized [21]. Discussion will be focused on the rational design, self-assembly, mechanistic aspect, and chemical manipulation of the transition metal-linked catenanes. 

Electronic Effects in the Self-Assembly of Pd(n)-Linked Catenanes... 

Another question on the self-assembly of metal-linked catenanes is whether it is possible to design various catenane molecules rationally on the basis of the same principle. To elucidate the major factors that control the self-assembly of inter-... 

From the results summarized in Table 1 we can conclude that the self-assembly of Pd(II)-linked catenanes is predictable. When the component rings have a cavity with an appropriate interplane distance (ca. 3.5 A), catenanes are obtained efficiently (e.g. from ligands 12, 22, 24, 26+27, or 29+30). If, however, the cavity is too large or too small, catenanes are not assembled (e.g. from 26, 27, 35, or 36). Thus two conditions must be satisfied if metal-incorporating catenanes are to be obtained by self-assembly. Firstly, component rings should contain reversible coordinate bonds. Second, both component rings should have interplane separation of approximately 3.5 A in the cavity. 

The discovery that DNA forms catenanes and knots, some of them extremely complex, initiated a new field of research which has been called Biochemical Topology [21]. In 1967, Vinograd and co-workers detected in HeLa cell mitochondria isolable DNA molecules that consist of independent, double-stranded, closed circles that are topologically interlocked or catenated like the links in a chain [22, 23]. A few years later, catenanes had been observed everywhere that circular DNA molecules were known [24] and the first knot was found by Liu and coworkers in single-stranded circular phage fd DNA treated with Escherichia coli co-protein [25]. In 1980, knots could also be generated in double-stranded circular DNA [26]. 

The spectroscopic data of the amide-linked catenanes revealed that the cir-cumrotation [6 a] of the two interlocked cycles is hindered, i.e. the steric demand... 

Figure 8. Synthesis of the first amide-linked [2]catenane consisting of two different macrocycles.

Recently Vogtle et al. in cooperation with Sessler et al. synthesized a Leigh-type amide-linked catenane 29 which coordinates anions (Figure 14) [26]. 

After the synthesis of the first amide-linked [2]rotaxanes, Vogtle et al. set out to study the limits of molecular recognition, which in terms of Emil Fischer means to discover if the lock (macrocycle) is specific to a certain key, or if several keys (monoamide threads) fit. It turned out that - in contrast to catenane formation - rotaxane synthesis is very tolerant towards the variation of the building... 

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