Functional catenanes

Three doubly spin-labelled [2]catenanes with different sizes were studied by 4-pulse DEER.52 The experimental distribution of interspin distances was compared with a theoretical pair-correlation function computed based on geometrical constraints. In chloroform solution the medium and large catenanes were close to fully expanded, but in glassy o-terphenyl they were partially collapsed. For the smaller catenane there was a higher population of shorter interspin distances, which was attributed to interactions between unsaturated sections of the molecule. 

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... 

After chromatographic removal of 3, the cleavage of the catenane, 4, to 3 and 1 was critical evidence of the structure. Numerous blank experiments excluded other possibilities. Figure 1 essentially describes a double-labeling experiment deuterium for one ring and the acyloin function for the other, the labels detected by their infrared absorption [6], The few milligrams of purified 3 could not be crystallized. The name catenane was based on the Latin catena for chain . 

The sulfonamide macrocycle proved to be a hospitable template, and at the same time the sulfonamide functionality bears the possibility of farther derivatiza-tion of [2]rotaxanes (see Section 8.4 Chemistry with Amide-Based Catenanes and Rotaxanes). Furthermore the expansion of the cavity did not afford a change of the blocking group - no disassembly of wheel and axle was detected. 

The great variety of incorporable building blocks also offers the synthetic chemist many potential structural and functional design possibilities. The insertion of, e.g., photo-responsive elements, groups with further supramolecular derivatization potential, or sulfonamide units which enable subsequent inter- and intramolecular linkage of catenanes and rotaxanes might render good service in the development of molecular switches [64] and devices [65]. 

This is particularly true, since rotaxanes and catenanes have entered a stage in their development, in which they have undergone the transition from beautiful structures with ever-new topologies to those which possess function. Many so-called molecular devices have been synthesized and studied with respect to a controlled motion of the two components relative... 

Table 10.4 Yields of [2] catenanes produced via the type of reaction shown in Scheme 10.14 as a function of crown ether ring size.

The existence of interlocked molecules such as 10.95 can be established by a variety of spectroscopic techniques. Most importantly, mass spectrometry provides very characteristic patterns for catenanes. The mass spectra for catenated species are very different from those of covalently linked precursors (such as 10.94) but are more than the sum of their two individual components. Catenane mass spectra are characterised by the appearance of peaks at high mlz corresponding to the parent species as well as fragments corresponding to the transfer of hydrogen atoms from one macrocycle to the functional... 

Although structurally and functionally appealing, the synthetic ways to build these species remained a hard task until mid 80s. Then, a strategy was developed by Sauvage and his co-workers that used coordination to a Cu(I) ion prior to the macrocyclization in the catenane synthesis (Figure la). The metal that served as a template with tetrahedral coordination geometry was then removed by demetallation with cyanide [8],... 

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