Rings-on-a-string

Fig. 3.11. Folding of T4 DNA by the addition of the gemini surfactant. Distributions of the long-axis length of T4 DNA at different concentrations [cs] of the surfactant. Coil, partially folded, and completely folded states are distinguished by the different colorings. Also shown are FM and AFM images with the corresponding schematic representation of the partially folded state ([cs] =0.2 pM) and completely folded state ([cs] = 1.0pM). The FM and AFM observations are of the same DNA molecules attached to a mica surface. A rings-on-a-string structure is clearly seen for the partially folded DNA, while the completely folded DNA assumes a network structure composed of many fused rings (see [19] for more details)...

Switches can be realized on the basis of the supramolecular design known as ro-taxane. A rotaxane consists of a ring threaded on a string or axis around which it can move freely. The ends of the axis, however, are blocked by bulky headgroups so that the ring will not fall off (Fig. 2.17. 

Lateral Translation of a Ring on the Molecular String on which it is Threaded Electrochemically-driven Motion... 

White composite cultured blister pearls can also be dyed (Fig. 9.3). Pearls are drilled li t throng the centre for stringing as beads, or part way throu to be mounted and glued on a pin, in, for example, a ring mount. 

The introduction of bis(p-chlorophenyl) and bis(methylthio) substituents in the 4 - and 4"-positions of quinquepyridine (35) (that is, on the backbone of the second and fourth pyridyl rings in the string) was reported to have little effect on the coordination behaviour of these ligands relative to that of the unsubstituted analogue (see earlier). Namely, with nickel(II) and copper(I) and/or copper(II), doublehelical 2 2 complexes were obtained, while cobalt(II) forms both a 1 1 and a 2 2 complex in the solid state (although, in solution only a mononuclear seven-co-ordinate complex occurs - with the solid state, double-helical structure interconverting to this form upon dissolution). 

Fig. 16A-D. Mechanical switching in rotaxanes. A Rotaxanes may exist in isomeric states by the movement of the ring component between dissymmetric sites on the string component. B A redox- or pH-switchable [2]rotaxane. While the cyclophane complexes the native benzidine site (spectrum, curve a), the reduced or protonated benzidine repels the cyclophane, causing it to move to the dioxybiphenylene site (spectrum, curve b). C An azobenzene-based switchable [2]rotaxane. The cyclodextrin ring complexes the azobenzene site in the trans-state, but it is repelled from the ds-azobenzene. The state of the system is measurable by circular dichroism (plot). D A pH-switchable rotaxane. When the amine on the string component is protonated, it complexes the crown ether ring by hydrogen-bonding interactions (40a). When the amine is deprotonated, however, the ring component moves to the bipyridinium unit, where it is complexed by n donor-acceptor interactions (40b). The plots in B and C are adapted from [67] and [69], respectively, with permission...

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