Biomedical Engineering Reference
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Fig. 11 Schematics of the extension-contraction motion in a rotaxane dimer ( top left box ) and the
contraction of 12 2+ upon demetalation and remetalation with Zn(II) ions
3.5
Other Systems
3.5.1
A Molecular Muscle
An exciting development in the fi eld of artifi cial linear molecular motors was the
construction of a rudimentary molecular-scale muscle (Jiménez-Molero et al. 2000,
2002 ). The idea started from the topology of a rotaxane dimer which, if suitably
designed, can undergo contraction and stretching movements, as schematized in
the box of Fig. 11 . The synthesized system (Fig. 11 ) is a rotaxane dimer, 12 2+ , that
contains two copper(I) metal ions and two identical ring-and-axle components.
Each of these components consists of a macrocyclic ring containing a bidentate
phenanthroline unit, an axle containing a bidentate phenanthroline (PHEN), a ter-
dentate terpyridine (TPY), and a bulky stopper unit. Each one of the two Cu(I)
metal ions present in the rotaxane dimer is coordinated to two bidentate chelates
since Cu(I) prefers a four-coordination arrangement. Under these conditions, the
system is “extended” (Fig. 11a , state 0; length 8.3 nm, estimated from molecular
models). Upon electrochemical oxidation of Cu(I), it was expected that the system
contracts since Cu(II) prefers a fi ve-coordination arrangement and therefore should
be surrounded by a bidentate and a terdentate ligand. This change in the coordina-
tion environment, in fact, had been previously observed for rotaxanes and cat-
enanes of the same family (Dietrich-Buchecker et al. 2003 ). In the case of 12 2+ ,
however, electrochemical oxidation had no effect and contraction could be obtained
only by chemical extraction of Cu(I) with a large excess of cyanide ions (Fig. 11b )
and successive remetalation of the free ligand with zinc(II) ions (Fig. 11c , state 1).
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