Fig. 1.17 Transitions between helical and extended linear conformations via sequential protonation and deprotonation.

shown in Chapter 10, nucleic acids have proven particularly useful in this respect. The foldamers described in this chapter have also been much exploited, particularly the ''fully predictable'' foldamers presented in Section 1.3.

Leaving aside the simplest transitions such as, for example, the dynamic equilibrium between right-handed and left-handed helical conformations, which may be biased in various ways [25, 34, 47], more dramatic transitions between helical and fully extended linear conformations have been implemented in various fully predictable foldamers. For example, 180° rotation about aryl-NHCO and aryl-CONH linkages in aromatic amide foldamers can be effected via sequential protonation and deprotonation reactions [18, 37]. Dolain et al. have shown the folding diversity of a single foldamer capable of transforming between two different helical conformations via a linear conformation using sequential protonation. TFA was used to protonate diaminopyridine units of a helical foldamer (Fig. 1.17a) giving rise to an extended linear conformation (Fig. 1.17b). Subsequent protonation by a stronger acid of the alternating pyridinedicarboxamide units results in an ''inside-out'' helical conformation (Fig. 1.17c) [18]. An original aspect of this system is that the linear strand shown in Fig. 1.2b may convert into a helix either upon protonation or deprotonation. In an analogous fashion, Kanamori et al. demonstrated linear-to-turn transformations in unsymmetrically linked phenolic oligoamides. In this case, deprotonation was used to switch between the linear conformer (Fig. 1.17d, NH--OH and OH--O=C hydrogen bonding) and a bent conformer (Fig. 1.17e NH---O(oxyanion) hydrogen bonding) [12].

Lehn's group has also used metal coordination to effect conformational changes in oligoheterocyclic molecular strands. In this case, aryl-aryl and aryl-imine linkages undergo 180° rotations upon ion binding that result in linear-to-helical or helical-to-linear conformational transitions of oligomeric strands (Fig. 1.18) [82, 98, 149]. For example, the interaction of Pb(II) with helical oligo-pyridyl-pyrimidine strands generates polymetallic racks - or grid-type supramo-lecular architectures - by uncoiling of the oligomeric ligand. The reversible inter-

Fig. 1.18 Transitions between helical and extended linear conformations via ion binding and unbinding.

conversion between the helical free ligand and the linearly extended ligand was controlled by coupling the ion binding/unbinding process with a competing ligand (tris(2-aminoethyl)amine; tren), whose ion affinity can be modulated with pH. Additional conformational changes can be envisaged in the folded conformations stabilized by metal coordination. For example, electrochemical changes of the oxidation state of the metal may induce a rearrangement of its coordination sphere that, in turn, modifies the conformation of an oligomeric ligand [150].

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