Shifting into Reverse

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Science  28 May 2004:
Vol. 304, Issue 5675, pp. 1215
DOI: 10.1126/science.304.5675.1215a

The right-handed DNA helix can be changed into left-handed Z-DNA by complexation with achiral agents such as metal ions or polyamines such as spermine. Similarly, synthetic helical polymers have been synthesized that can change helicity with changes in temperature. Miyake et al. show that a small CoII complex can reverse its helical sense in the presence of an achiral molecule, the relatively small nitrate anion. An x-ray structure shows that a chiral tetradendate ligand coordinates CoII with two amine nitrogens and two amide oxygen atoms in a λcis-α configuration. In solution, addition of nitrate switches the sign of the molecule's circular dichroism (CD) signal, whereas ClO4 and BF4 anions had no effect. Two equivalents of nitrate were needed to completely reverse the CD spectrum and convert the molecule to the δ isomer. Binding of nitrate to the amide hydrogen creates steric repulsions that destabilize the λ isomer. Unlike many other systems that show helical inversion, the kinetically labile CoII complex undergoes this transition rapidly (in under 1 min) at room temperature. — PDS

J. Am. Chem. Soc. 10.1021/ja049130o(2004).

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