Water Lends a Hand

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Science  04 May 2007:
Vol. 316, Issue 5825, pp. 663
DOI: 10.1126/science.316.5825.663a

Although the global chirality of molecular aggregates is strongly influenced by the individual chirality of the building blocks, it is not generally a simple matter to predict one from the other. In a series of careful experiments, Johnson et al. uncover the subtle environmental factors that determine the helical handedness of rosette nanotubes assembled in solution from small organic heterocycles bearing a chiral side chain. The heterocycles (which are self-complementary toward hydrogen bonding) first form hexameric supermacrocycles, which in turn stack into a helical arrangement. Dissolution of a single enantiomer of this building block in methanol gives rise to one helical isomer, but addition of as little as 1% water to the solvent instead induces opposite helicity in the stacks. The authors show that the water-induced product is thermodynamically favored, but faces a larger kinetic barrier than its counterpart to formation in pure methanol (though aggregation in both senses appears to be accelerated in the absence of water). They further find that chiral inversion of the kinetic isomer in methanol can be catalyzed by the thermodynamic isomer. Inversion is also possible at an early stage by heating, though after 3 days the kinetic isomer becomes stereochemically locked, a result attributed in part to extensive solvation. — MSL

J. Am. Chem. Soc. 129, 5735 (2007).

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