Credit: © 2009 Wiley

The formation of helices and their stereomutation — conversion into a helix of opposite handedness — have been investigated for a variety of macromolecular structures. This conversion has typically been induced by chemical or thermal stimuli and previously occurred through the breaking, formation or isomerization of bonds. Now, Tamaki Nakano at Hokkaido University in Japan and co-workers have observed1 a stereomutation that is not accompanied by changes in the chemical structure of the polymer.

Nakano and co-workers use an achiral acrylate derivative that bears a biphenyl-based side group as a monomer. Its polymerization leads to the formation of an optically active compound. Circular dichroism and NMR spectroscopy, supported by molecular mechanics simulations, suggest that the polymer obtained consists of a polyacrylate chain adopting a helical conformation with a preferred handedness. In addition, the side groups adopt a twisted, rather than a coplanar, conformation.

The helical structure remains stable when heated, but undergoes rapid racemization on irradiation with light at 320 nm. Its chemical structure, however, is not affected, as revealed by a virtually unchanged ultraviolet spectrum. Nakano and co-workers suggest that this conformational stereomutation is triggered by the photoexcitation of the side chains, which causes their conversion from a twisted to a coplanar form. This in turn induces the racemization of the polymer.