Probing Molecular Chirality -- New Photoelectron Circular Dichroism Techniques developed in School of Chemistry and Soleil/DESIRS collaboration

A long-standing collaboration between scientists from the University of Nottingham (UK) and Soleil/DESIRS group has pioneered and developed a powerful new technique, Photoelectron Circular Dichroism (PECD) that exploits the ability to produce circularly polarized (left and right handed) synchrotron radiation to probe sensitively molecular handedness. Specifically, PECD detects the asymmetric motion of electrons through a chiral molecule upon photoionization by imaging the spatial distribution of emitted electrons (see Figure).

In a recent study [1] the team examined a number of small diols (double alcohols). These  OH groups prefer to align themselves head-to-tail within the molecule, creating the possibility for a mirror image pair even in the smallest, nominally symmetrical (so a priori non-chiral)  1,3 propanediol molecule. However, rotation of the OH groups is facile, so that these forms rapidly interconvert. Propanediol’s chirality is therefore only transient and the distinct forms cannot be isolated.

This study showed that in the larger 1,3 butanediol, which is already permanently chiral because of its molecular configuration, the transient sense of handedness induced by the OH groups’ conformation can nevertheless be the dominant chiral factor controlling the behaviour of the frontier electrons. So relatively weak interactions with a chiral substrate could stabilise or lock the OH rotations in a preferred conformation to provoke a chiral response in the electronic structure of the diol, seemingly independent of the molecule’s fixed absolute chiral configuration.  In effect the diol inherits a handedness induced by its interaction with an external chiral partner, with important consequences for so-called chiral recognition in biological systems.

1       Daly, S., Tia, M., Garcia, G. A., Nahon, L. & Powis, I. The Interplay Between Conformation and Absolute Configuration in Chiral Electron Dynamics of Small Diols. Angewandte Chemie International Edition 55, 11054-11058, doi:10.1002/anie.201603771 and 10.1002/ange.201603771 (2016).