PNNL

Abstract

Chirality is a fundamental molecular property that plays a crucialrole in biophysics and drug design. Optical circular dichroism (OCD) is a well-established chiral spectroscopic probe in the UV-visible regime. Chirality is mostcommonly associated with a localized chiral center. However, some compoundssuch as helicenes (Figure 1) are chiral due to their screwlike global structure. Inthese highly conjugated systems, some electric and magnetic allowed transitionsare distributed across the entire molecule, and OCD thus probes the globalmolecular chirality. Recent advances in X-ray sources, in particular the control oftheir polarization and spatial profiles, have enabled X-ray circular dichroism (XCD), which, in contrast to OCD, can exploit thelocalized and element-specific nature of X-ray electronic transitions. XCD therefore is more sensitive to local structures, and thechirality probed with it can be referred to as local. During the racemization of helicene, between opposite helical structures, the screwhandedness can flip locally, making the molecule globally achiral while retaining a local handedness. Here, we use the racemizationmechanism of [12]helicene as a model to demonstrate the capabilities of OCD and XCD as time-dependent probes for global andlocal chiralities, respectively. Our simulations demonstrate that XCD provides an excellent spectroscopic probe for the time-dependent local chirality of molecules.