Speaker
Description
Molecular chirality is central to biology and chemistry. Identification of enantiomers of chiral molecules is mostly based on optical circular dichroism (CD). It measures the absorption difference of left and right circularly polarised light by the sample. CD signals are intrinsically weak, usually 0.1% of the linear absorption, which makes them challenging to measure. Pushing CD spectroscopy into the X-ray domain promises several advantages: a) an enhanced signal (since the terms of light-matter Hamiltonian for CD are inversely proportional to the wavelength), b) element-selectivity; c) on top of the chemical shifts, an ability to identify identical but inequivalent atoms in a molecule. Although several theoretical studies have underlined these advantages of X-ray natural CD (XNCD), [1–5] its measurement on disordered media (powders) has remained a challenge. There are only a few studies on amino-acid residues and organic molecules in powder form. [6–8]
With the advent of the liquid microjet technique, [9,10] it is now possible to inject liquid samples into vacuum and investigate their soft X-ray absorption both in steady-state and time-resolved studies. Here, we will present the first XNCD spectra of chiral fenchone in ethanol. The measurements were carried out at the O K-edge of the molecule, which is not a chiral centre. The results show a good agreement with our theoretical results.[5]
Beyond linear XNCD, we have explored the use of (2) non-linear methods such as sum-/difference-frequency generation (S/DFG) to study chiral solutions.[5] Indeed, it was shown [11] that these methods are sensitive both to the interface and the bulk of a chiral sample. We will present our efforts at implementing at an X-ray Free Electron Laser, S/DFG methods combining an optical pulse tuned to a valence transition of the chiral molecule and an X-ray probe tuned to one of its core transitions.
References
[1] A. Jiemchooroj et al, Near-edge x-ray absorption and natural circular dichroism spectra of L-alanine: A theoretical study based on the complex polarization propagator approach, J. Chem. Phys. 127, 16 (2007).
[2] A. Jiemchooroj and P. Norman, X-ray absorption and natural circular dichroism spectra of C84: A theoretical study using the complex polarization propagator approach, J. Chem. Phys. 128, 234304 (2008).
[3] C. Brouder et al, Theory of X-Ray Natural Circular Dichroism, https://doi.org/10.1107/S090904959801680X.
[4] V. M. Freixas et al, X-ray and Optical Circular Dichroism as Local and Global Ultrafast Chiral Probes of [12]Helicene Racemization, J. Am. Chem. Soc. 145, 21012 (2023).
[5] Y. Nam et al, Linear and Nonlinear X-ray Spectra of Chiral Molecules: X-ray Circular Dichroism, Sum- and Difference-Frequency Generation of Fenchone and Cysteine, J. Phys. Chem. Lett. 16, 4652 (2025).
[6] M. Tanaka et al, First observation of natural circular dichroism for biomolecules in soft x-ray region studied with a polarizing undulator, Phys. Scr. 2005, T115 (2005).
[7] A. Agui et al., First operation of circular dichroism measurements with periodic photon-helicity switching by a variably polarizing undulator at BL23SU at SPring-8, Rev. Sci. Instrum. 72, 8 (2001).
[8] Y. Izumi et al, Characteristic oxygen K-edge circular dichroism spectra of amino acid films by improved measurement technique, J. Chem. Phys. 138, 7 (2013).
[9] M. Ekimova et al, A liquid flatjet system for solution phase soft-x-ray spectroscopy, Struct. Dyn. 2, 5 (2015).
[10] M. Fondell et al, Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates, Struct. Dyn. 4, 5 (2017).
[11] J. A. Giordmaine, Phys. Rev. Lett., 1962, 8, 19–20.