What is chirality?
Chirality refers to a property of molecules, whose mirror images cannot be superimposed. A classic example is the human hand: your left hand is a mirror image of your right hand, but they cannot be aligned on top of each other. Several such structures exist in Nature – for example, in the spiral shapes of snail shells or the arrangement of certain plant stems. A very large diversity of molecules exhibit chirality, and the left- and right-handed forms are called enantiomers.
Chirality in molecules: Amino acids and DNA
Chirality plays a crucial role in the function of biological systems. For example, amino acids, in the L-form are predominant in proteins, D-amino acids are rare but essential in certain biological processes. DNA itself is a chiral molecule, with a double helix structure that is a right-handed spiral, essential for its function in storing genetic information.
Molecules of taste and smell
Chirality is also present in the compounds responsible for taste and smell. For example, limonene exists in two enantiomeric forms: the right-handed gives like orange fragrance, while the left-handed form gives the lemon fragrance. The different forms of these molecules interact with our sensory receptors in different ways, affecting our perception of taste and smell.
Drugs: How does chirality affect their effect ?
Several well-known drugs have different effects depending on their chirality. For example:
- Ibuprofen: The R-form of ibuprofen is the active painkiller, while the S-form is less effective.
- Naproxene: The S-form of naproxen is a potent anti-inflammatory drug, while the R-form is inactive and has been associated with liver toxicity.
- Thalidomide: the R-form of this drug was effective in treating morning sickness, while the S-form caused severe birth defects.
Current methods for chirality identification: Racemic mixtures and challenges
Currently, chirality is often studied using methods such as optical rotation, a technique based on the interaction of plane-polarized light with chiral molecules, first developed in the 19th century by Jean-Baptiste Biot and later advanced by Louis Pasteur. While these methods remain foundational, they are limited in sensitivity. The main challenge is identifying and characterizing specific enantiomers (R and S) in complex mixtures, which is crucial for understanding their distinct biological and chemical roles.
CHIRAX and the search for an alternative (physical) method
The CHIRAX project aims to overcome current limitations in the detection of molecular chirality by developing and implementing advanced X-ray spectroscopic techniques for liquid phase systems, including X-ray circular and helical dichroism and non-linear X-ray methods. These element-selective methods will provide detailed insights into the structure and dynamics of chiral compounds, overcoming the challenges presented by conventional optical spectroscopy. Using synchrotron light sources and X-ray Free Electron Lasers (XFELs), CHIRAX aims to improve enantiomeric detection and structural analysis with unprecedented precision. This work will have broad applications in biology, catalysis and the chemical sciences.
