Macroscopic and microscopic dissymmetry plays a key role in life sciences. The unknown origin of homochirality in terrestrial life and the high enantioselectivity of processes involving chiral molecules of biological relevance are among the most intriguing questions in natural phenomena and chiral technologies to be answered. Perhaps, the most fascinating unsolved mystery is the origin of life on our planet. One of the viable hypothesis is the extraterrestrial uptake of biological material through meteorites. This is clearly supported by the discovery of organic content made of chiral amino acids (the building blocks of life) exhibiting enantiomeric excess (e.e.) of the same form (L) of the terrestrial protein amino acids (the Murchinson meteorite fell in Australia in 1969).
In the context of the abiotic generation of e.e. in meteoritic material, studying mechanisms potentially responsible for such a generation becomes crucially important, such as, for instance, the interaction between racemic samples of chiral molecules (balanced mixture of enantiomeric forms) and the circularly polarized radiation (CPL), this latter being present in interstellar media since it can be generated as synchrotron radiation (SR) from magnetic neutron stars. The ultraviolet CPL might therefore be responsible for enantiomeric selection in interstellar organic molecules.
Recently, the scientific community has started developing more and more effective spectroscopic and theoretical methodologies to study the asymmetry induced by the interaction chiral-matter (enantiomers)/chiral photon (CP-SR).
The research activity here proposed is aiming at combining in a synergism skills in different areas of Chemistry, namely stereoselective organic synthesis, ab initio quantum-mechanical theoretical modelling, nanomaterials synthesis, and SR-based photoelectron spectroscopies, to investigate intrinsic electronic properties of selected model chiral molecules at unprecedently achieved level.
One of the main goal of the proposed project is to extend the experimental and theoretical investigation of chiroptical properties by PECD and related quantum-mechanical methods to a large class of chiral molecular structures. This has hitherto been impeded by the severe experimental obstacles described in the previous section. In the PECD spectroscopy the measured asymmetry (dichroism coefficient Di(h)) in the angular distribution of photoelectrons emitted from pure enantiomers reveals the chiral molecular identity (e.g. absolute configuration of asymmetric carbon atoms). The Di(h) observable is a sensitive chiroptical probe, which can be associated to any photoionization process. It is sensitive to details of the molecular potential, being a powerful probe of molecular structures such as conformers, isomers, clustering, chemical group substitution, as well as internal vibrational motions [23]. Extending this powerful kind of chiroptical investigation to specifically designed chiral structures would represent a very important advancement of knowledge in molecular chirality, because of the interest hold by the PECD spectroscopy from a fundamental and analytical point of view, the strong presence of chiral species in the biosphere, and the importance of such molecules in food and medical industry [23].
The innovative contents of the proposed research resides in the synergism between the activities in the different areas of chemistry involved in the project.
The technological advancement in the scientific instrumentation, as described in the next section (tasks of the participants), will provide a much higher sensitivity of the spectroscopic technique with consequently much smaller quantities of chiral sample required for the measurements. This improvement will be paralleled by organo-synthetic and organo-analytical activities to provide chiral molecular structures ad hoc designed in laboratory, which are expected to be obtained in relatively modest quantity because of the cost and the difficulty of the required procedures.
A detailed and accurate interpretation of the spectroscopic data will be based on theoretical studies by means of quantum-mechanical ab initio computing methods, which will provide a proper description of the valence and core electronic structures of the chiral molecules, the equilibrium between stable conformers (for instance due to the rotational isomerism). Spectral simulations will be theoretically calculated at state-of-the-art level for the dichroism coefficients as a function of photon energy for both valence and core photoionizations. It is worth mentioning that only combining accurate experimental PECD measurements and computed predictions of the dichroism coefficients obtained at high level of theory provides an exceptionally powerful technique for the chiral recognition, a key aspect of the present project.
The made selection of very few chiral oxirane derivatives is intended as a first prototype study characterized by the abovementioned synergism. The near future plane is to extend this approach to investigate a much larger variety of chiral molecules, and to involve, for the first time, molecules whose chirality has a different origin than the presence of an asymmetric carbon atom (e.g. axial chirality and helix-type chirality), and to study the effects of conformational equilibrium and chemical group substitution on the chiral properties.
In addition to the pure gas-phase experiments highlighted above, an innovative development relies in extending such investigations into the nano-structured realm: (i) opportunely thiol-derivatized chiral oxiranes will be chemisorbed by wet-chemistry methods onto Au or Cu surfaces in order to investigate their self-assembling properties. (ii) The preparation of gold and silver nanoparticles functionalized with the chiral derivatives, in order to transfer chirality to a metal nanoparticle surface will be also undertaken.
The structure of the adsorbed/chemisorbed molecule and its interactions with the environment will be studied for emerging applications such as enantioselective catalysts, or chiral discriminators for biomacromolecules.
More generally, progressing from gas-phase isolate molecule to nanostructured ensembles will provide a mean to study the effect of chirality at the nanoscale and to understand if it is uniquely able to generate more intense responses than their organic chiral counterparts
23) L. Nahon et al., J. Electron. Spectrosc. Relat. Phenom., 204, 322 (2015)