Chiral selection through rotational and magnetic levitation forces
One of the greatest challenges of modern science is to understand the origin of the homochirality of life: why are most essential biological building blocks present in only one handedness. Efforts towards understanding this phenomenon ultimately have to rely on the effect of a chiral, external influence that, at some point during the evolution, has driven certain systems towards specific chiralities. Several possible chiral fields have been suggested, with scenarios based on circularly polarized light in photochemistry, the electroweak interaction, vortex motion and external electric and magnetic fields. However, to date only few of these scenarios have been experimentally demonstrated. In a joint effort of researchers from the High Field Magnet Laboratory (HFML), the CNR-IPCF Istituto per i Processi Chimico-Fisici (Messina, Italy) and the Dipartimento di Chimica Inorganica, Università di Messina (Italy), it is now shown that the combination of rotation and magnetically tuned effective gravity is able to direct the chirality of a self-assembly process. These findings were published in the journal Nature Chemistry.
Many chemical and physical systems can occur in two forms distinguished solely by being mirror images of each other. This phenomenon, known as chirality, is important in biochemistry, where reactions involving chiral molecules often require the participation of one specific enantiomer. Life on earth utilizes almost exclusively L-amino acids and D-sugars, a pattern that is known as the 'homochirality of life'. Somewhere during evolution mirror symmetry was broken; the origin of this is still unknown. Chemical processes can proceed enantioselectively if chiral molecules or catalysts are involved, or if some external chiral inﬂuence is present. As chiral reactants require an asymmetric production process themselves, efforts towards understanding biological homochirality have focused on external chiral influences.
In a recent experiment we showed that a proper combination of external forces is able to direct the formation of molecular assemblies of specific chirality from achiral porphyrins. The forces we employed are rotation of the reaction vessel around a vertical axis, magnetically tuned effective gravity and magnetic alignment. Clockwise rotation under normal gravity in the presence of a magnetic alignment force yields right-handed structures. Reversing either the rotation direction or the direction of the effective gravity yields left-handed structures. Reversing both does not change the chirality. Magnetic alignment is essential; in absence of a magnetic field only structures of random chirality are formed. This system is a rare example in which gravity is shown to influence the outcome of a chemical process. Our ability to tune the direction and magnitude of the effective gravity by means of diamagnetic levitation is therefore crucial.
The effect of effective gravity (Geff) on the surface of spinning water. The paraboloidal shape of the liquid roughly follows a 1/ Geff dependence: the curvature increases with decreasing Geff, is maximal around Geff = 0 when all water resides at the wall, and is inverted for negative Geff.
Our set of forces is chiral in the sense that it is non-superimposable on its mirror image. Image and mirror image are however not only connected by mirror symmetry, but also by time-reversal symmetry, making it falsely chiral. This implies that through thermodynamics no enantioselection can be achieved. It is however theoretically possible to induce chirality in a system which is under kinetic control and far from equilibrium. Indeed, our results show that the falsely chiral influence needs to be employed during the initial stage of the aggregation, far from equilibrium, while the formation of molecular aggregates of measurable chirality takes several days. This is the first example of enantioselection by a falsely chiral influence.
Non-chiral porphyrins (green boxes) can form chiral super structures. Employing rotation (red arrow) and magnetically tuned effective gravity (green arrow) during the initial stage of the self-assembly process allows for direction of the chirality towards right- or left-handed structures, as shown by circular dichroism (CD, right).
This work is published in N. Micali, H. Engelkamp, P. G. van Rhee, P. C. M. Christianen, L. Monsu-Scolaro and J. C. Maan, "Selection of supramolecular chirality by application of rotational and magnetic forces", Nature Chemistry. 2012