We dedicate this paper to Prof. Josep Mª Ribó, the ‘onlie begetter’ of our interest
in symmetry breaking and the emergence of molecular chirality.
Abstract
The generally accepted hypothesis to explain the origin of biological homochirality
(that is to say, the fact that proteinogenic amino acids are left-handed, and carbohydrates
right-handed, in all living beings) is to assume, in the course of prebiotic chemical
evolution, the appearance of an initial enantiomeric excess in a set of chiral molecular
entities by spontaneous mirror-symmetry breaking (SMSB), together with suitable amplification
and replication mechanisms that overcome the thermodynamic drive to racemization.
However, the achievement of SMSB in chemical reactions taking place in solution requires
highly specific reaction networks showing nonlinear dynamics based on enantioselective
autocatalysis, and examples of its experimental realization are very rare. On the
other hand, emergence of net supramolecular chirality by SMSB in the self-assembly
of achiral molecules has been seen to occur in several instances, and the chirality
sign of the resulting supramolecular system can be controlled by the action of macroscopic
chiral forces. These considerations led us to propose a new mechanism for the generation
of net chirality in molecular systems, in which the SMSB takes place in the formation
of chiral supramolecular dissipative structures from achiral monomers, leading to
asymmetric imbalances in their composition that are subsequently transferred to a
standard enantioselective catalytic reaction, dodging in this way the highly limiting
requirement of finding suitable reactions in solution that show enantioselective
autocatalysis. We propose the name ‘absolute asymmetric catalysis’ for this approach,
in which an achiral monomer is converted into a nonracemic chiral aggregate that is
generated with SMSB and that is catalytically active.
Our aim in this Account is to present a step-by-step narrative of the conceptual and
experimental development of this hitherto unregarded, but prebiotically plausible,
mechanism for the emergence of net chirality in molecular reactions.
1 Introduction: The Origin of Biological Homochirality and Spontaneous Mirror-Symmetry
Breaking
2 Experimental Chemical Models for Spontaneous Mirror-Symmetry Breaking: The Soai
Reaction and Beyond
3 Spontaneous Mirror-Symmetry Breaking in Supramolecular Chemistry: Plenty of Room
at the Top
4 Absolute Asymmetric Catalysis: An Alternative Mechanism for the Emergence of Net
Chirality in Molecular Systems
5 Experimental Realization of Top-Down Chirality Transfer to the
Molecular Level
6 Conclusions and Outlook
Key words absolute asymmetric catalysis - autocatalysis - biological homochirality - dissipative
systems - spontaneous mirror-symmetry breaking - supramolecular catalysis - systems
chemistry
