2D Ising Model for Adsorption‐induced Enantiopurification of Racemates

Abstract

Mechanisms for the spontaneous transformation of achiral chemical systems into states of enantiomeric purity have important ramifications in modern pharmacology and potential relevance to the origins of homochirality in life on Earth. Such mechanisms for enantiopurification are needed for production of chiral pharmaceuticals and other bioactive compounds. Previously proposed chemical mechanisms leading from achiral systems to near homochirality are initiated by a symmetry‐breaking step resulting in a minor excess of one enantiomer via statistical fluctuations in enantiomer concentrations. Subsequent irreversible processes then amplify the majority enantiomer concentration while simultaneously suppressing minority enantiomer production. Herein, equilibrium adsorption of amino acid enantiomer mixtures onto chiral and achiral surfaces reveals amplification of surface enantiomeric excess relative to the gas phase; i. e. enantiopurification of chiral adsorbates by adsorption. This adsorption‐induced amplification of enantiomeric excess is shown to be well‐describe by the 2D Ising model. More importantly, the 2D‐Ising model predicts formation of homochiral monolayers from adsorption of racemic mixtures or prochiral molecules on achiral surfaces; i. e. enantiopurification with no apparent chiral driving force.

An Account of Chiral Metal Surfaces and Their Enantiospecific Chemistry

“…Recently, we have conducted Monte Carlo simulations (Figure 6B&D, solid and dashed curves) using the 2D-Ising model to show that the autoamplification of enantiomeric excess can be attributed to nearest neighbor interactions. 57 Specifically, the amplification of enantiomeric excess arises from homochiral (D−D and L−L) interactions that are more attractive than heterochiral (D−L) interactions. Comparison of the data and the Monte Carlo results indicate that the difference in homochiral and heterochiral nearest neighbor interactions is ΔΔE exch D−L ≅ 2.5 kJ/mol.…”

“…This behavior is predicted by Monte Carlo simulation of the 2D Ising model with ΔΔ E exch d – l > 0 (solid curves). Reproduced with permission from ref . Copyright 2021 Wiley-VCH.…”

“…Recently, we have conducted Monte Carlo simulations (Figure B&D, solid and dashed curves) using the 2D-Ising model to show that the autoamplification of enantiomeric excess can be attributed to nearest neighbor interactions . Specifically, the amplification of enantiomeric excess arises from homochiral ( d – d and l – l ) interactions that are more attractive than heterochiral ( d – l ) interactions.…”

Enantiomer Adsorption in an Applied Magnetic Field: D‐ and L‐Aspartic Acid on Ni(100)

“…High performance liquid chromatography (HPLC) can be enantioselective when using chiral stationary phases (CSPs) such as chiral synthetic polymers, polysaccharides, cyclodextrin, proteins, and others, [15] and chiral HPLC is the most popular method for enantiomer separation [16] . Nonetheless, various other novel chiral surfaces and processes for enantiomer purification have been proposed and studied [17–24] . The crystallographic orientation of Cu( hkl ) single crystals has been shown to influence the kinetics and thermodynamics of adsorption and to do so enantiospecifically for chiral adsorbates on chiral surface orientations, Cu( hkl ) R&S .…”

“…[16] Nonetheless, various other novel chiral surfaces and processes for enantiomer purification have been proposed and studied. [17][18][19][20][21][22][23][24] The crystallographic orientation of Cu(hkl) single crystals has been shown to influence the kinetics and thermodynamics of adsorption and to do so enantiospecifically for chiral adsorbates on chiral surface orientations, Cu(hkl) R&S . Yun et al [19] were able to show that during exposure to racemic mixtures of DL-aspartic acid (Asp), D-Asp acid preferentially adsorbs on the Cu(3,1,17) S surface while L-Asp shows an equal but opposite preference for the Cu(3,1,17) R surface.…”