Possibility of chiral recognition by adsorption on enantiomorphous crystals: the impact of crystal surface polarity.

Chiral crystals remain one of the probable sources of first minute chiral symmetry breaking, a trigger that potentially causes an as-yet unknown type of asymmetric autocatalysis during the formation of chiral biopolymers under the conditions of the Archean Earth. Therefore, studying adsorption processes on the surface of such crystals may help improve the understanding of the nature of the initial chiral shift. The adsorptive activity of non-porous crystals with respect to the majority of organic molecules essentially depends on the ability of a crystal surface to engage in specific intermolecular interactions. In this work, the enantioselectivity provided by hippuric acid and phloroglucinol crystals, obtained under Viedma ripening conditions, was studied by the adsorption of menthol enantiomers from solutions and the adsorption of limonene and α-pinene enantiomers from vapors. To establish the reliability of chiral recognition, the experimental adsorption isotherms on chiral crystals were compared with the isotherms on achiral (racemic mixtures) crystals, obtained under similar conditions. The data obtained were confirmed using CD spectra, XRD patterns and SEM images. A t-test was used to assess the statistical significance of differences in adsorption. From the adsorption isotherms of vapors at different temperatures, the isosteric heats of adsorption and the differential entropies of adsorption were calculated. It was determined that the chiral recognition ability depends not only on the difference between enantiomers in the thermodynamic functions of adsorption, but also on the isosteric heats of adsorption at low coverages and the heat of liquefaction ratio. If intermolecular interactions between the enantiomer and the surface are too weak, then enantiomer layer formation becomes difficult. This reduces the enantioselectivity or even makes chiral recognition impossible. The physicochemical regularities revealed in this present work made it possible to formulate the requirements that enantiomorphous crystals must meet for satisfactory chiral recognition of molecules of different polarities.

Mechanism of chiral recognition by enantiomorphous cytosine crystals during enantiomer adsorption.

The quest to understand why life exhibits chirality has been far from successful. In the terrestrial theory of chirality emergence in living matter, one of the main possible mechanisms is the chiral recognition of organic molecules by enantiomorphic crystals. In this work, we studied the ability of enantiomorphic cytosine crystals, obtained by Viedma ripening, for chiral recognition by enantiomers adsorption. For this, we used MD calculations, inverse gas chromatography, and adsorption from solutions. The difference between the isotherms of enantiomers was determined using a t-test. We found that cytosine crystals were capable of chiral recognition only when the adsorbate concentration on the surface was sufficient for lateral interactions leading to layer formation. In order to approximate adsorption isotherms, Langmuir, Freundlich, BET, and Fowler-Guggenheim equations were used. The difference in lateral interactions between menthol enantiomers during their adsorption from a solution in n-heptane was established. A mechanism of chiral recognition of the adsorbed substance by cytosine crystals was proposed. The conditions under which chiral recognition could proceed were determined. We observed that, upon adsorption from a solution, chiral recognition manifested itself at higher coverages than in MD simulations. This was caused by the competitive adsorption of the solvent. The results obtained show that adsorption on enantiomorphic crystals could be the source of the first minute enantiomeric excess, providing an opportunity to understand the origin of chiral imbalance.

Thermodynamics of organic molecule adsorption on sorbents modified with 5-hydroxy-6-methyluracil by inverse gas chromatography.

The thermodynamic features of organic molecule adsorption from the gaseous phase of sorbents modified with 5-hydroxy-6-methyluracil (HMU) were studied. Molar internal energy and entropy of adsorption variation analyses showed that with every type surface, except for silica gel, layers of supramolecular structure have cavities equal in size with the ones revealed in HMU crystals by X-ray diffraction. Adsorption thermodynamics on HMU-modified sorbents depended on the amount of impregnated HMU and on the polarity, but not the porosity, of the initial sorbent. Polarity of the modified surface increased as a function of HMU quantity and initial sorbent mean pore size, but become appreciably lower if the initial surface is capable of hydrogen bonding.