Enantiospecific Adsorption and Decomposition of D- and L-Asp Mixtures on Cu(643)R&S

Abstract

The study of molecular chirality is essential to understanding the fundamentals of enantiospecific chemical interactions that are ubiquitous in the biochemistry of life on Earth. At a molecular level, there is insufficient understanding of chiral recognition and enantiomer-enantiomer interaction (aggregation) of chiral molecules adsorbed on surfaces. Here, using enantiospecific isotopic labelling and surface sensitive techniques, we show that when the two enantiomers of chiral aspartic acid (Asp) are adsorbed on the naturally chiral Cu(643)R&S surfaces, they decompose enantiospecifically depending on the chirality of the surface. The non-linear kinetics of the surface decomposition mechanism amplifies the difference between the decomposition rate constants of the two adsorbed enantiomers resulting in highly enantiospecific decomposition rates. Further, we also demonstrate that Asp enantiomers aggregate homochirally on several chiral and achiral surfaces, amplifying the enantiomeric excess on the surface with respect to that in the gas phase, |ees |>|eeg. Our results show that it is possible to discern the enantiospecific behavior of a complex adsorbate such as Asp and shed light on molecular level enantiospecific interactions on surfaces. The enantiospecific isotope labelling methods discussed in this paper allow probing of both the qualitative features of the Asp decomposition mechanism on Cu(643)R&S and quantitative aspects of the adsorption equilibria of enantiomer mixtures.

2D Ising Model for Adsorption‐induced Enantiopurification of Racemates

“…Diastereomerism, , is demonstrated in both cases. Similar adsorption‐induced auto‐amplification of chirality has also been observed during equilibrium adsorption on other chiral surfaces and using other adsorbates: Asp , [23] Asp , [24] and Pro (sections SI2 and SI3) [25] . Interestingly, in all of those systems the driving force for auto‐amplification of chirality appears to dominate over enantiospecific adsorption on the chiral surfaces.…”

Enantiospecific equilibrium adsorption and chemistry of d‐/l‐proline mixtures on chiral and achiral Cu surfaces

“…This experiment was then repeated using different The relationship between and reveals the tendency for homochiral aggregation, heterochiral aggregation, enantiospecific adsorption, or lack of enantiospecific organization. 28,52,53 FIGURE 6: Variation of ⁄ with total Pro coverage, , on Cu(643) R held at 400 K when the surface is exposed to fluxes of Dand L-Pro simultaneously.…”

“…The enantiospecific adsorption isotherm ( Figure 11) of versus for mixtures of D-/L-Pro on Cu(643) R&S is similar to one published for Asp/Cu(643) R&S . 28 Figure 11 shows that when one enantiomer is in the majority in the gas phase ( ≠ 0), the Cu(643) R&S surfaces are enriched in that enantiomer at equilibrium. In the case of a racemic gas phase mixture with = 0, the adsorbed monolayer remains racemic, � = 0� = 0, indicating that there are no enantiospecific interactions of the adsorbed D-and L-Pro with the chiral Cu(643) R&S surfaces.…”

“…A Langmuir adsorption model has been developed to account for the role of enantiospecific clustering on surfaces. 28,52,53 Conventionally, the Langmuir model of adsorption assumes no adsorbate-adsorbate interactions. However, the model can be modified to account for aggregation by introduction of an equilibrium constant, , for aggregation of adsorbed monomers into homochiral or heterochiral clusters and introduction of an This article is protected by copyright.…”

“…Naturally chiral surfaces have been used in the recent past to study enantiospecific phenomena such as adsorption energetics, reaction kinetics, adsorbate orientation, enantiomer separations, and enantiomer aggregation. [25][26][27][28][29][30] The rigidity of Pro has been employed in metal-catalyzed diastereoselective hydrogenation reactions to produce enantiopure hydrogenated products. [31][32][33][34] In these reaction schemes, Pro is used as a chiral auxiliary which forms an intermediate compound with the organic reactant.…”