471453 Chiral Nanoparticles for Enantioslective Adsorption and Separations
The surfaces of chemically synthesized Au nanoparticles have been modified with D- or L-cysteine to render them chiral and enantioselective for adsorption of chiral molecules. Their enantioselective interaction with chiral compounds and pharmaceuticals has been probed by optical rotation measurements during exposure to mixtures with varying enantiomeric excess of chiral compounds such as R- and S-propylene oxide, R- and S-2-butanol, or R- and S-propranolol. The ability of optical rotation to detect enantiospecific adsorption arises from the fact that the specific rotation of polarized light by R- and S-propylene oxide is enhanced by interaction with Au nanoparticles. This effect is related to previous observations of enhanced circular dichroism by Au nanoparticles modified by chiral adsorbates. More importantly, chiral Au nanoparticles modified with either D- or L-cysteine selectively adsorb one enantiomer of the probe molecules such as propylene oxide from a racemic solution, thus leaving an enantiomeric excess in the solution phase. Au nanoparticles modified with L-cysteine (D-cysteine) selectively adsorb the R-propylene oxide (S-propylene oxide).
A simple adsorption model and accompanying experimental protocol have been developed to enable optical rotation measurements to be analyzed for quantitative determination of the ratios of the enantiospecific adsorption equilibrium constants of chiral species on the surfaces of chiral nanoparticles, KLS / KDS = KDR / KLR. This analysis is robust in the sense that it obviates the need to measure the absolute surface area of the absorbent nanoparticles, a quantity that is somewhat difficult to obtain accurately. This analysis has been applied to optical rotation data obtained from solutions of R- and S-propylene oxide, R- and S-2-butanol, or R- and S-propranolol, in varying concentration ratios, with D- and L-cysteine coated Au nanoparticles, in varying concentration ratios.
Recently, tetrahexahedral (THH, 24-sided) Au nanoparticles modified with D- or L-cysteine (Cys) have been used as enantioselective separators of the chiral pharmaceutical propranolol in solution phase. Polarimetry has been used to measure the rotation of linearly polarized light by solutions containing mixtures of propranalol and Cys/THH-Au NPs with varying enantiomeric excesses of each. Polarimetry yields clear evidence of enantiospecific adsorption of propranalol onto the Cys/THH-Au NPs. This extends prior work using propylene oxide as a test chiral probe, by using the crystalline THH Au NPs with well-defined, crystal facets to separate a real pharmaceutical. This work suggests that chiral nanoparticles, coupled with a density separation method such as centrifugation, could be used for enantiomeric purification of real pharmaceuticals. The model developed earlier has also been used to extract the enantiospecific equilibrium constants for R- and S-propranalol adsorption onto the D- and L-Cys/THH-Au NPs.
The figure illustrates the reversible equilibrium adsorption of R- and S-propylene oxide (PO) on Au nanoparticles modified with L- or D-cysteine. The equilibrium constants are enantiospecific, KLS = KDR ¹ KDS = KLR.
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