Selective Growth and Distribution of Crystalline Enantiomers in Hydrogels

Abstract

The crystallization of sodium chlorate (NaClO3) is a classic example of spontaneous chirality, since it is achiral in solution but adopts a chiral form in the solid state. While crystal growth of NaClO3 from pure aqueous solutions yields a 50:50 statistical distribution of d- and l-crystals, large enantiomeric excesses of either d- and l-crystals can be achieved by crystal growth in agarose gel, a naturally occurring chiral polysaccharide. The influence of gel density (0.1-0.75 wt %), temperature, and the diffusion of cosolvents on crystal distribution was discerned from statistical data obtained from 752 gel-mediated crystallization experiments yielding 12,384 individual crystals. These studies demonstrate that the magnitude and direction of the bias can be selectively engineered toward either d- or l-forms by changing the gelation conditions. Aqueous agarose gels infused with 48 wt % NaClO3 at 6 degrees C, favored the growth of d-NaClO3 crystals, with ee's reaching 22% at the highest gel concentrations. Crystal growth under methanol diffusion favored deposition of the opposite enantiomorph, l-NaClO3. The bias in the crystal distribution is enhanced at higher temperatures. Aqueous gels at 24 degrees C infused with methanol cosolvent favored l-NaClO3, with ee's reaching 53%. The changing magnitude and direction of the enantiomorph bias can be ascribed to differences in the agarose conformation and intermolecular interactions between the gel and crystal surfaces that inhibit the formation of the two enantiomers to different extents.

Supramolecular Gels: Functions and Uses

“…Crystals grown from gel have improved physical properties compared to crystals grown from solutions and gels have been used as media to grow large organic, inorganic and macromolecular crystals of high optical quality. 54 Semi-wet peptide/protein microarrays of a supramolecular hydrogel derived from glycosylated aminoester has been used for highthroughput analysis of protein/enzymes. 55 A semi-wet sensor chip developed by incorporating artificial receptors within the same gel was useful for high throughput analysis of analytes (anions/metal ions).…”

Water, Solute, and Segmental Dynamics in Polysaccharide Hydrogels

“…The most relevant example of this kind of gel is probably provided by the agarose gels. [3] Agarose chains are alternate copolymers of 3,6-anhydro-a-L-galactose and b-D-galactose linked (1 ! 4) and (1 !…”

Anion-switchable supramolecular gels for controlling pharmaceutical crystal growth

“…[1][2][3][4][5] The improved physical characteristics of the resulting crystals (better optical quality, larger size and fewer defects) are usually ascribed to the suppression of convection currents, sedimentation and nucleation afforded by the viscous gel environment. [6][7] In many instances the gel is thought to act as an inert matrix within which crystal growth occurs, however in some cases, the gel structure has been shown to influence the polymorphism, enantiomorphism and habit of crystals. 2 Occasionally gel fibres may become incorporated into the crystal, giving rise to composite materials.…”