Growth inhibition of calcium oxalate monohydrate crystal by linear aspartic acid enantiomers investigated by in situatomic force microscopy

Abstract

The inhibitory effect of linear enantiomers of L- and D-Asp₆ on the growth of calcium oxalate monohydrate crystal has been investigated using in situ atomic force microscopy. The inhibitory magnitude of D-Asp₆ on the growth of the [00] step on the (010) face is about 10% larger than that of L-Asp₆. While no chiral effect is observed or expected on the growth of the [0] step on the (01) face by both enantiomers, their inhibitory effect on this step is much stronger than that on the [00] step on the (010) face. In both cases, the step morphology indicates that these enantiomers create the impurity pinning along the steps, while the dependence of step speed on supersaturation shows that they also produce a reduction of the step kinetic coefficients. Analysis of the step speed data within the context of an existing model for step pinning and kink blocking shows that the major impact of Asp₆ is to block active kink sites. The larger inhibition of the [00] step growth by D-Asp₆ over L-Asp₆ and the substantially larger inhibition of the [0] step over the [00] step by both enantiomers both result from larger affinity for adsorption to the (010) face and the (01) face, respectively. This is because the larger adsorption leads to a higher density of blocking kink sites along the steps. The estimated difference in binding energy of L- and D-Asp₆ to the respective faces from the kinetics model is consistent with the trend predicted by our molecular modeling of the enantiomer binding to the faces.