Identifying alkali metal inhibitors of crystal growth: a selection criterion based on ion pair hydration energy.

We show that alkali metals function as effective modifiers of calcium oxalate monohydrate (COM) crystallization wherein alkali-oxalate ion parings reduce the rate of crystal growth by as much as 60%. Our findings reveal a distinct trend in alkali metal efficacy that cannot be explained by colloidal theories or simple descriptors, such as ion size, but is consistent with a theoretical model that accounts for the ion pair's affinity for water.

Natural promoters of calcium oxalate monohydrate crystallization.

Crystallization is often facilitated by modifiers that interact with specific crystal surfaces and mediate the anisotropic rate of growth. Natural and synthetic modifiers tend to function as growth inhibitors that hinder solute attachment and impede the advancement of layers on crystal surfaces. There are fewer examples of modifiers that operate as growth promoters, whereby modifier-crystal interactions accelerate the kinetic rate of crystallization. Here, we examine two proteins, lysozyme and lactoferrin, which are observed in the organic matrix of three types of pathological stones: renal, prostatic, and pancreatic stones. This work focuses on the role of these proteins in the crystallization of calcium oxalate monohydrate (COM), the most prominent constituent of human kidney stones. Using a combination of experimental techniques, we show that these proteins, which are rich in l-arginine and l-lysine amino acids, promote COM growth. The synthesis and testing of peptides derived from contiguous segments of lysozyme's primary amino acid sequence revealed subdomains within the protein that operate either as an inhibitor or promoter of COM growth, with the latter exhibiting efficacies that nearly match that of the protein. We observed that cationic proteins promote COM growth over a wide range of modifier concentration, which differs from calcification promoters in the literature that exhibit dual roles as promoters and inhibitors at low and high concentration, respectively. This seems to suggest a unique mechanism of action for lysozyme and lactoferrin. Possible explanations for their effects on COM growth and crystal habit are proposed on the basis of classical colloidal theories and the physicochemical properties of peptide subdomains, including the number and spatial location of charged or hydrogen-bonding moieties.

Specificity of growth inhibitors and their cooperative effects in calcium oxalate monohydrate crystallization.

The molecular recognition and interactions governing site-specific adsorption of growth inhibitors on crystal surfaces can be tailored in order to control the anisotropic growth rates and physical properties of crystalline materials. Here we examine this phenomenon in calcium oxalate monohydrate (COM) crystallization, a model system of calcification with specific relevance for pathological mineralization. We analyzed the effect of three putative growth inhibitors--chondroitin sulfate, serum albumin, and transferrin--using analytical techniques capable of resolving inhibitor-crystal interactions from interfacial to bulk scales. We observed that each inhibitor alters surface growth by adsorbing on to distinct steps emanating from screw dislocations on COM surfaces. Binding of inhibitors to different crystallographic faces produced morphological modifications that are consistent with classical mechanisms of layer-by-layer crystal growth inhibition. The site-specific adsorption of inhibitors on COM surfaces was confirmed by bulk crystallization, fluorescent confocal microscopy, and atomic force microscopy. Kinetic studies of COM growth at varying inhibitor concentrations revealed marked differences in their efficacy and potency. Systematic analysis of inhibitor combinations, quantified via the combination index, identified various binary pairings capable of producing synergistic, additive, and antagonistic effects. Collectively, our investigation of physiologically relevant biomolecules suggests potential roles of COM inhibitors in pathological crystallization and provides guiding principles for biomimetic design of molecular modifiers for applications in crystal engineering.