Time-Resolved Dynamics of Calcium Oxalate Dihydrate Crystallization

Calcium oxalate is a common biomineral that forms in plants and humans. The deleterious effects associated with the pathological crystallization of calcium oxalate in diseases such as kidney stones and breast cancer depend upon the structural form of the crystal. The majority of studies in the literature focus on understanding and controlling the formation of calcium oxalate monohydrate (COM), which is the most prevalent form in vivo. Relatively fewer studies have examined the crystallization of calcium oxalate dihydrate (COD), which is a less thermodynamically stable hydrate and the focus of this study. Here, we investigate methods of preparing large COD crystals using poly(acrylic acid) as a growth modifier to promote dihydrate nucleation and to generate crystals with a size and morphology that allow for in situ measurements of crystallization in physiologically relevant growth media. Time-resolved dynamics of COD surface growth were tracked in supersaturated calcium oxalate solutions using atomic force microscopy (AFM) to visualize the birth and spreading of unfinished layers on COD {100} crystal surfaces in real time. Our findings reveal that COD growth involves the nucleation and advancement of two-dimensional islands, which is in stark contrast to COM surfaces that grow exclusively from screw dislocations. AFM studies performed over a range of supersaturation indicate that COD grows at a significantly slower rate with a kinetic coefficient that is more than an order of magnitude smaller than that of COM. Similar in situ experiments were performed in the presence of citrate, a known inhibitor of calcium oxalate crystallization. These studies reveal distinct differences in the impact of citrate on COM and COD growth, which may have implications for its efficacy as an administered preventative therapeutic for pathological crystallization.