Citrate modulates calcium oxalate crystal growth by face-specific interactions

Because of its ability to inhibit the growth of calcium oxalate monohydrate (COM) crystals, citrate plays an important role in preventing the formation of kidney stones. To determine the mechanism of inhibition, we studied the citrate-COM interaction using a combination of microscopic and simulation techniques. Using scanning confocal interference microscopy, we found that addition of citrate preferentially inhibits crystal growth in and, to a lesser extent, directions, suggesting that citrate adsorbs to the faces of COM in the order {100} > {121} > {010}. Scanning electron microscopy showed that the resulting crystals are plate shaped, with large {100} faces and rounded ends. Molecular-dynamics simulations predicted, however, that citrate interacts with the faces of COM in a different order, i.e. {100} > {010} > {121}. Our simulations showed that citrate molecules align with the rows of Ca2+ ions on the {010} face but do not form close contacts, presumably because of electrostatic repulsion by the carboxylate groups that project from the Ca2+-rich plane. We propose that this weak interaction is responsible for citrate’s limited inhibition of COM growth in directions. Overall, these findings indicate that electrostatic interactions with the Ca2+-rich faces of COM crystals are responsible for the growth-modulating properties of citrate.