Calcium oxalate monohydrate precipitation at phosphatidylglycerol Langmuir monolayers

Calcium oxalate monohydrate (COM) has been crystallized beneath Langmuir monolayers of four different phosphatidylglycerol lipids, allowing investigation of the influence of the density of headgroup packing and monolayer compressibility on heterogeneous precipitation. The COM crystals were nucleated from metastable solutions at Langmuir monolayers of the phospholipids dipalmitoylphosphatidylglycerol (DPPG), dioleoyphosphatidylglycerol (DOPG), palmitoyloleoylphosphatidylglycerol (POPG), and dimyristoylphosphatidylglycerol (DMPG). Crystal formation was monitored at the air/water interface using Brewster angle microscopy, and in transferred films with scanning electron microscopy. For each Langmuir monolayer it was observed that precipitation is heterogeneous and is selective with respect to the orientation and morphology of the precipitated crystals, with a majority of crystals having the (101̄) face oriented toward the monolayer interface. When the phospholipid monolayers are held at the same pressure, more crystals are observed under DPPG, which is the film with the smallest area per molecule. However, in apparent contrast, nucleation rates increase when the films are expanded. Also, when the four monolayers are held at the same area, nucleation occurs preferentially beneath films at lower surface pressures. The results suggest that the compressibility of the monolayer and the potential of the monolayer to achieve a small area per molecule are more important characteristics that lead to heterogeneous crystallization of COM than is providing a prearranged template. We also observe some evidence that domain boundaries between different monolayer phases can act as nucleation sites.