Effect of the enzymatically modified supported dipalmitoylphosphatidylcholine (DPPC) bilayers on calcium carbonate formation.

After an hour contact with a phospholipase A2 (PLA2) solution, only the outer leaflet of the dipalmitoylphosphatidylcholine (DPPC) bilayers supported on mica surface underwent hydrolysis whose products, i.e., palmitic acid and lysophospholipid, accumulated on the bilayer surface. Only calcite was present on the bare mica and enzymatically unmodified and modified supported DPPC bilayers soaked for 2 weeks at 25 and 37 °C in a solution of initial pH equals to 7.4 and 9.2 containing calcium and bicarbonate ions at their concentrations about those of human blood plasma. The DPPC bilayers accelerate the crystal growth at lower pH and favors CaCO3 nucleation at higher pH. Enzymatic modification of bilayers does not affect crystal morphology and its organization on the examined surface but causes a slight crystal size increase at lower pH and significantly reduces crystal size at alkaline pH. The temperature increase leads to the formation of bigger crystals under physiological pH and has almost no effect on crystal size at alkaline pH. The obtained results are probably attributed to Ca(2+) interaction with a specific polar site on the surface of the membrane and DPPC hydrolysis products acting as nucleation centers.