Face-Specific Occlusion of Lipid Vesicles within Calcium Oxalate Monohydrate

Intracellular membrane-bound vesicles play important roles in the formation of biominerals, such as calcium oxalate monohydrate (COM) crystals, through the interactions of the vesicles and different crystal faces. However, in situ kinetics and the mechanism of occlusion of diverse vesicles, which have similar compositions, into the ((1) over bar 01) and (010) faces of COM remain unknown. Here, using time-resolved in situ atomic force microscopy (AFM), we observe that negatively charged phosphatidylcholine vesicles are adsorbed preferentially and then occluded into the ((1) over bar 01) face, whereas positively charged (2,3-dioleoyloxy-propyl)-trimethylammonium vesicles are only occluded into the (010) face, and zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine vesicles are rarely incorporated into COM crystals. The free energies of binding between the lipid vesicles and COM crystal faces measured by AFM-based single-molecule dynamic force spectroscopy account for the vesicle-crystal face interaction through an electrostatic attraction. These in situ kinetics and energetic analyses may improve our understanding of the mechanisms of lipid occlusion.