Calcium-induced compaction and clustering of vesicles tracked with molecular resolution.

Theory and simulations predict the complex nature of calcium interaction with the lipid membrane. By maintaining the calcium concentrations at physiological conditions, herein we demonstrate experimentally the effect of Ca in a minimalistic cell-like model. For this purpose, giant unilamellar vesicles (GUVs) with a neutral lipid DOPC are generated and the ion-lipid interaction is observed with attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy providing molecular resolution. Firstly, Ca encapsulated within the vesicle binds to the phosphate head groups of the inner-leaflets and triggers vesicle compaction. This is tracked by changes in vibrational modes of the lipid groups. As the calcium concentration within the GUV increases, IR intensities change indicating vesicle dehydration and lateral compression of the membrane. Secondly, by inducing a calcium gradient across the membrane up to a ratio of 1:20, interaction between several vesicles occur as Ca can bind to the outer-leaflets leading to vesicle clustering. It is observed that larger calcium gradients induce stronger interactions. These findings with an exemplary biomimetic model reveal that divalent calcium ions not only cause local changes to the lipid packing, but also have macroscopic implications to initiate vesicle-vesicle interaction.