Membrane Charging and Swelling Upon Calcium Adsorption as Revealed with Monodisperse Lipid Nanodiscs
Orion Shih1*, Yi-Qi Yeh1, Kuei-Fen Liao1, Chun-Jen Su1, Pei-Hao Wu2, Tsyr-Yan Yu2, U-Ser Jeng1,3
1National Synchrotron Radiation Research Center, Hsinchu, Taiwan
2Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
3Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan
* Presenter:Orion Shih,
Calcium ions bind naturally to phospholipids and cause changes in membrane surface charges, which is a key factor in many cellular processes. However, the calcium-induced structural changes of cellular membrane and detailed binding behavior are still unclear. Here, we present fine structural changes of dimyristoylphosphatidylcholine (DMPC) nanodiscs upon calcium binding probed by size exclusion chromatographic (SEC) small-angle X-ray scattering (SAXS) with simultaneous UV absorption and refractive index (RI) detection. SAXS data analysis with a modified core-shell bicelle model indicates that calcium can bind onto the phosphate head group region, leading to membrane swelling and enhanced ordering of the bilayer structure due to increased electrostatic interactions. The binding constant K = 24.5 M-1 was determined based on Gouy-Chapman theory for diffuse double layer. At 75 mM of CaCl2, the binding is near saturated with 1:4 calcium to lipid number ratio and an induced membrane potential voltage of 3.9 mV. With nanodiscs providing mimic cellular membrane systems, these results have important implications for calcium-membrane interactions.

Keywords: Calcium, DMPC lipid bilayer, Nanodisc, High Performance Liquid Chromatography (HPLC), Small-angle X-ray scattering (SAXS)