Membrane Charging and Swelling upon Calcium Adsorption as Revealed by Phospholipid Nanodiscs
Orion Shih1*, Yi-Qi Yeh1, Kuei-Fen Liao1, Chun-Jen Su1, Pei-Hao Wu2, Richard K. Heenan3, Tsyr-Yan Yu2, U-Ser Jeng1,4
1X-ray Scattering Group, National Synchrotron Radiation Research Center, Hsinchu, Taiwan
2Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
3STFC ISIS Facility, Rutherford-Appleton Laboratory, Didcot, UK
4Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan
* Presenter:Orion Shih,
Direct binding of calcium ions (Ca2+) to phospholipid membranes is an unclarified yet critical signaling pathway in diverse Ca2+-regulated cellular phenomena. Here, high-pressure-liquid-chromatography, small-angle X-ray scattering (SAXS), UV-vis absorption, and differential refractive index detections are integrated to probe Ca2+-binding to the zwitterionic lipid membranes in nanodiscs. The responses of the membranes upon Ca2+-binding, in composition and conformation, are quantified through integrated data analysis. The results indicate that Ca2+ binds specifically into the phospholipid headgroup zone, resulting in membrane charging and membrane swelling, with a saturated Ca2+-lipid binding ratio of 1:8. A Ca2+-binding isotherm to the nanodisc is further established and yields an unexpectedly high binding constant K = 4260 M-1 and a leaflet potential of ca. 100 mV based on a modified Gouy­Chapman model. The calcium-lipid binding ratio, however, drops to 40% when the nanodisc undergoes a gel-to-fluid phase transition, leading to an effective charge capacity of a few µF/cm2.

Keywords: Calcium binding, Phospholipid nanodisc, Differential refractive index, Small-angle X-ray scattering, Membrane swelling