Membrane Charging and Swelling upon Calcium Adsorption as Revealed by Phospholipid Nanodiscs

Orion Shih^1*, Yi-Qi Yeh¹, Kuei-Fen Liao¹, Chun-Jen Su¹, Pei-Hao Wu², Richard K. Heenan³, Tsyr-Yan Yu², U-Ser Jeng^1,4

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, email:shih.orion@nsrrc.org.tw

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