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

Direct binding of calcium ions (Ca) to phospholipid membranes is an unclarified yet critical signaling pathway in diverse Ca-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 Ca-binding to the zwitterionic lipid membranes in nanodiscs. The responses of the membranes upon Ca-binding, in composition and conformation, are quantified through integrated data analysis. The results indicate that Ca binds specifically into the phospholipid headgroup zone, resulting in membrane charging and membrane swelling, with a saturated Ca-lipid binding ratio of 1:8. A Ca-binding isotherm to the nanodisc is further established and yields an unexpectedly high binding constant K = 4260 M and a leaflet potential of ca. 100 mV based on a modified Gouy-Chapman 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/cm.