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

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 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/cm2.