中央研究院 原子與分子科學研究所

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 head-group 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 mF/cm².

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