Differential dependencies on [Ca²⁺] and temperature of the monolayer spontaneous curvatures of DOPE, DOPA and cardiolipin: Effects of modulating the strength of the inter-headgroup repulsion

Biomembranes assume nonlamellar structures in many cellular events, with the tendency of forming a nonlamellar structure quantified by the monolayer spontaneous curvature, C₀, and with many of these events involving the acts of Ca²⁺. Despite this biologically important intimacy, how C₀ is affected by [Ca²⁺] is unknown. In this study, we use the X-ray diffraction technique and the reconstruction of electron density profiles to measure the C₀s of a zwitterionic phospholipid, DOPE, and two anionic phospholipids, DOPA and 18:1 (9Z) cardiolipin, at temperatures from 20 °C to 40 °C and [Ca²⁺]s from 0 mM to 100 mM; these phospholipids are chosen to examine the contributions of the electric charge density per molecule. While showing a strong dependence on temperature, C_0,DOPE is nearly independent of [Ca²⁺]. In contrast, C_0,DOPA and C_{0,cardiolipin} are almost unresponsive to the temperature change but affected by the [Ca²⁺] variation; and C_0,DOPA varies with [Ca²⁺] ∼1.5 times more strongly than C_{0,cardiolipin}, with the phase preferences of DOPA and cardiolipin shifting to the H_II phase and remaining on the L_α phase, respectively, at [Ca²⁺] = 100 mM. From these observations, we reveal the effects of modulating the strength of the inter-headgroup repulsion and discuss the mechanisms underlying the phase behaviour and cellular functions of the investigated phospholipids. Most importantly, this study recognizes that the headgroup charge density is dominant in dictating the phase behaviour of the anionic phospholipids, and that the unique molecular characteristics of cardiolipin are critically needed both for maintaining the structural integrity of cardiolipin-rich biomembranes and for fulfilling the biological roles of the phospholipid. This journal is