Phase transition characteristics of diphosphatidylglycerol (cardiolipin) and stereoisomeric phosphatidyldiacylglycerol bilayers: Mono- and divalent metal ion effects

Synthesis and phase transition characteristics of aqueous dispersions of the homologous (12 : 0, 14 : 0, 16 : 0) diphosphatidylglycerols (cardiolipins) and phosphatidyldiacylglycerols are reported. Electron microscopy of the negatively stained aqueous dispersions reveals a characteristic lamellar structure suggesting that these phospholipid molecules are organized as bilayers in the aqueous dispersions. The phase transition temperature ($\text{T}{}_{\mathrm{<mtext>m</mtext>}}$) and the enthalpy of transition ($\text{ΔH}$) increase monotonically with chain length in the cardiolipin and phosphatidyldiacylglycerol series; $\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ for phosphatidyldiacylglycerol is higher than that for cardiolipin of the same chain-length. The transition temperatures for the enantiomeric $\text{sn-3,3-}\text{and}\text{sn-1,1-}\text{phosphatidyldiacylglycerol}$ and for the diastereomeric, $\text{meso}\text{-sn-1,3-}\text{phosphatidyldiacylglycerol}$ are approximately the same. The molar enthalpy for the transition of cardiolipin-NH₄⁺ bilayers is approximately twice the value for the phosphatidylcholines of the same chain length, i.e., the molar enthalpy per acyl chain is approximately the same in the two systems. The transition temperatures for metal ion salts of C_{1 6}-cardiolipin exhibit a biphasic dependence upon the unhydrated ionic radii, i.e. the highest $\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ is observed for Ca²⁺- cardiolipin and decreases for the salts of ions with smaller and larger ionic radii than that of Ca²⁺. The lowest $\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ is observed for Rb⁺-cardiolipin. Monovalent metal salts of cardiolipin exhibit two phase transitions. This effect may result from different conformational packing of the four acyl chains due to differences in metal-phosphate binding.