Thermotropic mesomorphism in aqueous dispersions of 1-palmitoyl-2-oleoyl- and 1,2-dilauroyl-phosphatidylglycerols in the presence of excess Na+ or Ca2+

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Abstract
1-Palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol (POPG) and 1,2-dilauroyl-sn-glycero-3-phospho-rac-glycerol (DLPG) have been made by the conversion of the appropriate lecithins in the presence of crude phospholipase D and excess glycerol. Aqueous dispersions of the lipids have been examined by differential scanning calorimetry and 31P-nuclear magnetic resonance spectroscopy. When dispersed in excess Na+ at pH 7, POPG and DLPG underwent gel to liquid-crystalline phase transitions at −2.0 ± 1.3 and −3.7 ± 1.9 °C, respectively. The enthalpy of transition for POPG in excess Na+ was 6.2 ± 0.3 kcal∙mol−1 (1 cal = 4.1868 J), but the value for DLPG in Na+ was variable. In the presence of excess Ca2+ at pH 7, both phospholipids displayed complex thermotropic mesomorphism. For DLPG in the presence of excess Ca2+, thermal events did not occur at the same temperatures as did those for POPG in excess Ca2+. The complexity of the observed behaviour may have arisen because of the formation of intermediate forms which slowly interconverted to more stable forms.