The distribution of ubiquinone-10 in phospholipid bilayers. A study using differential scanning calorimetry.

The thermal behaviour of mixed aqueous dispersions of ubiquinone-10 with dipalmitoylglycerophosphocholine, dimyristoylglycerophosphocholine or egg phosphatidylcholine has been examined by differential scanning calorimetry. Ubiquinone-10 was found to undergo a series of thermal transitions in the heating process in the temperature range 290-300 K prior to a major melting endotherm at 317 K. The temperature of this endotherm was unaltered when the coenzyme was codispersed with phospholipid but enthalpy determinations showed that up to about 5% (mol/ol) and 20% (mol/mol) of ubiquinone-10 was removed from the transition in mixtures with dimyristoyl and dipalmitoyl derivatives of phosphatidylcholine respectively. Likewise the pre-transition and main endothermic transitions of the saturated phospholipids were largely unperturbed by the presence of ubiquinone-10 but up to 22% (mol/mol) and 8% (mol/mol) of the dimyristoyl and dipalmitoyl derivatives of phosphatidylcholine respectively were removed from the endotherm. These data suggest that at molar percentages less than 20% ubiquinone-10 a high proportion of the coenzyme is molecularly mixed within the phospholipid bilayer but as the proportion increases there is a greater tendency for the coenzyme to form aggregates that display typical melting and crystallization behaviour.