The Distribution of Ubiquinone‐10 in Phospholipid Bilayers

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Abstract
The thermal behaviour of mixed aqueous dispersions of ubiquinone‐10 with dipalmitoylglycerophospho‐ choline, 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/mol) 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.