Ionization Constants pK(a) of Cardiolipin

Cardiolipin is a phospholipid found in the inner mitochondrial membrane and in bacteria, and it is associated with many physiological functions. Cardiolipin has a dimeric structure consisting of two phosphatidyl residues connected by a glycerol bridge and four acyl chains, and therefore it can carry two negative charges. The pK(a) values of the phosphate groups have previously been reported to differ widely with pK(a1) = 2.8 and pK(a2) = 7.5–9.5. Still, there are several examples of experimental observations from cardiolipin-containing systems that do not fit with this dissociation behavior. Therefore, we have carried out pH-titration and titration calorimetric experiments on two synthetic cardiolipins, 1,1′,2,2′-tetradecanoyl cardiolipin, CL (C14∶0), and 1,1′,2,2′-tetraoctadecenoyl cardiolipin, CL (C18∶1). Our results show that both behave as strong dibasic acids with pK(a1) about the same as the first pK(a) of phosphoric acid, 2.15, and pK(a2) about one unit larger. The characterization of the acidic properties of cardiolipin is crucial for the understanding of the molecular organization in self-assembled systems that contain cardiolipin, and for their biological function.