Potentiometric titration of monomeric and micellar acylcarnitines

Carnitine (β‐hydroxy‐γ‐trimethylammonium butyrate) enzymatically combines with fatty acids to facilitate their transport through mitochondrial membranes and thus their metabolism. In view of this fact, a study of the physical‐chemical properties of some acylcarnitines was conducted. Potentipmetric titrations were carried out at concentrations at which the acylcarnitines are monomeric and micellar. Unlike the monomeric species, the micellar acylcarnitines do not have a constant pK value but rather one that is dependent on the degree of protonation and hence the net positive charge of the micelle. The pK value for the zwitterionic micelle is about 4.85 and decreases linearly with β at low degrees of protonation. The pK of the zwitterionic micelle was taken as the intrinsic pK of a carboxyl group, and pK differences at any value of β were attributed to electrostatic interactions. This permitted calculation of the surface potential of the micelle. The dependence of the observed pK and the calculated surface potential on β and on ionic strength was shown to be qualitatively consistent with the Debye‐Hückel theory for a spherical impenetrable particle.