Solubility of long-chain fatty acids in phosphate buffer at pH 7.4

The solubility of the saturated fatty acids lauric, myristic, palmitic, and stearic acid and the unsaturated oleic acid at 37°C in phosphate buffer (pH 7.4) was estimated by using two independent methods. The one was a conventional solubility technique measuring the concentration of dissolved fatty acid in buffer by using radioactive compounds. The other was a dialysis exchange technique monitoring possible aggregation of solvated fatty acid anions by measuring the rate of diffusion of labelled compound across a dialysis membrane under conditions of chemical equilibrium. It was found that the results were strongly dependent on the radiochemical purity of the fatty acids. Using highly purified samples of radioactively labelled fatty acids, the solubility of monomeric laurate was shown to be greater than 500 μM, whereas the solubility of monomeric myristate was found to be 20–30 μM. Palmitate, stearate, and oleate solutions, on the other hand, showed a tendency to aggregation even at concentrations below 1 μM. Special attention was given to palmitate, as a reference compound for long-chain fatty acids, and the solubility of monomeric palmitate was estimated to be lower than 10 −10 M.