Partition of fatty acids.

The partition ratios of radioactive fatty acids between n-heptane and a physiological buffer at 37 degrees C were measured. The fatty acids included the saturated acids with an even number of carbons from 10 to 18 and the unsaturated acids oleic, linoleic, and linolenic. In addition, the partition ratios of decanoate, myristate, and palmitate were determined over a wide pH range. Any single plot of partition ratio vs. aqueous concentration of an acid gave a nearly straight line, a finding consistent with very little association in the aqueous phase. In the case of the acids with 16 and 18 carbon atoms, however, comparison of the constants calculated from these plots with the assumption of no aqueous phase association revealed several inconsistencies. These inconsistencies cannot be resolved completely by assuming the existence of fatty acid association in the aqueous solution. We believe that at least some of the deviations are due to the presence of trace quantities of radioactive impurities in the labeled fatty acids. For example, purification of a sample of supposedly pure [1-(14)C]myristate by a series of solvent extractions increased the partition ratio by a factor of 1.5. Although all of the observations cannot be explained by this interpretation, we believe that our studies suggest that there is no appreciable association of fatty acids under the usual physiological conditions.