Observations on the critical micellar concentration of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine and a series of its homologs and analogs

In a study designed to explore the physical chemical characteristics of platelet activating factor (PAF), or 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, the critical micellar concentration of this compound, as well as the propionyl, butyryl and hexanoyl homologs was determined. In addition, an analogous series of compounds, in which the 1-O-alkyl was replaced by a 1-O-hexadecanoyl or a 1-O-octadecanoyl group, were examined for their critical micellar concentration. A variety of analytical techniques including NMR, gas liquid chromatography, infra-red spectrometry, thin layer chromatography, phosphorus, choline, glyceryl ether and fatty acid analyses were used to confirm the high purity of the individual derivatives. A dye binding assay and a surface tension technique were compared as to their suitability for determination of the critical micellar concentration of these compounds. Whereas the dye binding method proved highly variable, the surface tension procedure proved to be a facile, reproducible technique and was the assay of choice. The critical micellar concentration of the 1-O-alkyl and the 1-O-acyl derivatives showed comparable values for each short chain substituent at carbon 2, with values, in μM, ranging from 1.3 ± 0.03 for 1-O-hexadecanoyl-2-acetyl-sn-glycero-3-phosphocholine and 1.1 ± 0.10 for 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine to 0.22 ± 0.01 for 1-O-hexadecyl-2-hexanoyl-sn-glycero-3-phosphocholine. The data show that at the molar concentration usually employed in biological studies with these compounds, i.e., 1 × 10^-7 to 10^-11 M,one can assume that they will be present as monomolecular species. Thus, it seems unlikely that the widely diverse biological activities of these compounds can be explained by this physical parameter.