Interactions between amines and phospholipids: A chromatographic study on immobilized artificial membrane (IAM) stationary phases at various pH values

The chromatographic capacity factors (log k') fur 23 amines were measured b y High performance Liquid Chromatography (HPLC) on a stationary phase compo sed of phospholipids, the so-called 'Immobilized Artificial Membrane' (IAM) . The chromatographic behaviour of the compounds, which consist of primary, secondary, and tertiary amines, and compounds with endocyclic amino functi ons was studied with eluents at various pH values (7.0, 5.5, and 3.0). The results were compared both to the octanol/buffer partition values of neutra l forms (log P) and to those of mixtures of neutral and ionised forms, exis ting at the three pH values above mentioned (log D-7.0, log D-55, and log D -30). At pH 7.0, the log k' of all secondary and tertiary amines overlapped with those previously observed fur neutral isolipophilic compounds. This b ehaviour was also observed for primary amines, but only for compounds fully ionised at this pH. In contrast, the partially ionised primary amines at p H 7.0 and the compounds with an endocyclic amino function both showed stron ger interactions with phospholipids than expected on the basis of log P. Th e changes in retention observed with eluents at pH 5.5 indicated that reten tion varies with the ionisation degree of the analytes. At pH 3.0, the inte raction between phospholipids and the ionised forms of the amines considere d was impaired probably by a change in the charges on the IAM surface, The present study indicates that phospholipids are a partitioning phase that be tter accommodates the neutral forms of primary amines than does octanol. Mo reover, the phospholipid phase is much less sensitive to the ionisation of analytes than octanol, but only at pH 7.0 and 5.5: indeed, the ionised form s of all the amines considered are retained to the same extent as expected for hypothetical neutral isolipophilic compounds. We can thus conclude that , for amines, the partition scale in phospholipids is distinct from the one in octanol.