Interactions between Amines and Phospholipids: A Chromatographic Study on Immobilized Artificial Membrane (IAM) Stationary Phases at Various pH Values

Abstract

The chromatographic capacity factors (log k′) for 23 amines were measured by High Performance Liquid Chromatography (HPLC) on a stationary phase composed 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 functions, 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 neutral forms (log P) and to those of mixtures of neutral and ionised forms, existing at the three pH values above mentioned (log D7.0, log D5.5, and log D3.0). At pH 7.0, the log k′ of all secondary and tertiary amines overlapped with those previously observed for neutral isolipophilic compounds. This behaviour was also observed for primary amines, but only for compounds fully ionised at this pH. In contrast, the partially ionised primary amines at pH 7.0 and the compounds with an endocyclic amino function both showed stronger interactions with phospholipids than expected on the basis of log P. The changes in retention observed with eluents at pH 5.5 indicated that retention varies with the ionisation degree of the analytes. At pH 3.0, the interaction between phospholipids and the ionised forms of the amines considered was impaired probably by a change in the charges on the IAM surface. The present study indicates that phospholipids are a partitioning phase that better accommodates the neutral forms of primary amines than does octanol. Moreover, 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 forms 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.