Can protonated beta-blockers interact with biomembranes stronger than neutral isolipophilic compounds? A chromatographic study on three different phospholipid stationary phases (IAM-HPLC).

The chromatographic capacity factors (k') of 10 beta-adrenoceptor antagonists ("beta-blockers") were measured on three different immobilized artificial membrane-phosphatidylcholine (IAM-PC) HPLC stationary phases, namely IAM-PC-MG, IAM-PC-DD2, and IAM-PC-DD. The two former phases are made of phosphatidylcholine as found in biomembranes and differ each other in end-capping of free propylamino residues whereas the latter is made of single-chain phosphatidylcholine analogues. On IAM-PC-DD2 we found that structurally unrelated neutral compounds give a single correlation between logk' values and the respective octanol/water partition coefficients (logP), as previously observed on IAM-PC-MG phase. This was not observed on the IAM-PC-DD phase. IAM chromatography was performed at a pH of the aqueous eluent (7.0) close to the physiological pH 7.4. Retention on all IAM phases showed a biphasic pattern, being proportional to logP(N) (lipophilicity of neutral forms) for more lipophilic congeners (logP(N)>1.90), and quite constant for the others. The comparison of beta-blocker retention with that of neutral compounds on IAM-PC-MG and IAM-PC-DD2 suggests that they can interact with phospholipids as strongly or more strongly than neutral isolipophilic compounds, despite their being more than 98% in their ionized form. Therefore, we hypothesize that electrostatic interactions play a pivotal role in the interactions between beta-blockers and membrane phospholipids.