Interaction and translocation of cysteamine (mercaptoethylamine) with model membranes: a 15N-NMR and 1H-NMR study

We investigated by 15N-NMR (nuclear magnetic resonance) spectroscopy the interactions of 15N-labeled cysteamine ([15N]mercaptoethylamine (MEA)), a radioprotecting aminothiol, with model membranes of egg yolk phosphatidylcholine (EPC) and phosphatidic acid (EPA). We prepared large unilamellar vesicles (LUVs) with a pH gradient between the intravesicular space and the bulk medium. Over the pH range from 4.8 to 8.1, the observations show a MEA incorporation into the vesicles. This result is consistent with a specific dissociation balance at the lipid-water interface. We carried out another 1H-NMR experiment with unlabeled MEA in the presence of small unilamellar vesicles (SUVs). It revealed that the interactions of MEA within the hydrophobic core of the phospholipidic bilayer vary with external pH value over the range 4.0–7.1. Through these experiments, and others compiled from the literature, it was concluded that 15N-NMR spectroscopy is particularly successful for transmembrane transport analysis.