Topological distribution of atropine in dipalmitoylphosphatidylcholine liposomes

The topography of atropine entrapped in sn-3-(dipalmitoyl)phosphatidylcholine (DPPC) liposomes was determined by the electron spin resonance (ESR) technique using 5-, 7-, and 16-nitroxy-stearic acid probes. The liposome preparations, with or without entrapped atropine, were passed through a Sepharose-4B column; the entrapment efficiency and lipid recovery were determined using [3H]-atropine and [14C]-DPPC as tracers. It was found that approximately 31 per cent of the added atropine was entrapped in the liposomes. The ESR results established that the temperature range of the gel-to-liquid crystalline transition of DPPC liposomes was not altered significantly by the encapsulation of atropine. The entrapped atropine also caused no significant change in the order of hydrocarbon chains of DPPC vesicles. These results were confirmed by data obtained from differential scanning calorimetry. On the basis of these experiments, it was concluded that atropine does not interact with the hydrophobic region of the lipid bilayers but is exclusively localized within the entrapped aqueous compartment of DPPC liposomes.