Preliminary μSR studies on the incorporation of steroidal drug molecules into liposomes

Muon spin relaxation of two liposome samples consisting of 100% DPPC (dipalmitoylphosphatidylcholine) and 50:50 mole% DPPC:Cholesterol at a final lipid concentration of 100 mg/mL are compared. The observation of a significant loss in the muonium fraction from 11% to 6% when cholesterol is present is compatible with radical formation in the second sample. Also reported are measurements on pure solid cholesterol where a clear T1 minimum at ca 100 K, corresponding to a dynamic process with activation energy of ca. 1.89 kJ mol−1 and an attempt frequency of 5.89×1010 s−1, is observed. An estimate of a hyperfine interaction of 255.4 MHz for this radical predicts a ΔM=1 ALC transition at 0.94 T.