Cholesterol oxidation in a heterogeneous system initiated by water-soluble radicals

Generation of C-7 allylic cholesterol oxidation products was followed in a heterogeneous system with cholesterol solubilized in Tween-20 micelles in water. Initiation of oxidation by the water-soluble free radical initiator 2,2′-azobis(2-amidinopropan) dihydrochloride (AAPH) lead to accumulation of 7-ketocholesterol as the dominant oxysterol. The isomeric forms of 7-hydroxycholesterol which were also formed, although in minor amounts, were found to dehydrogenate to 7-ketocholesterol through a two-step radical reaction. 7α-hydroxycholesterol, which was found in higher concentration than 7β-hydroxycholesterol, was dehydrogenated in the oxidating system to 7-ketocholesterol at a rate approximately half of that for 7β-hydroxycholesterol. The higher rate of dehydrogenation of 7β-hydroxycholesterol may be explained by a more direct exposure of the β-surface of the sterol to the water phase and thus a more direct contact between the hydroxy group and the free radicals generated in the water phase.