Abstract
A model reaction was studied for the interaction between cholesterol (1) and molecular oxygen in biological matrices. An aqueous colloidal solution (60 ml) of 1 (2.7×10-4 mol) dispersed with sodium stearate (1.8×10-4 mol) was adjusted to pH 8 and energetically stirred at 85° without air-bubbling. Relative amounts of the sterols present in the reaction mixtures were determined every one hour by quantitative TLC and time-course of the oxyfunctionalization of 1 was followed for 9 hours. The major product in the early period was the 7-hydroperoxide (2) as an epimeric mixture of α : β≒1 : 2. The amounts of the 7-ketone (3) and 7-hydroxide (4) increased with decrease in that of 2 which was regarded as the intermediate. After passage of 7 hours the autoxidation system arrived at an apparent final state where consumption of 1 and formation of products were suspended and 2 was no longer detectable. More than 70% of 1 was likely to be consumed and the relative amounts of the oxygenated products (3, 4, and the 5, 6-epoxide) and the unchanged substrate (1) were in an approximate ratio of 45 : 25 : 5 : 25. The induction period was lost when a small amount (1×10-5 mol) of the 7α-hydroperoxide (2a) was initially present in the reaction mixture. From these results, the autoxidation of 1 under the conditions was assumed to have an ordinary pattern of the chain reaction initiated by the decomposition of hydroperoxide.
