Mechanisms of the Liebermann Burchard and Zak color reactions for cholesterol

Abstract

Correlation of SO2 and Fe2+ measurements with new spectral data indicates that the Liebermann Burchard (L-B) and Zak color reactions for cholesterol have similar oxidative mechanisms, each yielding as oxidation products, a homologous series of conjugated cholestapolyenes. These studies further suggest that the colored species observed in these 2 systems are enylic carbonium ions formed by protonation of the parent polyenes. Thus, the red (λmax, 563 nm) product typically measured in the Zak reaction is evidently a cholestatetraenylic cation, and the blue green product in the L-B reaction (λmax near 620 nm) is evidently the pentaenylic cation. The effects of rate of carbonium ion formation and sulfuric acid concentration on sensitivity and color stability are discussed. A solvent extraction procedure is described for specifically converting cholesterol to 3,5 cholestadiene. Incorporating this step into the typical L-B method can increase the L-B sensitivity for cholesterol by several fold.