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Clinical Chemistry 20: 794-801, 1974;
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Clinical Chemistry, Vol 20, 794-801, Copyright © 1974 by the American Association for Clinical Chemistry

Mechanisms of the Liebermann-Burchard and Zak Color Reactions for Cholesterol

R. W. Burke 1, B. I. Diamondstone 1, R. A. Velapoldi 1, and O. Menis 1

1 Analytical Chemistry Division, Institute for Materials Research, National Bureau of Standards, Washington, D. C. 20234.

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 two systems are enylic carbonium ions formed by protonation of the parent polyenes. Thus, the red (lgrmax, 563 nm) product typically measured in the Zak reaction is evidently a cholestatetraenylic cation, and the blue-green product in the L-B reaction (lgrmax, 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.


Key Words: cholestapolyenes • conjugated double bonds • carbonium ions from polyenes • factors affecting color • SO2 and Fe2+ measurement • equivalent ratios

Submitted on January 4, 1974
Accepted on May 7, 1974




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Copyright © 1974 by the American Association for Clinical Chemistry.