Improvements in the thin-layer chromatography of natural products. I. Thin-layer chromatography of the aflatoxins.

During the period which has elapsed since the aflatoxins were first isolated, one of the main problems has been the separation of the individual aflatoxins in pure form from aflatoxin-containing extracts. This separation has been best effected by thin-layer chromatography, and in this paper we describe how some of the difficulties may be overcome by using an appropriate combination of solvent system and silica gel preparation. For the examination of aflatoxin-containing extracts from the mycelia of Aspergillus flavus moulds, an initial freeze-drying step has been found to improve appreciably the quality of the chromatograms obtained.

Biological activity and electronic structure of the aflatoxins.

In theoretical studies of aromatic hydrocarbons, Pullman and Pullman (1969) used the molecular orbital method to correlate electronic structure with biological activity. They suggested that the interaction between carcinogens and their molecular receptors must occur through the K region of the carcinogenic molecule and involve a strong chemical binding of the type of an addition reaction. In the present work the electronic structures of aflatoxins B(1), G(1), 4-20 dehydro B(1) and of versicolorin A have been determined by the simple Hückel molecular orbital method using a computer, in order to see whether the correlation between electronic structure and biological activity is applicable to these compounds also. Calculations show that the 2-3 pi-bond, which has the highest bond order of the aflatoxin molecules, should be the most susceptible to electrophilic attack and is the most probable location of the K region. This is in agreement with the experimental observation of Dutton and Heathcote (1968) that aflatoxins B(1) and G(1) hydrate rapidly in dilute acid to the hydroxyaflatoxins B(2a) and G(2a) with an apparent total loss of carcinogenicity. The calculations also show that aflatoxins B(1) G(1) and M(1) have no suitable site for an L region and this probably accounts for their highly carcinogenic nature.