Extracts from herbal materials are inherently complex, and Chinese licorice is no exception. To date, close to 40 different compounds have been characterized in this extract, most of which have a rather unusual structure (see Figure 1). The main components are isoflavonoids; the isoflavans licoricidin and licorisoflavan A account for more than 15% of the extract. A number of other isoflavans, isoflavanones and isoflavones; licoricone and licoisoflavone B; pterocarpans; flavones such as liquiritigenin; and 2-arylbenzofurans have also been described (see Chinese Licorice Compounds).1 In addition, the extract contains coumarins, coumestans, fatty acids, glycosides of liquiritigenin and isoliquiritigenin, and triterpene glycosides, i.e., saponins which give licorice roots its typical sweet taste.
One characteristic of most of these flavonoids is the addition of one or two isoprene units to the base structure. Isoflavanoids are rather rare in nature, and some of the subgroups, e.g., the coumestans and 2-arylbenzofurans, have less than 100 structures reported to date—compared with more than 1,000 known flavones and more than 1,500 flavonols. The addition of one or two isoprene units makes for an even more unusual chemical structure. The isoprene moiety can be open, as with licoricidin, or form a pyran-ring, e.g., licoisoflavone B. Publications on the relationship between the chemical structure and antibacterial activity of isoflavonoids have shown that isoprene substituents are important for such activity.1, 2 Molecules with two isoprene substituents have better activity than those with only one, which in turn have better activity than isoflavonoids without a prenyl moiety. An isoprene unit in position C-6 seems to be particularly important, as is a hydroxyl substituent in position C-7. In cases where the isoprene unit is fused to form a pyran ring, the activity noticeably decreases.2 The antimicrobial effect of isoflavones has been attributed to their phenolic hydroxyl groups, which allow the molecules to act as inhibitors of microbial enzymes. The presence of isoprene groups increases the lipophilicity of the isoflavonoid, which seems to be a requirement to disrupt the function of the protoplasmic cell membrane of Gram-positive bacteria and thus interfere with the transport of nutrients into the cell.3
Glycyrrhiza uralensis (Chinese Licorice) Sources
As a background, Glycyrrhiza uralensis is found in dry, grassy plains and on sunny mountainsides in northeastern and northwestern China, as well as in Siberia and Mongolia. Most of the current supply comes from northwest China. The annual production of herbal material from this plant is more than 60,000 tons and since its natural habitats are severely disturbed due to overharvesting, the plant is now under protection in China. Thus, this market supply comes from extensive cultivation.4 The aerial parts are usually cut in autumn and the roots are dug up, cleaned and dried using heat exchangers or left at ambient temperature; the optimum drying temperature is 35–40°C.5 Much of the material is sold to Japan as peeled root for infusions.