| Research Abstract |
The fact that high concentrations of tannins in plant tissues without their crystallizing out or precipitation do not inhibit the growth of the plants suggests that certain form(s) of the tannins play a role for their solubilization in the tissues. The investigation of the existing forms of tannins should be important in clarification of the functions of the tannins in plants, and also give valuable information for utilization of the tannin-containing plants for medicine or health care.
Geranium thunbergii, a representative medicinal herb in Japan, contains more than 10% of a crystalline tannin, geraniin, in its dried leaves. In spite of low solubility of crystals of geraniin in water, the tannin is easily extracted from the leaves with hot water, and is hardly crystallized from its aqueous solution even if in highly concentrated solution. Our present study revealed that its complexation with co-existing substances in the extract affects its solubilization.
In an experiment using rats, a fraction containing geraniin as only a phenolic component, which was obtained from an aqueous extract of G.thunbergii, showed an antidiarrheic effect remarkably stronger than that of crystalline geraniin.
Analysis of the fraction showed that it contains amino acids and metallic ions. Conditions for solubilization of geraniin by complexation with several materials including these substances and polysaccharides have also been studied. However, soluble complexes has not yet been obtained, due to in part its structural change, degradation or precipitation.
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