PROPERTIES OF or- AND IB-GLYCYRRHIZINS 183 ot-G has a transconfiguration, while IB-G is cis in which D and E rings of the triterpene moiety are bent at the C•7-C•8 bond. These isomers have no structural difference ex- cept at this point and the configuration of the hydrogen at C•8. One of the most important physicochemical properties of or- and IB-G is that both have interfacial ac- tivity as shown in Figure 2. The interfacial tension between water and liquid paraffin was diminished by 25-30 dyn/cm by addition of each G. This activity is based on the peculiar structure having a triterpene moiety as a lipophilic group and glucuronic acid moiety as a hydrophilic group. Although C.M.C. values of both compounds are almost equal (ca. 2-3 x 10 -4 mol/1), the physicochemical natures of the two isomers were remarkably dissimilar. The aqueous solution of IB-G formed an extremely rigid gel in acidic media, whereas ot-G showed no sign of gelation (Figure 3). It is also notable that, even in dilute solution below 0.1 wt%, in spite of being such a small molecule (MW = 840), I•-G formed a rigid gel in acidic media below pH 4.5. SOLUBILIZING AND GELLING MECHANISMS OF GLYCYRRHIZINS PROPERTIES OF VARIOUS DERIVATIVES OF GLYCYRRHIZINS Various derivatives of Gs were prepared, and their gelling properties were examined in order to investigate the structural factors responsible for the gelling mechanisms. The results are shown in Table 1. It was confirmed by preliminary IR and NMR investiga- tions that salt formation occurred first at carboxyl groups of the glucuronic acid moiety and subsequently at that of the triterpene moiety. Table 1, representing the gelling behavior of several derivatives of Gs along with their chemical structures, demonstrates that the free carboxyl group of the triterpene moiety is indispensable for gelation by IB-G and that the gelation never occurs when all of the I 35 4 5 I I I • I I 200 180 ppm 200 180 ppm Figure 6. Effect of Eu(fod) 3 on •3C-NMR spectra of Gs (10 wt% D20 solution), Eu/G = 0.05 (molar ratio). c•-G was measured at 25.0øC and •-G at 80.0øC.

184 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS carboxyl groups and/or some of the hydroxyl groups of the glucuronic acid moiety are blocked by salt formation or esterification. Therefore, the factors essential for gelation are as follows: 1. The carboxyl group of the triterpene moiety should be free. 2. At least one of the carboxyl groups of the glucuronic acid moiety should be free. 3. Some of the hydroxy groups of the glucuronic acid moiety should be free. The potassium salt of tx-glycyrrhetin, which takes a more planar conformation, was ready to produce a coagel (soap-type gel), in contrast to the case of Gs, thereby sug- gesting that a rigid gel produced by I3-G is not of a coagel structure (Figure 4). BEHAVIOR OF GLYCYRRHIZIN MOLECULES IN MICELLAR STATE Dynamic behavior of G molecules in a miceliar state was examined by •3C- NMR(CMR). Temperature dependence of CMR spectra of individual Gs is shown in Figure 5. The spectra of tx-G was slightly affected by temperature, while that of I3-G varied markedly with the elevation of temperature. At 25.0øC, all signals of I3-G can hardly be discerned. This undoubtedly reflects the strong restriction of molecular mo- tion of I3-G due to gel formation. Further experiments were conducted on the effect of Figure 7. Scanning electron micrograph image of lyophilized gel of I3-G aqueous solution (!.0 wt%).