Issue in Honor of Prof. Atta-ur-Rahman ARKIVOC 2007 (vii) 65-75
inhibition against the odor-producing cyanobacterium O. perornata compared to activity against the green alga S. capricornutum. Specifically, the dichloromethane extract of the aerial portions of L. myrianthum as well as the hexane extract of the roots of I. helenium both demonstrated selective inhibitions with lowest-complete-inhibition concentration (LCIC) values of 10 µg/mL and 100 µg/mL against O. perornata and S. capricornutum, respectively.
The results of evaluation of the extract fractions are provided in Table 2. Fractions B and C from the dichloromethane extract of the aerial portions of L. myrianthum did reveal selective toxicity and LCIC values of 10 µg/mL for O. perornata, and, therefore, these extracts were selected to pursue bioassay-guided isolation of the active constituent(s). Further fractionation and purification of fractions B and C using HPLC resulted in the isolation of four compounds, nepodin (7a), torachrysone (7b), chrysophanol (8a), and physcion (8b) (Figure 1). Structure elucidation for all compounds was performed using combinations of GC-MS, high-resolution MS, and one- and two-dimensional NMR spectroscopy. Final structure confirmation was accomplished by comparing 1H and 13C NMR data with that reported in the literature.6-9 Chrysophanol (1,8-dihydroxyanthraquinone) was the most active of the four compounds isolated from fractions B and C from the L. myrianthum extract with a LCIC of 10 µg/mL towards O. perornata and at least 10X more toxicity when compared with the LCIC results (>100 µg/mL) of
S. capricornutum. Although fraction C from the hexane extract of the roots of I. helenium (LCIC of 10 µg/mL towards O. perornata) did not show selective toxicity, bioassay-guided isolation for active compounds was performed anyhow to determine if the isolated pure compounds might reveal selective toxicity. The LOEC and LCIC values for Copper Control were 1 µg/mL for each test organism, thereby indicating the lack of selective toxicity for the positive control. The isolated pure compounds from fraction C of the crude extract from I. helenium were alantolactone (1), isoalantolactone (4), and 11aH,13-dihydroisoalantolactone (6) (Figure 1). These compounds did not reveal selective toxicity towards O. perornata and their lowest-observed-effect concentration (LOEC) and LCIC values were at or above 100 µg/mL (Table 3). Furthermore, none of the synthetic compounds evaluated were selectively toxic towards O. perornata (Table 3). These synthetic compounds [11,13-dihydroxyalantolactone (3), 5a-epoxyalantolactone (2) and 4(15)aepoxyisoalantolactone (5)] were included in the screening process since they are isomers of several of the isolated pure compounds from I. helenium and were readily available from a previous study.9
ISSN 1424-6376 Page 67 ©ARKAT
