Hibiscus sabdariffa (Malvaceae) is a plant that is widely recognised for its antihypertensive properties; however the constituent(s) responsible for this biological activity are presently unknown. The aim of this study was to identify the potential compounds that are responsible for the vasorelaxant activity of H. sabdariffa. Thereafter, the mechanisms involved in producing the vasorelaxation were investigated. The plant was extracted consecutively with hexane, ethyl acetate and methanol. The methanolic extract was subjected to bioassay-guided fractionation in order to isolate pure compounds that possessed vasorelaxant activity. The vascular effects of the pure compounds were studied on the rat aorta in vitro using myography techniques. Hibiscus acid produced a concentration-dependent relaxation of the rat aorta precontracted with either phenylephrine (3 µM) or KCl (60 mM), irrespective of the presence of the endothelium. When the tissue was pre-contracted with phenylephrine, the concentration required to produce 50% relaxation (IC 50), was 0.09 ± 0.01 mg/ml. Hibiscus acid had no effect on the phasic contraction induced by phenylephrine in Ca 2+-free physiological solution; but it did affect the component of the contraction that is due to Ca 2+ influx. In parallel studies, garcinia acid, a diastereoisomer of hibiscus acid, was found to have an almost identical vasorelaxant effect. The vasorelaxant action of both compounds is most likely due to the inhibition of Ca 2+ influx via voltage-dependent Ca 2+ channels.

Trypanosomiasis and leishmaniasis are a group of neglected parasitic diseases caused by several species of parasites belonging to the family Trypansomatida. The present study investigated the antitrypanosomal and antileishmanial activity of chalcones and flavanones from Polygonum salicifolium, which grows in the wetlands of Iraq. The phytochemical evaluation of the plant yielded two chalcones, 2′,4′-dimethoxy-6′-hydroxychalcone and 2′,5′-dimethoxy-4′,6′-dihydroxychalcone, and two flavanones, 5,7-dimethoxyflavanone and 5,8-dimethoxy-7-hydroxyflavanone. The chalcones showed a good antitrypanosomal and antileishmanial activity while the flavanones were inactive. The EC50 values for 2′,4′-dimethoxy-6′-hydroxychalcone against Trypanosoma brucei brucei (0.5 μg/mL), T. congolense (2.5 μg/mL), and Leishmania mexicana (5.2 μg/mL) indicated it was the most active of the compounds. None of the compounds displayed any toxicity against a human cell line, even at 100 µg/mL, or cross-resistance with first line clinical trypanocides, such as diamidines and melaminophenyl arsenicals. Taken together, our study provides significant data in relation to the activity of chalcones and flavanones from P. salicifolium against both parasites in vitro. Further future research is suggested in order to investigate the mode of action of the extracted chalcones against the parasites.

The biologically active title compounds have been isolated from Hibiscus sabdariffa plants, hibiscus acid as a dimethyl sulfoxide monosolvate [systematic name: (2S,3R)-3-hydroxy-5-oxo-2,3,4,5-tetrahydrofuran-2,3-dicarboxylic acid dimethyl sulfoxide monosolvate], C6H6O7C2H6OS, (I), and hibiscus acid dimethyl ester [systematic name: dimethyl (2S,3R)-3-hydroxy-5-oxo-2,3,4,5- tetrahydrofuran-2,3-dicarboxylate], C8H10O7, (II). Compound (I) forms a layered structure with alternating layers of lactone and solvent molecules, that include a two-dimensional hydrogen-bonding construct. Compound (II) has two crystallographically independent and conformationally similar molecules per asymmetric unit and forms a one-dimensional hydrogen-bonding construct. The known absolute configuration for both compounds has been confirmed