Bioactivity studies and isolation of chemical constituents from the leaves of Calliandra tergemina

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of infection in hospital settings and it is resistant against all commercially available β-lactam antibiotics. Over the years, the spread of MRSA has become an emerging concern as there are few therapeutic options to treat MRSA. Vancomycin is commonly used to treat MRSA but can cause serious side effects and is expensive. Recently, vancomycin resistant Staphylococcus aureus (VRSA) had also emerged. Thus, there is a need to find new therapeutic drugs. Phytochemicals isolated from plants could be used as alternative drugs to treat MRSA. In this study, the bioactivities and chemical constituents of Calliandra tergemina L. (Benth) were investigated. This is the first study on the isolation of chemical constituents and bioactivities of C. tergemina. Antibacterial testing on C. tergemina crude extracts revealed that its leaves have significant anti-MRSA activity. Bioassay-guided isolation of the active leaves extracts of C. tergemina afforded three new galloylated flavonol rhamnosides, namely, quercetin-3-O-(3,4-di-O-galloyl)-rhamnopyranoside (1), quercetin-3-O-(2,3,4-tri-O-galloyl)-rhamnopyranoside (2), and kaempferol-3-O-(2,3,4-tri-O-galloyl)-rhamnopyranoside (3). Five known galloylated and non-galloylated flavanol rhamnosides were also isolated. The known flavonol rhamnosides were quercetin-3-O-(3-O-galloyl)-rhamnopyranoside (4), quercetin-3-O-rhamnopyranoside (5), kaempferol-3-O-(2,3-di-O-galloyl)-rhamnopyranoside (6), kaempferol-3-O-(3-O-galloyl)-rhamnopyranoside (7) and kaempferol-3-O-rhamnopyranoside (8). The structures and molecular formulae of these isolated compounds were established using various spectrospopic techniques. For the known compounds, the spectroscopic data were compared to previously reported data. The anti-MRSA activity of the isolated compounds evaluated using a microboth dilution method had revealed the structure-activity relationship of galloylated flavonol rhamnosides. The functional side groups of these flavonol rhamnosides were more important in conferring anti-MRSA activity compared to their parent structures. Flavonol rhamnosides with multiple esterifications of their side groups showed anti-MRSA activity with a minimum inhibitory concentration (MIC) of 256 µg/mL. These flavonol rhamnosides also showed lytic effect on the MRSA cells when observed under scanning electron microscope (SEM). The antioxidant activity of the isolated compounds was examined as flavonoids are known to have good antioxidant properties. The antioxidant activity of these isolated flavonol rhamnosides was investigated by determining their ability to scavenge free radicals using DPPH free radical scavenging assay. All the isolated flavonol rhamnosides have strong free radical scavenging capabilities (IC50max < 8.0 ug/ml, except for quercetin rhamnosides) with medium kinetic reactions. This study also showed that hydroxyl groups from the galloyl groups of these flavonol rhamnosides played an important role in scavenging free radicals. A sugar-like compound, 3-O-methyl-D-chiro-inositol (D-pinitol), was isolated from the methanol leaves extracts of C. tergemina. This compound was reported to have larvarcidal activity. However, larvacidal activity was not observed in the crude methanol extract, from which D-pinitol was isolated. In conclusion, the bioactivity studies and chemical isolation of C. tergemina leaves extracts had led to the discovery of three new compounds with anti-MRSA activity and free radical scavenging capabilities. This study showed the potential of C. tergemina in the search for new and beneficial therapeutic compounds and revealed the structural-activity-relationship of galloylated flavonol rhamnosides.