Density functional theory studies of Hypaphorine from Erythrina mildbraedii and Erythrina addisoniae : structural and biological properties

This study was aimed at isolating hypaphorine from Erythrina mildbraedii Harms (Fabaceae) and Erythrina addisioniae Hutch. & Datziel (Fabaceae) in order to determine its structural and antibacterial effects. Density functional theory (DFT) calculations and X-ray crystallographic analysis of the isolated hypaphorine was determined. The antibacterial effects of hypaphorine against a number of Gram positive and Gram negative bacterial strains were investigated. The root mean square deviation between the experimental and calculated bond lengths and bond angles of hypaphorine were found to be 0.046 Å and 1.5° respectively. The highest occupied molecular orbital (HOMO) of hypaphorine was delocalised on the indole moiety whereas the lowest occupied molecular orbital (LUMO) was delocalised on the –N(CH 3 ) 3 group and the HOMO–LUMO gap of hypaphorine was 4.65 eV. Hypaphorine inhibited the growth of the Gram-positive bacteria tested, namely Bacillus cereus , B. subtilis , Staphylococcus aureus and S. epidermidis. The lowest minimum inhibitory concentration (MIC) value of 2 mg/mL was exhibited against Mycobacterium smegmatis , Staphylococcus aureus and B . subtilis . The theoretical and experimental results from this study showed that hypaphorine is capable of forming quadrupole moments thus explaining its antibacterial effects on Gram positive bacteria. Graphic abstract