Elucidating the interaction of Spathodea campanulata leaf extracts mediated potential bactericidal gold nanoparticles with human serum albumin: spectroscopic analysis.

Nanomaterials in different form have been thoroughly used in the area of pharmaceutics and medicine for drug delivery. The large scale of nanoparticles (NPs) synthesis from plant extract is much safe, cheap and eco-friendly. Here, we demonstrated a new, one-step, ultra-fast biosynthesis of gold nanoparticles (sc-AuNPs, 19.54 nm) by using aqueous Spathodea campanulata leaf extracts as a reducing and capping agent. And also, we presented the synthesis of citrate capped gold nanoparticles (cit-AuNPs) of approximately same size (19.66 nm). These two NPs were characterized by UV-Visible, dynamic light scattering, transmission electron microscope and energy dispersive X-ray spectroscopy. Fourier transform infrared spectroscopy confirmed that the functional groups like OH, NH, OH of COOH and CO were contributed in the sc-AuNPs formation. The negative zeta potential (-20.5, -22.8 mV) established the stability and dispersion of the sc- and cit-AuNPs. The anti-bacterial activity of the sc- and cit-AuNPs were checked against Escherichia coli (DH5-Alpha). Minimum inhibitory concentration was 2.4 and 3.0 nM, respectively for sc- and cit-AuNPs. The interaction study of the sc-AuNPs/cit-AuNPs-human serum albumin (HSA) system was done by UV-Visible absorbance, fluorescence, circular dichroism, time resolved fluorescence spectroscopy and the measurement of zeta potential. Absorbance, three dimensional fluorescence, synchronous fluorescence and circular dichroism spectroscopy showed a minor conformational change of HSA upon interaction with the sc-AuNPs compared to cit-AuNPs. The present comparative study will advance our knowledge about the binding mode, mechanism and conformational change of the protein upon interaction with green synthesized sc-AuNPs and cit-AuNPs. Communicated by Ramaswamy H. Sarma.