Interaction between protoporphyrin IX and tryptophan silver nanoparticles

Silver nanoparticles (AgNPs) have been intensively studied for several purposes including therapeutic applications in cancer. When prepared with tryptophan and photoreduction, silver nanoparticles (TrpAgNPs) become an alternative to conventional anticancer drugs. In this study, the anticancer activity of synthesized TrpAgNPs against MCF-7 breast cancer cells was evaluated, and the inhibitory concentration (IC50) was found to be ~3.4 mg/mL. Since the protoporphyrin IX (PPIX) concentrations in tumor cells are elevate compared to normal cells, the PPIX-TrpAgNP interaction was studied to investigate if it could contribute for cell apoptosis. The investigation was performed using PPIX solution (0.9 μg/mL) with different TrpAgNP concentrations (from 0 to 13 mg/mL). PPIX was characterized by UV-Vis spectroscopy, steady-state and time-resolved fluorescence spectroscopy. The results have shown that the presence of spherical TrpAgNps with 16-nm diameter quench the PPIX fluorescence intensity. This quenching is strongly dependent on the concentration of the TrpAgNPs, and it is caused by a combination of a static and a dynamic process. The chemical binding leads to oxidation of tryptophan and formation of kynurenine, observed in the emission spectra around 470 nm. The strong reduction of the PPIX fluorescence decay lifetime with nanoparticle increasing concentration confirms the quenching processes due to charge transfer from the excited PPIX states to the resonant silver states. The present study confirms the anticancer activity of TrpAgNPs on the human breast cancer cell line (MCF-7) in vitro and indicates that PPIX-AgNP interaction could contribute with MCF-7 apoptosis.