Protein structural changes by nanoparticles interactions as revealed by Trp phosphorescence

Nanoparticles (NPs) are promising materials for industrial and biomedical applications. Progress in these fields, however, has raised concerns about human health and environmental hazards. It is now known that proteins form a coat around NPs, but the biological impact of this interaction has not been fully assessed. The characterization of protein/NP complexes contributes to evaluate the biological hazards of NPs. In this study we utilized Trp fluorescence and phosphorescence to examine possible perturbations of the protein native fold induced by glutathione-coated CdS nanocrystallites. Measurements were conducted on model proteins varying in the degree of burial of the Trp probe and on quaternary structure. Fluorescence measurements showed that the interaction can change the emission properties of both protein and nanoparticle suggesting they influence each other. Protein conformational changes were indicated also by phosphorescence lifetime measurements, which showed that NPs interact with proteins forming stable complexes. In some proteins, the protein structure resulted significantly altered, particularly in the superficial regions. A rough determination of the affinities of proteins-NP complexes was derived from the change of the phosphorescence lifetime at increasing NP concentration.