This article describes, for the first time, the effect of three different sizes of silver nanoparticles on the binding of curcumin to lysozyme as examined by spectroscopic and zeta potential techniques at physiological conditions. The binding constants of curcumin to lysozyme in the presence of silver nanoparticles were measured. Based on the results of synchronous fluorescence and three-dimensional fluorescence spectroscopy, the presence of the different sizes of silver nanoparticles caused conformational changes in lysozyme during the binding of curcumin. Such changes were also observed when increasing the curcumin concentration. The results of fluorescence resonance energy transfer theory indicated that different sizes of silver nanoparticles could change the binding distance between curcumin and lysozyme. Based on the red edge excitation shift approach, we concluded that the limited mobility around the Trp residues decreased in the presence of silver nanoparticles with bigger size. Under resonance light scattering, the aggregation of curcumin on lysozyme in the presence of silver nanoparticles can play a major role in functional proteins. Communicated by Ramaswamy H. Sarma.

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The binding of ciprofloxacin (CIP) to human holo-transferrin (HTF) in the presence of silver nanoparticles (AgNPs) has been investigated by fluorescence quenching and circular dichroism (CD) techniques as well as resonance light scattering under physiological conditions. It was determined that the intrinsic fluorescence of HTF was quenched by CIP in the presence of AgNPs through static quenching, thus confirming that a CIP-HTF complex was formed in both the binary and ternary systems. However, the analysis of HTF fluorescence quenching in these binary and ternary systems indicated that the AgNPs were affected upon complex formation between CIP and HTF and that the binding affinity between them became more substantial when the AgNPs coexisted with the drug. Fluorescence quenching proved that HTF had one class of binding sites for CIP in both binary and ternary systems. CD spectra indicated that the secondary structure of HTF changed when increasing the CIP concentration and during the simultaneous presence of CIP and AgNPs, which led to decreased contents of