Inhibition of lysozyme amyloid fibrillation by curcumin-conjugated silver nanoparticles: A multispectroscopic molecular level study

Abstract

Inhibition of amyloid fibrillation of hen egg white lysozyme (HEWL) was targeted by using curcumin-conjugated silver nanoparticles (c-AgNPs). Fluorescence assays (thioflavin T, ANS binding, tryptophan (Trp) fluorescence), circular dichroism and microscopic imaging techniques (HRTEM and fluorescence) were adopted to study fibrillation inhibition. Results suggested that c-AgNPs having hydrodynamic radius ∼ 22 nm can inhibit fibrillation of HEWL and restricted the protein fibrillation to smaller fibrils. Moreover, in the form of c-AgNPs, curcumin remained water-soluble and was more effective than curcumin. Further, interactions between HEWL and c-AgNPs were studied using fluorescence and circular dichroism spectroscopic techniques. The nanoparticles had caused static quenching of Trp fluorescence of native HEWL. This was also confirmed through determination of excited state lifetime of Trp of HEWL in absence and the presence of c-AgNPs. The association and quenching constant for the conjugation of HEWL with c-AgNPs were determined ∼ 107 M−1. Hydrophobic interactions were found to be involved in HEWL-c-AgNPs complex. Polarity of Trp microenvironment in HEWL was not perturbed by c-AgNPs as supported by synchronous and three-dimensional fluorescence spectroscopy. Far-UV CD spectra suggested that the secondary structure of native HEWL was not affected by c-AgNPs.