Effect of gold nanoclusters (Au-NCs) on the circular dichroism (CD) spectra and enzymatic activity of ot-chymotrypsin (ChT) (towards hydrolysis of a substrate, N-succinyl-L-phenylalanine p-nitroanilide) are studied. The CD spectra indicate that on binding to Au-NC, ChT is completely unfolded, resulting in nearly zero ellipticity. alpha-chymotrypsin (ChT) coated gold nano-clusters exhibit almost no enzymatic activity. Addition of glutathione (GSH) or oxidized glutathione (GSSG) restore the enzyme activity of alpha-chymotrypsin by 30-45%. ChT coated Au-NC exhibits two emission maxima-one at 480 nm (corresponding to Au-10) and one at 640 nm (Au-25). On addition of glutathione (GSH) or oxidized glutathione (GSSG) the emission peak at 640 nm vanishes and only one peak at 480 nm (Au-10) remains. MALDI mass spectrometry studies suggest addition of glutathione (GSH) to ct-chymotrypsin capped Au-NCs results in the formation of glutathione-capped Au-NCs and ot-chymotrypsin is released from Au-NCs. CD spectroscopy indicates that the conformation of the released ct-chymotrypsin is different from that of the native ce-chymotrypsin. (C) 2017 Elsevier Inc. All rights reserved.