The water soluble glutathione capped metal nanoparticles (M-GS, where M = Pd, Pt, Au and Ag; GS = glutathione) with size 2.4 +/- 0.2 nm were synthesized by borohydride reduction of metal ions in the presence of glutathione as capping ligand and used as catalyst for the hydrogenation of nitroaniline in aqueous phase. The rate of catalytic hydrogenation was dependent on metal type and the trend of catalytic activity over these M-GS nanoparticles was found to be Pd-GS (k(app) = 0.0227 (+/- 3 x 10(-4))) s(-1) >> Pt-GS (k(app) = 0.0043 (+/- 1 x 10(-4)))s(-1) > Au-GS (k(app) = 0.0015 (+/- 0.2 x 10(-4))) s(-1) > Ag-GS (k(app) = 0.0008 (0.2 x 10-4)) s-1. The similar trend of catalytic activity was found for the hydrogenation of nitrobenzene. Our experimental results, along taking into account the theoretical calculations done by other research groups, suggest that the observed catalytic activity trend is attributed to the "different rates of H-2 molecule adsorption and dissociation" on the M-GS nanoparticles. The "high rate of H-2 molecule adsorption" and "highly oxidized surface" make Pd-GS nanoparticles an ideal candidate for the rapid hydrogenation. On the basis of our experimental results, we proposed that small gaps between less densely packed branched thiol "glutathione molecules" provide the access to metal nanoparticle surface for the hydrogenation reaction. (C) 2014 Elsevier Inc. All rights reserved.