The present study was undertaken to better understand the complexation of mercury (II) by cysteine, histidine, tryptophan, and their di- and tri-peptides. Their mercury (II) binding affinities and associated thermodynamic parameters are evaluated by isothermal titration calorimetry. Cysteine S-donor atoms form the strongest complexes, which can be attributed to a more exothermic Hg-S soft acid and soft base interaction. These thiol S-donor peptide ligands show two sequential binding for mercury (II). Their stability constants for the first binding (10(8) M-1 to > 10(10) M-1) are largely due to favorable contribution of the enthalpy term to the free energy of complexation. As more mercury (II) ions are added, this enthalpy contribution decreases and the free energy of the second binding (10(5) M-1 to 10(6) M-1) is partially compensated by the entropy term. The dependency of the fluorescence intensity for these peptides on mercury (II) concentration shows two different Stern-Volmer plots, which corroborates the calorimetric data and supports the formation of two types of stable complexes. (C) 2009 Elsevier B.V. All rights reserved.