Influence of acidity on the reaction between [PdCl(dien)](+) and L-cysteine or glutathione in the presence of sodium dodecyl sulfate micelles
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The reaction mechanism and thermodynamic parameters of the complex formation between [PdCl(dien)](+) and sulfur-containing ligands L-cysteine and glutathione (GSH) were investigated in the presence of sodium dodecyl sulfate micelles in the pH range 0.5-3.5. The reaction rates were determined under pseudo-first-order conditions (ligand in excess) in the temperature range 276-299 K by using the stopped-flow technique. A reaction mechanism was proposed that consisted of at least two parallel paths involving protonated and zwitterionic forms of thiols. The pH effects on reaction rate were interpreted in terms of the electrostatic interactions between the negatively charged micelle surface and different ionic forms of the ligand species. The calculation of pH-dependent activation parameters (Delta H-not equal and Delta S-not equal) revealed the considerable catalytic effects on the rate of complexation. The entropy of activation is strongly negative in the presence and absence of micelles, ...which is compatible with an associative reaction mechanism. Copyright (c) 2004 John Wiley T Sons, Ltd.