We obtained the binding free energy of the complex [Co(C2O4)(3)](3-) to the peptide H-Lys-Gly-(Lys-Gly)(9)-Lys-NH2, and to the monomers (aminoacids) forming the peptide, using the electron transfer reaction between [Ru(NH3)(5)pz](2+) and [Co(C2O4)(3)](3-) as the probe. The polymerization of the monomers increases the negative free energy of binding and changes its character, non-cooperative for the monomers and anti-cooperative for the peptide. This increase in the negative free energy represents a driving force for the polymerization process that produces the peptide from the aminoacids (monomers) in such a way that the polymerization in the presence of the cobalt complex is more favourable, from a thermodynamic point of view, in 16.5 kJ/mol. (C) 2003 Elsevier B.V. All rights reserved.