Behavior of glutathione as lead-ligand

The presence and mobilization of heavy metal cations represents under many aspects a current and important problem in the environmental field. In this research, as cation lead (II) ion was studied. On the other hand, glutathione (GSH), present in plants, is potentially able to form complexes with cations, in this case with lead (II), so that it could be useful for a possible improvement of environmental remediation technique (phytoremediation). The formation of complexes between glutathione (below indicated with L in its completely deprotonated form) and lead (II) was studied at 25° C and in 1.00 mol dm-3 NaCl as ionic medium by means of measurements of electromotive force (e. m f.) of the following galvanic cells: Pb (Hg)/Solution Test/ R.E. and R.E./ Solution Test/ G.E. where Pb (Hg), and G.E. are amalgam of lead and glass electrodes respectively, whereas R.E. is the reference electrode. Beside to the e. m. f. measurements, spectrophotometric and N.M.R. measures were carried out to obtain information on the type of links existing between L and Pb (II). Previously, it was necessary to study the protolytic behavior of glutathione in the same experimental conditions of temperature and ionic medium. The experimental data, obtained from e.m.f. measurements performed with a glass electrode, are well explained assuming the following species: H4 L +, H3 L, H2 L-, HL 2- The relative protonation constants defined as: [Hn L] = h Kn [Hn-1 L] where square parentheses indicate the free concentrations of the inside species and h is the free concentration of hydrogen ion. The following values are obtained: log K1= 9.55 ± 0.03, log K2= 8.46 ± 0.02, log K3= 3.45 ± 0.05, log K4= 2.25 ± 0.10. The e. m. f. experimental data obtained in a wide range of both analytical excess of concentration hydrogen ions, and -log h, and of total concentration of lead (II) and of glutathione were explained by assuming the presence of mono-nuclear complexes in lead (II), but with the participation of the hydrogen ion. The formation of Pb H L, Pb H2 L and Pb H3 L (omitted charges), with the relative stability constants, explains the experimental data very well. Spectrophotometric measurements carried out with the Job method confirm the presence of complexes in the ratio 1: 1 between cation and ligand. The N.M.R. data show the formation of a chelate between Pb (II) and carboxyl and sulfur present in the glutathione molecule.