Studies of metal-organic interactions with model synthetic and natural ligands applicable to natural waters

Citation

Piatina, Tatiana Borisovna (2001) Studies of metal-organic interactions with model synthetic and natural ligands applicable to natural waters. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/57TJ-PE22. https://resolver.caltech.edu/CaltechTHESIS:10122010-103734735

Abstract

Metal speciation significantly influences the geochemical cycling of trace metals and can control metal bioavailability and toxicity. This study addressed some unresolved questions concerning metal speciation in natural waters in several complementary ways. Metal competition for model and natural organic ligands was examined by coupling size-exclusion chromatography (SEC) with inductively-coupled plasma mass spectrometry. The method was validated with well-defined organic ligands in ligandcompetition studies with a single metal and a binary metal mixture. The measured concentrations of metal-ligand species corresponded to the calculated equilibrium speciation. However, the method is subject to kinetic limitations. For metal complexes that are partially labile during chromatographic separation, the rate constant for complex dissociation and the concentration of the (initial) complex were estimated based on a mathematical model. Application of this method to studies of copper complexation by Suwannee River humic acid demonstrated that copper complexes are kinetically labile on the SEC column for copper additions comparable to the background concentrations in the humic acid samples. This suggests that the copper-binding sites that form complexes detectable by this method are present in humic acids at very low concentrations and are not available to bind added copper. Investigation of copper complexation with a synthetic analog of the natural metalbinding peptide phytochelatin has demonstrated that Cu(II) is not stable in the presence of phytochelatin but that the peptide fonns strong complexes with Cu(I). Complexation of Cu(I) by phytochelatin was studied using a spectroscopic technique in which bathocuproine was added as a competing ligand. The method was validated in experiments with glutathione. Titrations of bathocuproine and phytochelatin with Cu(I) and of Cu(I) and bathocuproine with phytochelatin were explained by formation of 1:1 and 1:2 Cu(I)phytochelatin complexes. To account for the experimental observations, a polynuclear (1:2:1) phytochelatin-Cu(I)-bathocuproine complex was introduced in modeling the titration data. Consistent values of conditional stability constants were obtained in Cu(I) and phytochelatin titrations. However these results were not consistent with those obtained in titrations of Cu(I) and phytochelatin with bathocuproine possibly due to the slow ligand exchange kinetics of the pre-formed Cu(I)-phytochelatin complexes.

Thesis Files