INTERACTIONS BETWEEN AQUEOUS TRANSITION METALS (AU, ZN) AND ORGANIC MOLECULES PROBED BY ESI MASS SPECTROMETRY AND DENSITY FUNCTIONAL THEORY
The primary objective of this study was the application of ESI FT-ICR mass spectrometry and DFT calculations in the investigation of molecular metal-biomolecule complexes. Three independent ESI-MS experiments were performed, in which aqueous solutions of AuCl3:cysteine, ZnCl2:cysteine and ZnCl2:thymidine were analyzed. In the case of gold-cysteine system, the formation of disulfides and gold(I)-disulfide species was observed. Structural and energetic data of Au(I) complexes were calculated at the M06 level theory, and using TD-DFT and PCM/TD-DFT computations, UV-Vis properties of these species were predicted. The major goal of the second project was to deliver speciation, structures and stability of aqueous zinc-cysteine complexes. ESI-MS of ZnCl2:cysteine solution resulted in formation of mono- and polynuclear Zn complexes. DFT calculations further showed that tetranuclear Zn-cysteine species can be considered as potential intermediates during sphalerite formation. In the third study the focus was on the complexation behavior of zinc with nucleoside thymidine. IRMPD-MS analysis of the most abundant [Zn(dT)(dTH)]+ showed that the most favorable dissociation channel for zinc-thymidine complexes is the neutral removal of 2’-deoxyribose and the subsequent fragmentations proceed via cross-ring bond raptures of thymine. The M06 calculations showed that zinc-thymidine complexes retain symmetric structures and their predicted gas-phase stability was found to reflect ESI-MS experimental results.
In each project the ion abundance and distribution of gaseous metal-free and metal-containing species were found to be critically dependent on the i) solution concentration; and ii) time, at which feed solution was electrosprayed. Therefore, from a geochemical perspective, ESI mass spectrometry is considered as an excellent tool for delivering new insights into metal speciation in bulk solution.