The research described in the following dissertation focuses on the use of ESI FT-ICR mass spectrometry for the analysis of complex environmental systems. The projects we have chosen are multidisciplinary by design including the fields of analytical, physical, inorganic, environmental and biochemistry. To begin with, the unique fragmentation pathways resulting from oxidative stress to angiotensin II are characterized using surfaced induced dissociation, RRKM modeling and molecular dynamics calculations. The final chapters focus on the study of metal sulfide nucleation in aqueous environments using thiol capping agents to interrupt particle growth and gas phase ion-molecule reactions of metal salt clusters with hydrogen sulfide.
The gas phase fragmentation reactions of singly charged angiotensin II (AngII, DR+VYIHPF) and the ozonolysis products AngII+O (DR +VY*IHPF), AngII+3O (DR+VYIH*PF), and AngII+4O (DR +VY*IH*PF) were studied using SID FT-ICR mass spectrometry, RRKM modeling, and molecular dynamics. Oxidation of Tyr (AngII+O) leads to a low-energy charge-remote selective fragmentation channel resulting in the b4 fragment ion. Modification of His (AngII+3O and AngII+4O) leads to a series of new selective dissociation channels. For AngII+3O and AngII+4O, the formation of [MH+3O] +-45 and [MH+3O]+-71 are driven by charge-remote processes while it is suggested that b5 and [MH+3O]+-88 fragments are a result of charge-directed reactions. Energy-resolved SID experiments and RRKM modeling provide threshold energies and activation entropies for the lowest energy fragmentation channel for each of the parent ions. Fragmentation of the ozonolysis products was found to be controlled by entropic effects. Mechanisms are proposed for each of the new dissociation pathways based on the energies and entropies of activation and parent ion conformations sampled using molecular dynamics.
Developed through collaboration with the College of Marine Studies, our focus shifted to the study of metal sulfide clusters. The initial experiments include the observation of metal salt cluster growth in solution using ESI mass spectrometry. Relative intensities of the observed clusters ([Cd x(NO3)2x+1]-, x = 1-5) shift to larger clusters following dilution of aqueous solutions with methanol. The time-dependent variance in the relative signal intensity is evidence of salt nucleation in water-methanol binary solvent systems suggesting larger clusters may persist in low dielectric constant solutions as found in supercritical aquatic systems.
With the methodology in place to effectively elecrospray cadmium salt clusters, observing metal sulfide cluster formation became the primary focus. The first approach was to use capping agents to interrupt particle growth at various stages of nucleation. Sulfidic metal clusters were observed for reactions of cadmium nitrate with hydrogen sulfide using 2-mercaptopyridine as a molecular capping agent. While NO3 ligands were substituted by sulfate for larger clusters ([Cdx(NO3)2x+1]-, x = 2-5), no reaction products were observed for the single cadmium nitrate complex.
Finally, gas phase ion-molecule reactions between metal salt clusters and hydrogen sulfide were used to understand metal sulfide cluster formation. A simplified sequential pseudo first-order kinetic model was used to describe reactions. Anionic clusters were shown to react with low reaction efficiencies (kx/kc < 5%) with the exception of [Cd4(CH3COO)9]- which displayed enhanced reactivity following formation of the [Cd4S(CH3COO) 7]- intermediate. No reaction products were observed for any anionic cadmium chloride cluster. Large cationic cadmium acetate clusters ([Cdx(CH3COO)2x-1]+, x = 2-4) were shown to react fast initially with greatly reduced efficiencies as the reaction proceeds. All other metal salt solutions (Cd(NO3) 2, CdCl2 and ZnCl2) generate solvated single metal complexes which react with H2S to form bisulfide species. While an increase in the number of solvent molecules causes a decrease in reaction efficiencies, solvent molecules persist throughout the reaction suggesting they play no role in the mechanism of metal sulfide formation. Additionally, counter ions were shown to affect reaction rates with NO3 - > OH- > Cl- in terms of reaction efficiency.