Synthesis, characterization, and reactivity of iron(II)- and zinc(II) complexes of imidazole thione and selone ligands: Investigations into oxidation mechanisms

Fe(II) localized on DNA can reduce hydrogen peroxide (H 2O2) to form hydroxyl radical, resulting in oxidative DNA damage. This oxidative DNA damage is an underlying leading cause of cardiovascular diseases, neurodegenerative diseases, and cancer. Sulfur and selenium antioxidants have been investigated for their prevention of metal-mediated oxidative DNA damage, and sulfur- and especially selenium-containing antioxidants prevent metal-mediated oxidative DNA damage, likely by radical scavenging and sacrificial oxidation via metal coordination.;To determine how iron coordination results in DNA damage inhibition, Fe(II) complexes with imidazole-thione and --selone ligands were synthesized to examine their iron coordination modes (Chapter 2), electrochemistry (Chapter 2), and reactivity with H2O2 (Chapter 3). N,N' -dimethylimdiazole thione (dmit) and --selone (dmise) ligands were used since they resemble ergothioneine and selenoneine, thione and selone-containing antioxidants naturally found in plants and animals. Dmit and dmise coordination to Fe(II) increases the zwitterionic character of these ligands, and binding occurs primarily through sigma and pi donation. Fe(II)-thione and selone complexes of the formulae Fe(L)2Cl2, [Fe(L)2(NCCH 3)2]2+, and [Fe(L)4]2+ complexes (L = dmit or dmise) have ligand-based oxidation potentials ranging from 0.36 - 0.69 V, significantly higher than their observed Fe(III/II) oxidation potentials. Thus, the thione and selone ligands should undergo oxidation prior to Fe(II), possibly preventing iron-mediated oxidative DNA damage.;The oxidation of Fe(dmit)2Cl2 and Fe(dmise) 2Cl2 complexes by H2O2 was examined to determine potential antioxidant mechanisms (Chapter 3). Treatment of Fe(dmit/dmise) 2Cl2 with H2O2 showed sacrificial oxidation of the thione and selone ligands; however, only dmise significantly protected Fe(II) from oxidation to Fe(III). If similar iron-thione and --selone complexes form in vivo, these complexes may similarly scavenge H2O2 to inhibit iron-mediated oxidative DNA damage.;A comparative mechanistic study required synthesis and characterization of Zn(L)2Cl2 and [Zn(L)4]2+ complexes (L = dmit or dmise) as well as H2O2 oxidation studies of Zn(dmit/dmise)2Cl2 complexes (Chapter 4). Dmit and dmise coordination to non-redox-active Zn(II) occurs thorough sigma and pi binding, similar to their Fe(II) analogs. However, the dmit and dmise ligand oxidation potentials decrease compared to unbound dmit and dmise, indicating stabilization of the chalcogenones, opposite the effect observed upon Fe(II) coordination. Treatment of Zn(dmit)2Cl2 with H 2O2 showed dmit oxidation; however, dmit does not undergo complete oxidation, since dmit-OH species were observed by mass spectrometry. Unlike dmit, when Zn(dmise)2Cl2 was treated with H 2O2 complete oxidation of dmise was observed, similar to that observed for its Fe(II) complex analog. These results indicate dmit oxidation is dependent upon the redox-activity of the coordinated metal ion, whereas dmise oxidation is not metal dependent.;Novel thione- and selone-containing imidazoles were synthesized and characterized with various substituents on the imidazole ring (Chapter 5). A novel synthetic approach was also implemented for the synthesis of thione derivatives to avoid the use of air-sensitive techniques. Addition of chlorine atoms in place of the imidazole ring hydrogen atoms in dmit increases its zwitterionic character compared to dmit; however, no similar difference in zwitterionic character is observed for its selenium analog compared to dmise. This difference suggests that the nucleophilicity of the Se compared to the S in these chloro-substituted imidazole thiones is significant, with a larger partially negative charge on the Se atom compared to its S analog even when not bound to a metal. Use of trityl as a thione sulfur protecting group is a novel synthetic route that allows facile synthesis of novel thione derivatives. Addition of a methyl-keto substituent on an imidazole nitrogen atom may increase metal binding affinity of the resulting thione due to an increase in zwitterionic character and a possible an increase in ligand denticity through the available S and O atoms compared to methimazole and dmit.;Methimazole and its derivatives were also investigated for their ability to coordinate Fe(II) and Zn(II) (Chapter 6). Since methimazole and 2-mercaptoimidazole oxidize to their respective disulfides, the synthesis of Fe(II) and Zn(II) complexes with these oxidized ligands was also investigated. When bound to Fe(II) and Zn(II), methimazole and 2-mercaptoimdazole increase in zwitterionic character and bind solely through the S atom via sigma and pi donation. The oxidized thione and selones bind through their available nitrogen atom, forming seven-membered ring systems. These studies have enhanced our understanding of the possible antioxidant mechanisms of thiones and selones as well as provided proof that iron coordination is a major mechanism in prevention of metal-mediated DNA damage.