Pulsed laser time-resolved ligand-substitution photochemistry for (DTO)W(CO)4, (DTN)W(CO)4, and (NP)Mo(CO)4 (DTO = 2,2,7,7-tetramethyl-3,6-diathiaoctane; DTN = 2,2,8,8- tetramethyl-3,7-diathianonane; NP = l-diethylamino-2- diphenylphosphinoethane) proceeds via initial fission of the W-S and Mo-P bonds, affording Cs and C4v five-coordinate intermediates for DTN and NP but largely Cs for DTO. The rates of reaction of these intermediates, via chelate ring closure and competitive bimolecular interaction with Lewis bases (= L, alkylphosphines and alkyl phosphites) for the Cs intermediates and via bimolecular interaction of L with the C4v intermediates, together with activation parameters for these processes have been determined. The rates of interactions at the Cs intermediates are significantly faster than at the C4v intermediates.

Thesis (Ph.D.)--McMaster University, 2005. / Supervisor: Ken Coley. Includes bibliographical references (leaves 204-211).

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In recent years, hydrogen sulfide (H2S) has garnered interest as the third addition to the gasotransmitter family. Essential to human physiology, H2S has roles in the cardiovascular, nervous, and respiratory systems and perturbations in physiological H2S levels have been correlated to a variety of diseases. As a result, there has been significant interest in the development of H2S-releasing compounds (H2S donors) that can mimic slow, enzymatic production for research and therapeutic applications. While a large library of H2S donors exists, several common drawbacks persist, such as: lack of spatial and temporal control, poorly understood mechanisms of release, uncontrolled kinetics, and low efficiency. These issues significantly limit the biological applications of many H2S donors. This dissertation describes recent work to provide biocompatible H2S donors with controllable release kinetics using a robust, novel strategy for H2S delivery that relies on rapid enzymatic hydrolysis of carbonyl sulfide (COS) to H2S by the ubiquitous mammalian enzyme carbonic anhydrase (CA). Self-immolative thiocarbamates can be designed to release COS by a variety of stimuli, and in biological milieu this COS is rapidly converted to H2S by CA. This strategy has enabled the development of the first analyte-replacement fluorescent probe for H2S and has become a popular strategy for H2S delivery in a variety of applications. Additionally, the unexpected cytotoxicity profile of enzyme-activated COS/H2S donors has piqued interest in COS chemical biology, and these donors are being used as tools for studying COS itself. This dissertation includes previously published and unpublished coauthored work. / 2021-04-30

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Thesis (Ph. D.)--University of Hong Kong, 2002. / Includes bibliographical references.

Oxidative degradation of lipids, not only leads to the quality deterioration in foodstuffs but also associates with a multitude of physiological processes. One of the causations involved in these damaging effects is the generation of reactive carbonyl species (RCS) in lipid peroxidation process. RCS are notorious toxins that possess reactivity towards biological nucleophiles (such as proteins and DNA) with potential functional alternation in these biomolecules. Therefore, the exogenous intervention is required to inhibit the toxicity related to RCS/lipid peroxidation. In present study, the screening for effective natural polyphenols to trap two representative RCS, acrolein (ACR) and 4-hydroxy-trans-2-nonenal (HNE), was performed with mechanism elucidation. It was found that the polyphenols from the categories of flavan-3-ols, theaflavins, cyanomaclurins and dihydrochalcones were effective scavengers of ACR/HNE. Subsequently, facilitated by HPLC, LC-MS/MS and NMR analysis, the characterization of polyphenols’ as sacrificial nucleophiles towards these two electrophiles products was accomplished. Michael addition at A ring of polyphenols’ to the C=C bond of ACR/HNE was suggested to be responsible for trapping of these two RCS and thus render their active sites unavailable to covalently modify critical biomolecules. Further investigation of phloretin’s effect to attenuate ACR-induced modification on lysine residue and proteins was carried out. Phloretin’s protective effect against ACR’s toxicity was clearly reflected by its inhibition of the formation of Nε-(3-formyl-3,4-dehydropiperidino) lysine (FDP-lysine), blocking the electrophilic site in FDP-lysine, lowering protein carbonylation in bovine serum albumin (BSA) and lessening protein oligomerization in bovine pancreas ribonuclease A. Such protection might be mediated by phloretin’s directly trapping of ACR and consequently deactivation of ACR in covalent modification of amino acids and proteins. The biological relevance of polyphenols’ trapping activity of ACR was explored in a cell culture model. Natural polyphenols including phloretin, EGCG and quercetin were proved to be active to inhibit ACR-induced cytotoxicity in human neuroblastoma SH-SY5Y cells. The cytoprotection of phloretin (as the most potent one in alleviation of ACR stress) was suggested to be achieved through the reduction of the increased cellular protein carbonyl level as revealed by Western blotting analysis. In the final part of this study, an in vitro system containing metal-catalyzed fatty acids and BSA was established to study the modification on protein induced by lipid peroxidation and possible inhibitory effects conferred by some natural polyphenols. The protective effects of these polyphenols against lipid peroxidation-induced modification on BSA was manifested by the observation of reduced levels of high-molecular-weight proteins, MDA-related fluorescent substances and protein carbonylation. However, poor correlations were found between such protection and antioxidant activities, suggesting alternative mechanisms were existed such as carbonyl-scavenging. In conclusion, findings from the present study highlighted certain kinds of natural polyphenols as promising agents in counteracting RCS/lipid peroxidation-induced toxicity in amino acid, protein and cell models. / published_or_final_version / Biological Sciences / Doctoral / Doctor of Philosophy

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