Thioester Hydrolysis Reactivity of Metal Complexes

• Main Author: Danford, James Justin
• Format: Others
• Published: DigitalCommons@USU 2010
• Subjects: thioester; hydrolysis; reactivity; metal complexes; Inorganic Chemicals
• Online Access: https://digitalcommons.usu.edu/etd/571; https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1567&context=etd

Glyoxalase II is one of two metalloenzymes found in the glyoxalase pathway and is responsible for catalyzing the hydrolysis of a thioester substrate. Its bimetallic active site is found to contain a variety of metal combinations, including Fe(III)Zn(II). A recent report indicates that human glyoxalase II, while containing a Fe(II)Zn(II) center, is catalytically active as a mononuclear Zn(II) enzyme. Detailed mechanistic studies of glyoxalase II enzymes are limited due to uncertainty in the metal ion content of recombinantly prepared samples. The research presented in this thesis is focused on gaining mechanistic insight into thioester hydrolysis promoted by well-characterized metal complexes The initial research is focused on studies involving a Fe(III)Zn(II) complex supported by the 2-{[bis(2-pyridylmethyl)amino]methyl}-6-[{[2-hydroxyphenyl)methyl]-(2- pyridylmethyl)amino}methyl]-4-methylphenol) ligand. Thioester hydrolysis reactions were examined by following the loss of a deuterium-labeled thioester (hydroxyphenyl thioacetic acid S-methyl(d3) ester) over time using 2H NMR as the monitoring method. Based on kinetic data and spectroscopic investigations (UV-vis and EPR), a reaction pathway for thioester hydrolysis promoted by the aforementioned Fe(III)Zn(II) complex has been proposed. An important feature of this pathway is the formation of a precursor complex wherein the deprotonated α-hydroxy group of the thioester coordinates to the Zn(II) center prior to nucleophilic attack by an Fe(III)-OH moiety. Of relevance to human glyoxalase II, the thioester hydrolysis reactivity of a mononuclear zinc complex containing the N,N-bis(2-pyridylmethyl)-tert-butylamine ligand, (bpta)Zn](ClO4)2⋅0.5H2O, has been examined. Based on kinetic data, it is proposed that thioester hydrolysis promoted by this complex proceeds via a bimolecular pathway, with a Zn-OH moiety being the nucleophile for attack on the thioester carbonyl. Activation parameters are reported for the zinc complex-promoted thioester hydrolysis reaction and are compared to those of OH- promoted thioester hydrolysis reactions. In a separate area of investigation, the chromium chloride complex {(C6H11N2)[CrCl3]}n has been isolated and characterized by elemental analysis and X-ray crystallography. This complex has been proposed as the catalyst responsible for high yield conversion of glucose to 5- hydroxymethylfurfural (HMF), which is an important reaction toward using renewable resources as feedstock chemicals.