No description available.

No description available.

No description available.

Thesis (Ph.D.)--McMaster University, 2005. / Supervisor: Paul Harrison. Includes bibliographical references (leaves 192-207).

Graduation date: 1996

Graduation date: 1996

Thesis--Auburn University, 2009. / Abstract. Vita. Includes bibliographic references (p.49-58).

Bacterial type I modular polyketide synthases (PKS) are large multifunctional enzyme systems responsible for the biosynthesis of complex polyketide natural products such as erythromycin, pikromycin, and borrelidin. Type I systems are comprised of a loading module which generally selects an appropriate acyl group starter unit, and multiple discrete extension modules, responsible for each single round of acyl group incorporation into the final polyketide core structure. These modules can exist naturally as either single discrete polypeptides, such as modules 5 and 6 from the pikromycin PKS (PikA3 and PikA4 respectively), or as multimodular polypeptides fused together by short intrapolypeptide linkers such as the loading module and the first and second extension modules of the erythromycin and pikromycin PKSs (DEBS1 and PikAI respectively). While short peptide linkers between modules on the same polypeptide facilitate the transfer of polyketide intermediates from one module to the next via their close proximity to one another, docking domains found at the C-terminus of one module and the N-terminus of the next subsequent module facilitate the needed protein-protein interactions for the passage of biosynthetic intermediates between modules on separate polypeptides. The ability to utilize docking domains in place of intrapolypeptide linkers was explored in the pikromycin and erythromycin PKSs by dissecting the tri-modular PikAI and DEBS1 polypeptides with matched docking domains. It has been shown that PikAI can be separated into two proteins at either of these linkers, only when matched pairs of docking domains from a heterologous modular phoslactomycin PKS are used in place of the intrapolypeptide linker. In both cases the yields of pikromycin produced by the S. venezuelae host mutant, which is a PikAI deletion strain were 50% of that of an S. venezuelae strain expressing the native trimodular PikAI. Additionally, expression of module 2 as a monomodular protein fused to a heterologous N-terminal docking domain was also observed to give almost a 10-fold improvement in the in vivo generation of pikromycin from a synthetic diketide intermediate. The utilization of docking domains to separate linked modules was also demonstrated in the erythromycin PKS. Expression of the first protein involved in erythromycin biosynthesis (DEBS1) with the DEBS thioesterase fused to the C-terminal (DEBS1-TE) in S. venezuelae results in the production of triketide lactone products. Separation of DEBS1-TE resulted in 50% triketide lactone production, consistent with the observations in the pikromycin system. Published work has shown that the DEBS loading module has relaxed substrate specificity, and is capable of incorporating acetate, butyrate and isobutyrate in addition to the normally observed propionate starter unit, which typically predominates. However, in the current study when the DEBS loading module is separated from module 1 with matched docking domains, a dramatic shift in the starter unit, favoring the isobutyrate derived tri-ketide lactone is observed. This apparent shift in starter unit preference for a dissected PKS system has resulted in insights into the kinetics of acyl group loading, off loading, as well as the hydrolysis and transfer from the AT to ACP domains. In addition to the separation of multimodular PKS polypeptides with docking domains, it has also been shown that the individual catalytic domains of single discrete module, BorA5 from the borrelidin PKS can be expressed as stand alone proteins while retaining catalytic functionality in vitro. This work has provided a basis for future studies of this module, which has been proposed to function iteratively, catalyzing three rounds of chain elongation.

A long-standing research interest of the Leighton group has been the utilization of strained silanes in the rapid and efficient synthesis of polyketides. Recently, we have been interested in how the effects of strain might manifest itself in the conductance and functionality of silicon-based molecular junctions. As electronic components continue to miniaturize to the point where transistor size and structure begin to resemble small molecules, understanding the principles that guide charge transport in single molecule junctions will be crucial. Herein, we describe our studies on a series of single molecule junctions formed by strained silicon wires. We demonstrate that high conductance pathways are accessed for the cis diastereomers of conformationally locked 1,2-disilaacenaphthenes via a bipodal binding motif which provides a stable electrical contact between the Si—Si bond and the gold electrodes. We then elucidate the mechanism of voltage-induced breakdown in silicon-based single molecule junctions. We show that the naphthalene bridge provides a parallel conductance pathway to the silicon backbone, altering bond rupture behavior of the Si—Si bond. We further investigate the bond rupture mechanism through DFT and molecular dynamics calculations and conclude that breakdown occurs by the excitation of vibrational modes in the molecular junction by tunneling electrons, leading to homolytic Si—Si bond rupture.

The thesis entitled "Synthesis of Natural Products Based on Cyclohexadienes" consists of two chapters. Chapter 1 is divided into two parts. Part I gives a brief introduction to the structure, synthesis, biosynthesis and biological activities of some naturally occurring phthalides (eg. mycophenolic acid 1, zinniol2, phthalides 3 & 4). A general strategy for the preparation of highly substituted phthalides is also described. Cycloaddition of 1,s-dimethoxycoclohexa-1, 4-diene with dimethylacetylenedicarboxylate(DMAD) followed by an Alder-Rickert reaction gave the diester 5 which upon hydrolysis with KOH and refluxing with acetic anhydride gave the phthalic anhydride 6. Regioselective reductions of the anhydride 6 gave the phthalides 7 and 8. Using a similar strategy the phthalides 11 & 12 were prepared from 2,6dimethoxytoluene through the intermediates 9 & 10. The aromatic ethers 13 & 14 upon Birch reduction followed by Diels-Alder reaction with maleic anhydride gave the bicyclic anhydrides 15 & 16 respectively. Attempts to dehydrogenate 15 using variety of conditions failed. But refluxing 15 in nitrobenzene gave a poor yield of 17 which is an important intermediate in the synthesis of mycophenolic acid. Part II describes the first total synthesis of zinniol 2, phthalide-1 3 & phthalide-2 4. Thus the diene 18, obtained from 2-methylcyclohexane-1,3dione, upon Diels-Alder and Alder-Rickert with DMAD gave the diester 19. Prenylation of 19 afforded the diester 20 which was convened into 21 upon hydrolysis and DCC treatment. DIBAL reduction of 20 gave Zinniol 2 which on oxidation provided the phthalides 3 & 4 (7:3 ratio respectively). The anhydride 21, on selective reduction, gave the same phthalides in 2:8 ratio which could be readily separated and characterized.