A novel synthetic method for oxygen-17 labelling of metal-dioxygen complexes

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Authors
Palmer, James
Keyword
Oxygen , Oxygen-17 NMR , Metal-dioxygen complexes , Water electrolysis , NMR , Transition metal complexes , Myoglobin
Abstract
This thesis investigates the synthesis of 17O-labelled metal-dioxygen complexes by utilizing 17O2 gas produced from H217O electrolysis as a source of 17O isotopes. Because of the high cost of H217O, a miniature water electrolysis device was built to collect 17O2 gas at a rate of 6.1 x 10-4 mol O2/hr (~15 mL/hr) over water. Two 17O-labelled dinuclear cobalt(III)-dioxygen complexes, [(en)2Co(μ-17O2)(μ-OH)Co(en)2]3+ where en is ethylenediamine (1b) and [(NH3)5Co(μ-17O2)Co(NH3)5]4+ (2b) were successfully synthesized for the first time. Their 17O Nuclear Magnetic Resonance (NMR) signals were found to appear at 242 and 208 ppm for 1b and 2b, respectively. These 17O chemical shifts represent the most shielded values to date for metal-dioxygen complexes. We also discovered that the μ-hydroxo ligand in [(en)2Co(O2)(μ-OH)Co(en)2]3+ can be readily 17O-labelled by exchange with H217O, thus producing [(en)2Co(O2)(μ-17OH)Co(en)2]3+ (1c). We found that the 17O and 1H NMR signals for the μ-hydroxo ligand in 1c are highly shielded ((17O) = -239 ppm and (1H) = -2.43 ppm). Furthermore, we report new 17O NMR data for a 17O-labelled mononuclear metal-dioxygen complex, Pd(PPh3)2(2-17O2) (4b), δiso(17O) = 561 ppm. Lastly, a preliminary investigation of the crystallization and stability of oxymyoglobin (oxyMb) was conducted. Optimal crystallization of oxyMb was observed in a Tris-HCl buffer at pH 6.5 with 75% saturated ammonium sulfate. Optimal storage conditions of oxyMb microcrystals were found in a Tris-HCl buffer at pH 8.5 with 75% saturated ammonium sulfate, where the autooxidation rate of oxyMb microcrystals was determined to be (8.1 ± 0.8) x 10-4 hrs-1 at 253 K, corresponding to t1/2 = 850 hrs. We concluded that oxyMb is a suitable candidate for 17O-labelling via the new H217O-electrolysis route.
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