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Hydrogen oxidation by Azospirillum brasilense Sp7 was studied in N(,2)-fixing and NH(,4)('+)-grown batch cultures. The K(,m) for H(,2) of O(,2)-dependent H('3)H oxidation in whole cells was 9 uM. The rates of H('3)H and H(,2) oxidation were very similar, indicating that the initial H(,2) activation step in the overall H(,2) oxidation reaction was not rate-limiting and that H('3)H oxidation was a valid measure of H(,2)-oxidation activity. Hydrogen-oxidation activity was inhibited irreversibly by air. In N-free cultures the O(,2) optima for O(,2)-dependent H(,2) oxidation, ranging from 0.5-1.25% O(,2) depending on the phase of growth, were significantly higher than those of C(,2)H(,2) reduction, 0.15-0.35%, suggesting that the H(,2)-oxidation system may have a limited ability to aid in the protection of nitrogenase against inactivation by O(,2). Oxygen-dependent H(,2) oxidation was inhibited by NO(,2)('-), NO, CO, and C(,2)H(,2) with apparent K(,i) values of 20, 0.4, 28, and 88 uM, respectively. These inhibitors also affected methylene blue-dependent H(,2) oxidation, presumably by acting on the hydrogenase directly. The CO inhibition was easily reversible; the NO(,2)('-) and NO inhibitions were irreversible; and the C(,2)H(,2) inhibition was not readily reversible. Hydrogen-oxidation activity was 50 to 100 times higher in denitrifying cultures when the terminal electron acceptor for growth was N(,2)O rather than NO(,3)('-), possibly due to the irreversible inhibition of hydrogenase by NO(,2)('-) and NO in NO(,3)('-)-grown cultures. THe expression of the H(,2)-oxidation system was independent of nitrogenase expression, did not require added H(,2) (and probably not endogenous H(,2)), was not affected by low concentrations of carbon substrates (less than 30 mM malate), and required low O(,2) concentrations (microaerobic or anaerobic conditions).

The role of redox reactions in the formation of aroma volatiles, colour and other non-volatiles in the Maillard Reaction was investigated. The electrochemical properties of individual reactants and Maillard model mixtures were monitored via ORP (oxidation-reduction potential) and oxygen electrodes. All models exhibited unique electrochemical activities represented by their corresponding ORP profiles. Investigation into the redox potentials of several model systems demonstrated that the increased negative value of a redox potential is not necessarily correlated with its browning potential. An optimal redox potential range, where browning is favoured, was found to represent a balance between carbonyl and hydroxyl moieties in the structure. Adjustment of this redox potential by introducing reducing or oxidizing species can shift this balance resulting in modifications in browning capacities. However, it was concluded that there is a clear relationship between browning ability and reducing capacity of the model systems. Furthermore, a novel oxidative pyrolysis technique was developed to study the role of oxidative environment on the product distribution during pyrolysis and to investigate the mechanism of formation of non-volatiles through 13C and 15N-label incorporation. Application of this technique to glucoselglycine model system have indicated that most non-volatile Maillard reaction products can arise from glucose oxidation intermediates such as glucosone, gluconic acid and deoxyglucosones. To study the specific role of redox reactions in the formation of non-volatiles, a post-pyrolytic derivatization technique was developed and optimized. Several non-volatile end products were identified and mechanistically confirmed to involve oxidation and reduction reactions for their formation, such as lactones, hydroxylated benzenes and hydroxylated pyrazine. The latter was identified and confirmed to be generated via the dimerization of glycine and subsequent oxidation. In addition, the formation of different volatiles such as pyrazole, imidazole and oxazole was mechanistically confirmed to depend on redox reactions.

Amorphous alloys have a range of desirable ferromagnetic, electrical, mechanical and chemical properties. For instance, the application of Fe-based soft ferromagnetic amorphous alloys as transformer core materials can cut the transformer core losses to about 1/4, with considerable energy saving. However, during manufacture, heat treatment and in service, amorphous alloys may need to be exposed to moderately high temperatures for a period of time, with possible degradation caused by oxidation and crystallisation. There has been almost no previous study of oxidation behaviour and the relationship between oxidation and crystallisation in amorphous alloys. Eight important amorphous alloys and an industrial crystalline silicon steel have been studied in the present work; amorphous Fe78Si9Bl3, Fe40Ni40B20, Fe40Ni40P14B6, Co58NilOFe5SillB16, Fe32Ni36Crl4P12B6, Co66Fe4NilSil5B14, Co76Fe2Mn4Si6B12 and Ni78Si8B14, and crystalline Fe94Si6. A combination of thermogravimetry, optical and electron microscopy, electron probe microanalysis, X-ray diffractometry and differential scanning calorimetry has been used to investigate the oxidation and crystallisation kinetics, oxide structure and composition, oxidation and crystallisation mechanisms and the effect of crystallisation on the oxidation behaviour. The results show that the oxidation resistance at 350 C in air increases in the order Fe40Ni40- P14B6 < Fe94Si6 < Co66Fe4NilSil5B14 < Co58NilOFe5SillB16 < Co76MnFe2- Si6B12 < Fe40Ni40B20 < Si78Si9B13 < Ni78Si8B14 < Fe32Ni36Crl4P12B6. Most of the amorphous alloys obey a parabolic oxidation rate law, but the oxidation kinetics, oxide growth mechanism and resulting oxide structure change sharply when crystallisation takes place in the amorphous alloys. Amorphous Fe78Si9B13 and Fe40Ni40B20 have better oxidation resistance than the corresponding crystalline alloys, while amorphous Fe40Ni40P14B6 and Co58NilOFe5Sil1B16 have poorer oxidation resistance than the crystalline counterparts. Most of the crystalline alloys also obey a parabolic oxidation rate law, except for the crystalline Co based alloys and Fe40Ni40B20, which obey a logarithmic rate law. In most cases, the amorphous and crystalline alloys oxidise to form a fine-scale multiphase oxide scale, except for amorphous Fe40Ni40P14B6, which oxidises to form a whisker-like thick layer of Fe203- In general, ion diffusion through fast transport paths such as grain boundaries and dislocations is the rate controlling process for oxide growth. Different oxidation kinetics and oxide growth mechanisms in amorphous and crystalline alloys of the same composition are caused by micro-chemical segregation of the alloying elements during crystallisation.

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