Selenide intermediates determination

Electrochemical methods can be applied to the determination of the composition of solid phases as well as mixtures of solids [224-228], The first situation is illustrated in Fig. 4.1, where cathodic voltammograms of CuS, CuSe, and a solid phase of composition CuSeoASo.e reported by Meyer et al. [227] are shown. This last can be described as a solid solution formally regarded as a copper sulfide, in which 40% of sulfide ions have been replaced by selenide ions. The new phase produces a voltammetric peak at a potential intermediate between those for CuS and CuSe. 

The effects of aryl groups were kinetically analyzed by comparing the rate constants of both steps (ki for oxidation step and /C2 for elimination step) which were determined by NMR analysis of the concentration of vinyl selenides, the intermediate selenoxide, and allenic sulfones [16b]. This kinetic study indicates that the rates of both oxidation and elimination steps were accelerated by the introduction of an electron-withdrawing group. Such acceleration has been known in the overall selenoxide elimination as well as in the selenoxide elimination step of alkyl aryl selenides. As a result, it was disclosed that the ratio of these rate constants (/C1//C2) was closely related to the enantiomeric excess of the products the smaller the ratio, the larger the enantiomeric excess becomes. Thus, the introduction of o-nitrophenyl group as an aryl moiety, which suppresses sterically the racemization of the intermediate chiral selenoxide and accelerates the selenoxide elimination step, is necessary to achieve a higher asymmetric induction. 

The metabolism of selenium is now fairly well understood. To become incorporated into selenium-specific proteins (e.g., glutathione peroxidase, thioredoxin reductase, iodothyronine 5 -deiodinase) through a cotranslational mechanism requires that selenium be in the form of selenide (Sunde 1990). All forms of selenium can be transformed to selenide, although the rates of transformation vary. For example, selenate is not converted to selenide as readily as selenite. The formation of selenide from selenocysteine requires a specific enzyme, selenocysteine (3-lyase, which catalyzes the decomposition of selenocysteine to alanine and hydrogen selenide. Excess selenium is methylated and exhaled or excreted in the urine in both humans and animals. Further research is required to determine which selenium metabolites or intermediates lead to toxicity. 

Most of the methods for the determination of selenium in human materials require some sample preparation or pretreatment. The biotransformation of selenium in man, which is characterized by a step-wise biochemical reduction, leading to the binding to or direct incorporation of the element into proteins, apparently involves the formation of intermediate volatile species. Dimethyl selenide as well as many other organic forms of selenium and its halides are relatively volatile. 

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