CYSTEINE is spontaneously oxidized to cystine at neutral pH1,2; at acid pH. this spontaneous reaction does not take place. Under the influence of ionizing radiation, cysteine is oxidized, even at acid pH.3–7. In the presence of oxygen this radiation conversion of cysteine to cystine is very fast, and a G value of 74 has been reported8, although the exact rate of conversion depends on the cysteine concentration and oxygen tension9. It has also been reported that, in the presence of hydrogen, radiation could induce the reduction of cystine to cysteine9. Markakis and Tappel10 analysed a number of products after irradiating cystine in oxygen-saturated and nitrogen-saturated solutions, and in both cases cysteine was one of the products. Yalow11 also reported the presence of cysteine after deuteron irradiation of dry cystine. However, Cavallini et al.12 invariably found that no SH was produced after irradiating a mixture of disulphides, including cystine.
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DEWEY, D., BEECHER, J. Interconversion of Cystine and Cysteine induced by X-rays. Nature 206, 1369–1370 (1965). https://doi.org/10.1038/2061369a0
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