Sulphur labelled isotopes

Dithiazoles. - Stable 1,3,2-dithiazolyl (404) and 1,3,2-dithiazolidinyl radicals (405), potentially useful spin probes and labels, result from reaction of SJ N2 with acetylenes435 436 and strained alkenes, respectively. Isotopic double labelling with 15N and 33S demonstrated the equivalence of the ring sulphur atoms. E.S.R. spectra have been analysed and M.0. calculations performed. 

The exchange of labelled sulphur can be promoted by enzyme catalysts, instead of heating. Bird egg yolk and the cysteine desulphydrase that it contains catalyse the exchange of sulphur-35 from Na2 S to L-cysteine, L-cystine and L-cysteic acid. In a typical experiment, 150 ml of a buffer solution containing 2 millimoles of cysteine-HCl, 2 millimoles of Naa S and 500 mg of cysteine desulphydrase preparation is incubated at 38°C for 15 h. A mixture of 74-4% cystine- S and 25-3% cysteine- S is obtained. L-Cystine- S is subsequently reduced electrolytically to cysteine- S. The total yield of L-cysteine- S obtained by isotope exchange is 70%. 

Although isotope exchange by virtue of its simplicity and ability to form compounds of high specific activity is the method of choice for the labelling of tautomeric thiols, a synthetic method is often better suited to other thiols. For example, heating a-toluenethiol with sulphur- S in benzene at 135-140°C for 6-12 h, yields ct-toluenethiol- S with a specific activity of only 2-9%. However, the synthesis of the compound from benzyl-magnesium chloride and sulphur- S yields a-toluenethiol- S... 

The stable isotopes of carbon, nitrogen, oxygen, and sulphur are available commercially in a variety of chemical forms. In this section, an outline is given of the various methods that are currently used for the separation of stable isotopes. It will be clear that the primary source of isotope, and therefore, in most cases the cheapest available form, is determined by the separation procedure adopted. A discussion of some aspects of the synthesis of labelled compounds follows. Although in a review of this size, this cannot be comprehensive, the methods used and the problems that are raised will be outlined. Some recent developments that increase the choice of chemical form available will be mentioned. 

By means of isotopes it is possible to mark a compound at one or more positions in the molecule and to follow the fate of these labelled atoms. With the introduction of isotopes of the most conunon elements in the biosphere (hydrogen, carbon, nitrogen, oxygen, phosphorous, sulphur), the method has proved most fruitful. 

In a double-labelled experiment it was shown that when cystine was fed to P. chrysogenum the isotope ratio of the starting material was retained unchanged in the isolated benzylpenicillin. This demonstrated that cystine (rather than penicillamine) was the source of the thiazolidine sulphur atom. Further evidence on the origin of the penicillin sulphur atom was provided by Stevens etal, 212) who showed that the label from p S]-cysteine was better incorporated into benzylpenicillin than label from P S]-sulphate. In competition experiments it was shown that L-cysteine or L-cystine were utilised in preference to p S]-sulphate, whereas D-cysteine and DL-penicillamine had no effect on the incorporation of label from sulphate into benzylpenicillin. 

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