Simple Thiols Research Articles

ESR Studies on Irradiated Frozen Aqueous Solutions of Sulfur Compounds

Radiation-induced ESR centers in frozen aqueous solutions of sulfur compounds have been studied. Special attention has been focused on secondary radical reactions and the relative importance of direct and indirect effects. Solutions were rapidly frozen at 77°K and irradiated in vacuum at this temperature. All ESR spectra were recorded at 77°K. The irradiated samples were heat treated, either for increasing lengths of time at 110°K, or for the same period of time at successively increasing temperatures. The ESR spectra of aqueous solutions irradiated at 77°K are composed of the ``water resonance'' and a resonance ascribed to radicals in the solute. The latter are found primarily to be induced by the direct action of radiation. This observation implies that H atoms which already have reacted at 77°K have no or very little effect in producing solute radicals. The remaining ESR centers in water, including OH radicals, are released in the temperature range 100° to 130°K. The spectral changes in this temperature range indicate that recombination processes dominate and only a minor fraction of the ``water radicals'' interacts with the solute. Thus, in a 10% solution of glutathione, about 15% of the ``water radicals'' become effective in producing solute radicals. The radicals induced in the solute and the subsequent secondary radical reactions seem to be more or less the same as those found when the solute was irradiated in the solid state. For solutions of some simple thiols like penicillamine the secondary reactions yield sulfur radicals above 170°K, in accordance with the results for solid penicillamine. On the other hand, no sulfur resonance appears for solutions of glutathione in spite of the fact that sulfur radicals are formed in the solid state. The resonance of the solution disappears at a lower temperature than that found necessary for the formation of sulfur raducals in solid glutathione.

Radiation-Induced Free Radicals in Sulfur Compounds. ESR Studies in the Polycrystalline State

The secondary processes occurring in organic sulfur compounds, irradiated with ionizing radiation in the dry state, have been followed by ESR spectroscopy. Samples were irradiated in vacuum at 77°K and the ESR spectra recorded at this temperature before and after stepwise heat treatment at successively increasing temperatures. The resonance spectra seem to indicate that after irradiation at 77°K, several kinds of ESR centers are formed. No or very few sulfur radicals seem to be formed at this temperature. The results show that few secondary processes proceed efficiently in solids in the temperature range from 77° to about 110°K. Depending upon the compound, decay processes seem to dominate in the temperature range varying from about 110°K and up to about 250°K. Upon further heat treatment, secondary reactions take place which lead to the formation of sulfur radicals. The threshold temperature for these reactions is about 170°K for simple thiols and disulfides, 240°K for peptides like glutathione, and about 280°K for proteins. The formation of sulfur radicals is found to coincide with an increase in the total number of observable ESR centers. Thus, in the case of penicillamine, the number of unpaired spins observed after annealing to 295°K exceeds the initial value at 77°K by about 70%. Evidence is presented that diffusion-controlled intermolecular reactions take place in irradiated solids, and it is suggested that the observed formation of sulfur radicals may in part be explained by such a mechanism. The resonance pattern due to polyoriented radicals of the type R—CH2–S· (e.g., cysteine, cysteamine, glutathione, proteins) was found to exhibit a temperature dependent hyperfine splitting which is not found for radicals of the type R—CX2—S· (e.g., penicillamine). This observation supports the model for organic sulfur radicals proposed by Kurita and Gordy.

The reaction of thiol and disulphide groups with mercuric chloride and methylmercuric iodide. I. Simple thiols and soluble proteins

The polarographic behaviour of MeHgCl, MeHgI, HgCl2, neohydrin (3-chloromercuri-2-methoxypropylurea) and its iodo-derivative are described. Their chemical reactivity and -SH specificity have been investigated and MeHgI shown to have advantages as an -SH reagent on account of its high reactivity, stability, and " ideal " polarographic behaviour. The reaction of protein -SS- groups with Na2SO3 proceeds slowly to completion in the presence of HgCl2 or MeHgI and it is shown that with bovine plasma albumin, ribonuclease, and insulin the polypeptide chains may be separated for preparative or structural studies under milder conditions than are customary. In the presence of urea at pH 9 the reaction is sufficiently rapid to form the basis of amperometric titration procedures for determining -SS- groups in eight intact proteins. The methods are especially valuable for proteins which have been previously converted to their -SR, -SO3-, or -SSO3-; derivatives since the destructive hydrolytic step may be avoided. The mechanism and stoichiometry of the reactions are discussed.

The Sulfur Chemistry of Proteins

Publisher Summary This chapter discusses the chemical aspects of proteins. The problems involved in studying the behavior of any of the functional groups in proteins are great. The minimum requirements appear to to be a full understanding of methods of analysis and of the properties of the groups in simple molecules. The chapter deals with the methods of estimation of thiols, disulfides, and total sulfur. It explores the capabilities and the limitations of the different methods, as well as the reactions of simple thiols, disulfides, and thioethers. The chapter also discusses the sulfur chemistry of a number of proteins. It is concerned with the behavior of SH and SS groups as little work has yet been done on the chemistry of methionine in proteins. The differences in reactivity of SH and SS groups, which occur are discussed and some tentative conclusions are drawn as to the possible functions of these groups. The symbols “SH” and “SS” have been used, when referring to the groups themselves. The words “thiol” and “disulfide” refer to the compounds containing SH or SS groups.