3,6-Dioxa-1,8-octanedithiol

Thioredoxins are small redox proteins and have the active sites of Cys-Xaa-Yaa-Cys; they are overexpressed by many different cancer cells. Cisplatin and Pt(II) analogues could bind to the active sites and inhibit the activities of the proteins, as demonstrated by other researchers. Platinum(IV) anticancer drugs are often regarded as prodrugs, but their interactions with thioredoxins have not been studied. In this work, 3,6-dioxa-1,8-octanedithiol (dithiol) was chosen as a model compound for the active sites of thioredoxins, and its reactions with cis-[Pt(NH(3))(2)Cl(4)] and trans-[PtCl(2)(CN)(4)](2-) (cisplatin prodrug and a model complex) were studied. The pK(a) values for the dithiol were characterized to be 8.7 ± 0.2 and 9.6 ± 0.2 at 25.0 °C and an ionic strength of 1.0 M. The reaction kinetics was followed by a stopped-flow spectrophotometer over a wide pH range. An overall second-order rate law was established, -d[Pt(IV)]/dt = k'[Pt(IV)][dithiol], where k' stands for the observed second-order rate constants. Values of k' increased several orders of magnitude when the solution pH was increased from 3 to 9. A stoichiometry of Δ[Pt(IV)]/Δ[dithiol] = 1:1 derived for the reduction process and product analysis by mass spectrometry indicated that the dithiol was oxidized to form an intramolecular disulfide, coinciding with the nature of thioredoxin proteins. All of the reaction features are rationalized in terms of a reaction mechanism, involving three parallel rate-determining steps depending on the pH of the reaction medium. Rate constants for the rate-determining steps were evaluated. It can be concluded that Pt(IV) anticancer prodrugs can oxidize the reduced thioredoxins, and the oxidation mechanism is similar to those of the oxidations of biologically important reductants by some reactive oxygen species (ROS) such as hypochlorous acid/hypochlorite and chloramines.

The mechanism of the new Radical Ring-opening Redox Polymerization (R3P) of 3,6-dioxa-1,8-octanedithiol (DODT) by triethylamine (TEA) and dilute H2O2 was investigated. Scouting studies showed that the formation of high molecular weight polymers required a 1:2 molar ratio of DODT to TEA and of DODT to H2O2. Further investigation into the chemical composition of the organic and aqueous phases by 1H-NMR spectroscopy and mass spectrometry demonstrated that DODT is ionized by two TEA molecules (one for each thiol group) and thus transferred into the aqueous phase. The organic phase was found to have cyclic disulfide dimers, trimers and tetramers. Dissolving DODT and TEA in water before the addition of H2O2 yielded a polymer with Mn = 55,000 g/mol, in comparison with Mn = 92,000 g/mol when aqueous H2O2 was added to a DODT/TEA mixture. After polymer removal, MALDI-ToF MS analysis of the residual reaction mixtures showed only cyclic oligomers remaining. Below the LCST for TEA in water, 18.7 °C, the system yielded a stable emulsion, and only cyclic oligomers were found. Below DODT/TEA and H2O2 1:2 molar ratio mostly linear oligomers were formed, with <20% cyclic oligomers. The findings support the proposed mechanism of R3P.

A considerable quantity of an alkylation by-product is observed when using 3,6-dioxa-1,8-octanedithiol as a scavenger during acidic release of peptides containing the thioether amino acid methionine from the solid support. Adjustment of the cleavage conditions by replacement of 3,6-dioxa-1,8-octanedithiol with ethane dithiol or by using methionine sulfoxide as an alternative to methionine resulted in no such impurity. The by-product was detectable by liquid chromatography and mass spectrometry and characterised by NMR spectroscopy of an isolated model peptide. It could be effectively removed in a separate post cleavage step by treatment with dilute aqueous acid at 37 °C.