Reactive sulfur species: Kinetics and mechanism of the oxidation of cystine by hypochlorous acid to give N,N'-dichlorocystine

Cystine and HOCl (a neutrophil-derived oxidant) react to form an intermediate that has a half-life of ca. 5 min at pH 7.5. The intermediate subsequently decomposes to eventually yield a mixture of cystine, higher oxides of Cys, and other uncharacterized species. Spectral titrations, transitory H-1 NMR and UV-vis spectra, and the reaction properties of the intermediate are consistent with a formulation of N,N'-dichlorocystine {NDC = [-SCH2CH(NHCl)(CO2H)](2)}. The reaction of equimolar amounts of HOCl with cystine at pH 11.3 does not yield N-chlorocystine [NCC = (-O2C)(H3N+)CHCH2SSCH2CH(NHCl)(CO2H)] but rather a 1:1 mixture of NDC and cystine. This result could be explained by two mechanisms: rapid disproportionation of NCC to produce NDC and cystine or a faster reaction of the second equivalent of HOCl with NCC than the first equivalent of HOCl reacts with cystine. The latter mechanism is favored because of our observation by NMR spectroscopy that NDC decomposes via a species that we have assigned as NCC. Thus, disproportionation of NCC is apparently a relatively slow process. The rates of reaction of cystine(0) = [-SCH2CH(NH3+)(CO2-)](2)degrees, cystine(1-) [(-O2C)(H2N)CHCH2SSCH2CH(NH3+)(CO2-)](-), and cystine(2-) = [-SCH2CH(NH2)(CO2-)](2)(2-) have been investigated, and it is clear that cystine(0) is unreactive, whereas cystine(2-) is about four times more reactive than cystine(1-). Accordingly, the following mechanism is proposed (constants for 5 degrees C): HOCl = H+ + OCl-, pK(1) = 7.47; cystine(0) = cystine(1-) + H+, pK(2) = 8.15; cystine(1-) = cystine(2-) + H+, pK(3) = 9.00; cystine(1-) + HOCl -> NCC1- + H2O, k(4) = 4.3(2) x 106 M-1 s(-1); cystine(2-) + HOCl -> NCC2- + H2O, k(5) = 1.6(2) x 107 M-1 s(-1); NCC1- -> NCC2- + H+, k(6) = fast; NCC2- + HOCl -> NDC2- + H2O, k(7) = fast. At physiologic pH, the k(4) pathway dominates. The generation of long-lived chloramine derivatives of cystine may have physiological consequences, since such compounds are known to react with nucleophiles via mechanisms that are also characteristic of HOCl, electrophilic transfer Cl+.