Catalytic and direct oxidation of cysteine by octacyanomolybdate(V)

The oxidation of cysteine by [Mo(CN)(8)](3-) in deoxygenated aqueous solution at a moderate pH, is strongly catalyzed by Cu2+, to the degree that impurity levels of Cu2+ are sufficient to dominate the reaction. Dipicolinic acid (dipic) is a very effective inhibitor of this catalysis, such that with 1 mM dipic, the direct oxidation can be studied. UV-vis spectra and electrochemistry show that [Mo(CN)(8)](4-) is the Mo-containing product. Cystine and cysteinesulfinate are the predominant cysteine oxidation products, The stoichiometric ratio (Delta n(Mo(V))/Delta n(cysteine)) of 1.4 at pH 10.8 is consistent with this product distribution. At pH 1.5, the reaction is quite slow and yields intractable kinetics. At pH 4.5, the rates are much faster and deviate only slightly from pseudo-first-order behavior. With 2 mM PBN (N-phenyl-tert-butyl nitrone) present at pH 4.5, the reaction rate is about 20% less and shows excellent pseudo-first-order behavior, but the stoichiometric ratio is not significantly changed. The rates also display a significant specific cation effect. In the presence of spin-trap PBN, the kinetics were studied over the pH range 3.48-12.28, with [Na+] maintained at 0.09-0.10 M. The rate law is -d[Mo(V)]/dt = k[cysteine](tot)[Mo(V)], with k = {2(k(b)K(a1)K(a2)[H+] + k(c)K(a1)K(a2)K(a3))}/([H+](3) + K-a1[H+](2) + Ka1Ka2[H+] + Ka1Ka2Ka3), where K-a1, K-a2, and K-a3 are the successive acid dissociation constants of HSCH2CH(NH3+)CO2H. Least-squares fitting yields k(b) = (7.1 +/- 0.4) x 10(4) M-1 s(-1) and k(c) = (2.3 +/- 0.2) x 10(4) M-1 s(-1) at mu = 0.1 M (NaCF3SO3) and 25 degrees C. A mechanism is inferred in which k(b) and k(c) correspond to electron transfer to Mo(V) from the thiolate forms of anionic and dianionic cysteine.