Reactive sulfur species: kinetics and mechanism of the reaction of hypothiocyanous acid with cyanide to give dicyanosulfide in aqueous solution.

The chief sources of cyanide (CN(-)) in humans are tobacco and occupationally derived smoke, inflammation [vis-a-vis myeloperoxidase (MPO)-induced chlorination of glycine], and microbial cyanogenesis (including Pseudomonas aeruginosa infection of the cystic fibrosis lung). The human mucosae of healthy individuals are usually protected from infection by innate defense mechanisms that include the defensive peroxidase systems. In the oral cavity, salivary peroxidase and MPO catalyze the oxidation of the pseudohalide thiocyanate (SCN(-)) by hydrogen peroxide to produce the antimicrobial hypothiocyanite (OSCN(-)). Lactoperoxidase carries out the same reaction in the human lung (as does MPO during inflammatory response). In the present study, we show that OSCN(-) and CN(-) react with pH-dependent kinetics to produce SCN(-) and cyanate (OCN(-)) via dicyanosulfide (NCSCN), with the maximum rate occurring near neutral, physiological pH. In addition to presenting a detailed chemical mechanism, we discuss unresolved issues, including the possible biological relevance of the NCSCN intermediate.

Reactive sulfur species: kinetics and mechanism of the equilibrium between cysteine sulfenyl thiocyanate and cysteine thiosulfinate ester in acidic aqueous solution.

The kinetics and mechanism of the hydrolysis of cysteine sulfenyl thiocyanate (CySSCN) to give cysteine thiosulfinate ester (CyS(=O)SCy) have been investigated between pH 0 and 4. The reaction is reversible. The hydrolysis of CySSCN is second-order in [CySSCN] and inverse first-order in [H+] and [SCN-]. The following mechanism is proposed for the hydrolysis of CySSCN (where the charge depends upon the pH): CySSCN0/+ + H2O <==>CySOH0/+ + SCN- + H+, CySOH0/+ + CySSCN0/+ --> CyS(=O)SCy0/+/2+ + SCN- + H+; k1 = 3.36 +/- 0.01 x 10-3 s-1, K1k2 = 0.13 +/- 0.05 Ms-1 (which yields k2/k-1 = 39 M). The observed rate law rules out alternative mechanisms for 1 0.4 M). The following mechanism is proposed: CyS(=O)SCy2+ + H+ <==> CyS(OH)=SCy3+, Ka; CyS(OH)SCy3+ + SCN- --> CySOH+ + CySSCN+, k-2 = 0.239 +/- 0.007 M-2s-1/Ka M-1. Since cysteine sulfenic acids are known to play an important function in many enzymes, and SCN- exists in abundance in physiologic fluids, we discuss the possible role of sulfenyl thiocyanates in vivo.

Reactive sulfur species: hydrolysis of hypothiocyanite to give thiocarbamate-S-oxide.

Hypothiocyanite (OSCN-) hydrolyzes under alkaline conditions to give thiocarbamate-S-oxide (H2NC(=O)SO-, the conjugate base of carbamothioperoxoic acid) via a mechanism that involves rate-limiting nucleophilic attack of OH- on OSCN-, followed by fast protonation (with no net consumption of H+/OH- at pH 11.7). Thiocarbamate-S-oxide has been characterized by 13C NMR, 15N NMR, UV spectroscopy, and ion chromatography. It has also been independently synthesized by the reaction of thiocarbamate (H2NC(=O)S-) and hypochlorite (OCl-). The properties of thiocarbamate-S-oxide that is produced by hydrolysis of OSCN- and by oxidation of H2NC(=O)S- are the same. The possible relevance of thiocarbamate-S-oxide in human peroxidase defense mechanisms remains to be explored.

Reactive sulfur species: kinetics and mechanisms of the reaction of cysteine thiosulfinate ester with cysteine to give cysteine sulfenic acid.

The kinetics and mechanisms of the reaction of cysteine with cysteine thiosulfinate ester in aqueous solution have been studied by stopped-flow spectrophotometry between pH 6 and 14. Two reaction pathways were observed for pH > 12: (1) an essentially pH-independent nucleophilic attack of cysteinate on cysteine thiosulfinate ester, and (2) a pH-dependent fast equilibrium protonation of cysteine sulfenate that is followed by rate-limiting comproportionation of cysteine sulfenic acid with cysteinate to give cystine. For 6 < pH < 12, the rate-determining reaction between cysteinate and cysteine thiosulfinate ester becomes pH-dependent due to the protonation of their amine groups. Hydrolysis of cysteine thiosulfinate ester does not play a role in the aforementioned mechanisms because the rate-determining nucleophilic attack by hydroxide is relatively slow.

Kinetics and mechanism of the comproportionation of hypothiocyanous acid and thiocyanate to give thiocyanogen in acidic aqueous solution.

The kinetics of comproportionation of hypothiocyanous acid (HOSCN) and thiocyanate (SCN-) to give thiocyanogen ((SCN)2) in acidic aqueous solutions have been determined by double-mixing stopped-flow UV spectroscopy. Hypothiocyanite (OSCN-) was generated at pH 13 by oxidation of excess SCN- with hypobromite (OBr-), followed by a pH jump to acidic conditions ([H+] = 0.20-0.46 M). The observed pseudo-first-order rate constants exhibit first-order dependencies on [H+] and [SCN-] with overall third-order kinetics. The corresponding kinetics of hydrolysis of (SCN)2 have also been examined. Under conditions of high (and constant) [H+] and [SCN-], the kinetics exhibit second-order behavior with respect to [(SCN)2] and complex inverse dependences on [H+] and [SCN-]. Under conditions of low [H+] and [SCN-], the kinetics exhibit first-order behavior with respect to [(SCN)2] and independence with respect to [H+] and [SCN-]. We attribute this behavior to a shift in the rate-limiting step from disproportionation of HOSCN (second-order dependency on [(SCN)2]) to rate-limiting hydrolysis (first-order dependency on [(SCN)2]). Thus, we have determined the following equilibrium constant by the kinetic method: (SCN)2 + H2O HOSCN + SCN- + H+; Khyd = [HOSCN][SCN-][H+]/[(SCN)2] = khyd/kcomp = 19.8(+/-0.7) s-1/ 5.14(+/-0.07) x 103 M-2 s-1 = 3.9 x 10-3 M2.

Ligand effect on the kinetics of hydroperoxochromium(III)-oxochromium(V) transformation and the lifetime of chromium(V).

A macrocyclic superoxochromium complex L(2)(H(2)O)CrOO(2+)(L(2)=meso-Me(6)-[14]aneN(4)) is generated from L(2)Cr(H(2)O)(2)(2+) and O(2) with k(on)=(2.80 +/- 0.07)x 10(7) M(-1) s(-1). One-electron reduction of L(2)(H(2)O)CrOO(2+) produces a transient hydroperoxo complex that readily undergoes intramolecular conversion to L(2)Cr(v), k(1)= 1.00 +/- 0.01 s(-1) in acidic aqueous solutions, and 0.273 +/- 0.010 s(-1) at pH >7, with an apparent pK(a) of 5.9. The decay of L(2)Cr(v) in the pH range 1.3-6.2 obeys the rate law, -d[L(2)Cr(v)]/dt= (0.0080 (+/- 0.0049)+ 8.19 (+/- 0.13)[H(+)])[L(2)Cr(v)]. Both the kinetics of formation and lifetime of L(2)Cr(v) are significantly different from those for the closely related [14]aneN(4) complex. The X-ray structure of the parent Cr(iii) complex, [L(2)Cr(H(2)O)(2)](ClO(4))(3).4H(2)O, shows that the macrocyclic ligand adopts the most stable, "two up-two down" configuration around the nitrogens.

Mesoporous silica-supported uranyl: synthesis and photoreactivity.

A mesoporous silica-supported uranyl material (U(aq)O(2)(2+)-silica) was prepared by a co-condensation method. Our approach involves an I(-)M(+)S(-) scheme, where the electrostatic interaction between the anionic inorganic precursor (I(-)), surfactant (S(-)), and cationic mediator (M(+)) provides the basis for the stability of the composite material. The synthesis was carried out under acidic conditions, where the anionic sodium dodecyl sulfate provided the template for the uranyl cation and silicate to condense. Excitation with visible or near-UV light of aqueous suspensions of U(aq)O(2)(2+)-silica generates an excited state that decays with k(0) = 1.5 x 10(4) s(-1). The reaction of the excited state with aliphatic alcohols exhibits kinetic saturation and concentration-dependent kinetic isotope effects. For 2-propanol, the value of k(C)3(H)7(OH)/k(C)()3(D)7(OH) decreases from 2.0 at low alcohol concentrations to 1.0 in the saturation regime at high alcohol concentrations. Taken together, the data describe a kinetic system controlled by chemical reaction at one extreme and diffusion at the other. At low [alcohol], the second-order rate constants for the reaction of silica-U(aq)O(2)(2+) with methanol, 2-propanol, 2-butanol, and 2-pentanol are comparable to the rate constants obtained for these alcohols in homogeneous aqueous solutions containing H(3)PO(4). Under slow steady-state photolysis in O(2)-saturated suspensions, U(aq)O(2)(2+)-silica acts as a photocatalyst for the oxidation of alcohols with O(2).

Kinetics of acid-catalyzed O-atom transfer from a hydroperoxorhodium complex to organic and inorganic substrates.

Oxygen atom transfer from (NH(3))(4)(H(2)O)RhOOH(2+) to organic and inorganic nucleophiles takes place according to the rate law -d[(NH(3))(4)(H(2)O)RhOOH(2+)]/dt = k[H(+)] [(NH(3))(4)(H(2)O)RhOOH(2+)][nucleophile] for all the cases examined. The third-order rate constants were determined in aqueous solutions at 25 degrees C for (CH(2))(5)S (k = 430 M(-)(2) s(-)(1), micro = 0.10 M), (CH(2))(4)S(2) (182, micro = 0.10 M), CH(3)CH(2)SH (8.0, micro = 0.20 M), (en)(2)Co(SCH(2)CH(2)NH(2))(2+) (711, micro = 0.20 M), and, in acetonitrile-water, CH(3)SPh (130, 10% AN, micro = 0.20 M), PPh(3) (3.74 x 10(3), 50% AN), and (2-C(3)H(7))(2)S (45, 50% AN, micro = 0.20 M). Oxidation of PPh(3) by (NH(3))(4)(H(2)O)Rh(18)O(18)OH(2+) produced (18)OPPh(3). The reaction with a series of p-substituted triphenylphosphines yielded a linear Hammett relationship with rho = -0.53. Nitrous acid (k = 891 M(-)(2) s(-)(1)) is less reactive than the more nucleophilic nitrite ion (k = 1.54 x 10(4) M(-)(2) s(-)(1)).

Reduction and oxidation of hydroperoxo rhodium(III) complexes by halides and hypobromous acid.

Oxygen atom transfer from trans-L(H(2)O)RhOOH(2+) [L = [14]aneN(4) (L(1)), meso-Me(6)[14]aneN(4) (L(2)), and (NH(3))(4)] to iodide takes place according to the rate law -d[L(H(2)O)RhOOH(2+)]/dt = k(I)[L(H(2)O)RhOOH(2+)][I(-)][H(+)]. At 0.10 M ionic strength and 25 degrees C, the rate constant k(I)/M(-)(2) s(-)(1) has values of 8.8 x 10(3) [L = (NH(3))(4)], 536 (L(1)), and 530 (L(2)). The final products are LRh(H(2)O)(2)(3+) and I(2)/I(3)(-). The (NH(3))(4)(H(2)O)RhOOH(2+)/Br(-) reaction also exhibits mixed third-order kinetics with k(Br) approximately 1.8 M(-)(2) s(-)(1) at high concentrations of acid (close to 1 M) and bromide (close to 0.1 M) and an ionic strength of 1.0 M. Under these conditions, Br(2)/Br(3)(-) is produced in stoichiometric amounts. As the concentrations of acid and bromide decrease, the reaction begins to generate O(2) at the expense of Br(2), until the limit at which [H(+)] 2(NH(3))(4)(H(2)O)RhOH(2+) + O(2); i.e., the reaction has turned into the bromide-catalyzed disproportionation of coordinated hydroperoxide. In the proposed mechanism, the hydrolysis of the initially formed Br(2) produces HOBr, the active oxidant for the second equivalent of (NH(3))(4)(H(2)O)RhOOH(2+). The rate constant k(HOBr) for the HOBr/(NH(3))(4)(H(2)O)RhOOH(2+) reaction is 2.9 x 10(8) M(-)(1) s(-)(1).

1H NMR studies of ligand and H/D exchange reactions of cis- and trans-([14]aneN4)(H2O)RhH2+ in aqueous solutions.

Substitution and exchange reactions of cis- and trans-L(1)(H(2)O)RhH(2+) (L(1) = 1,4,8,11-tetraazacyclotetradecane = [14]aneN(4)) were studied in aqueous solutions by UV-vis and (1)H NMR spectroscopies. At pH 1 and 25 degrees C, the substitution of SCN(-) for the coordinated molecule of water is rapid and thermodynamically favorable. Spectrophotometric determinations yielded the equilibrium constants K = 1.49 x 10(3) M(-1) (cis) and 1.44 x 10(3) (trans). (1)H NMR studies in D(2)O revealed a rapid dynamic process, interpreted as the exchange between coordinated water and X(-) (X = Cl, Br, or I). On the other hand, no line broadening was observed for the strongly bound ligands CN(-) and SCN(-). The complex trans-L(1)(D(2)O)RhH(2+) undergoes a base-catalyzed H/D exchange of the hydride in D(2)O with a rate constant of (1.45 +/- 0.02) x 10(3) M(-1) s(-1). The exchange in the cis isomer is very slow under similar conditions. The complex cis-[L(1)ClRhH](ClO(4)) crystallizes in the centrosymmetric Ponemacr; space group, unit cell dimensions a = 8.9805(11) A, b = 9.1598(11) A, c = 10.4081(13) A, alpha = 81.091(2) degrees, beta = 81.978(2) degrees, gamma = 88.850(2) degrees. The rhodium atom resides in a slightly distorted octahedral environment consisting of the four N atoms of the cyclam, a stereochemically active hydrogen, and a chlorine atom.