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The reaction between cystine (CySSCy) and hydrogen sulfide ion (HS−) in alkaline solutions has been studied by amino acid analysis and uv–visible spectroscopy. The reaction occurs in two reversible steps to form cysteine (CySH), S-thiocysteine (CySS−), and disulfide (S22−), as represented by the reactions [Formula: see text] and [Formula: see text]. The equilibrium and rate constants were pH dependent due to the presence of various charge types of reactants and products. The rate constants at 25 °C, pH 10.0, and μ = 0.17 M were determined to be: k1 = 3.7 ± 0.4 M−1 min−1, k−1 = 5.5 ± 0.6 M−1 min−1, k2 = 6.1 ± 0.5 M−1 min−1, and k−2 = 122 ± 20 M−1 min−1. When the rate constant k1 is expressed as k1 = A exp (−Ea/RT), values of A = (4.7 ± 0.3) × 1011 M−1 min−1 and Ea = 15.8 ± 0.9 kcal mol−1 were obtained. The ionic strength and pH dependence of k1 were also studied.

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respect to the construction of both ion-sensing interfaces and electrosynthetic cells. Experimental SectionMaterials. Nickel electrodes consisted of Ni wire of 0.63-mm diameter (Alfa). Nickel plate electrodes used for IR studies were 0.125-mm thick (Alfa) having an area of 1 cm2. Inco 255 Ni powder was used for UV-vis studies. Reagent grade chemicals were used as derivatizing agents and supporting electrolytes. Na3Fe(CN),H20 was made by dissolving Na3Fe(CN),NH3 in distilled water at pH [Fe(CN)SHis]2-solution was made by stirring a mixture of 50 mL of 0.2 M Na3Fe(CN),H20, 50 mL of 6% H202, and 2.3 g of L-histidine in the dark for 45 min at about 5 OC. A catalytic amount of Mn02 was added at the end of the reaction to decompose excess H202.34 Electrochemistry. A single-compartment cell with a large Pt-mesh counterelectrode and SCE as reference was used. Measurements were made with a PAR Model 174A potentiostat and a PAR Model 175 programmer. Electrodes were abraded with 150 metalite cloth and washed with distilled water before derivatization. After derivatization, they were wiped with a Kimwipe and washed with distilled water to remove loosely adhering substances. Data were recorded on a Houston (35) Although unstable, in our hands the native nickel surface appears to give an oxidation peak at -+0.65 V vs. SCE for the oxidation of ascorbic acid. This is the same potential at which we observe a peak in the ascorbic acid oxidation wave using a platinum electrode.Ideal electrodes were made by potentiostating the Ni-wire electrodes at 1 .O V vs. SCE for 50 s in an electrolyte containing 0.1 M NaNO, and 0.005 M [Fe(CN)6]3-. By appropriate increase of potentiostating voltage, time, and concentration of [ Fe(CN)6]3-, larger coverage electrodes were obtained as desired.Spectroscopy. Diffuse-reflectance FT IR spectroscopy was carried out on a Digilab FTS.20C spectrometer with a diffuse-reflectance attachment. A derivatized Ni-plate electrode was used as the sample while a nonderivatized abraded Ni plate served as the reference. The model compounds were used as powders and referenced against KBr. UV-vis spectra were recorded on a Hewrett-Packard 8540A UV-vis spectrometer.Acknowledgement is made to t h e Dreyfus Foundation for partial support of this work through a newly appointed Young Faculty Grant. Professor Kalman Burger is thanked for informative discussions that helped fuel this work a t its early stages. The characterization and photochemistry of [Si02]-LRu(CO)4 and [Si02]-L3Ru3(CO)9 are reported, where [Si02]-represents high surface area (-400 m2/g) SiO,. Synthesis of [Si02]-LRu(C0)4 and [S~O,]-L,RU,(CO)~ is effected by reaction of Ru(CO)~(PP~~CH~CH,S~(OE~)~) or Ru~(CO)~(PP~~CH~CH~S~(OE~),), with a hydrocarbon suspension of [Si02]-. Solid-state I3C, 29Si, and ,'P CP/MAS NMR, FTIR, UV-vis photoacoustic spectroscopy, and elemental analyses establish the nature of the functionalized [Si02]-. Typical coverage of -LRu(CO), or -L , R U~( C O )~ is -10-'o-lO-'l mol/cm2.