The Reactions of Nitrosyl Complexes with Cysteine

Abstract

The reaction kinetics of a set of ruthenium nitrosyl complexes, {(X)5MNO}n, containing different coligands X (polypyridines, NH3, EDTA, pz, and py) with cysteine (excess conditions), were studied by UV-vis spectrophotometry, using stopped-flow techniques, at an appropriate pH, in the range 3-10, and T = 25 degrees C. The selection of coligands afforded a redox-potential range from -0.3 to +0.5 V (vs Ag/AgCl) for the NO+/NO bound couples. Two intermediates were detected. The first one, I1, appears in the range 410-470 nm for the different complexes and is proposed to be a 1:1 adduct, with the S atom of the cysteinate nucleophile bound to the N atom of nitrosyl. The adduct formation step of I1 is an equilibrium, and the kinetic rate constants for the formation and dissociation of the corresponding adducts were determined by studying the cysteine-concentration dependence of the formation rates. The second intermediate, I2, was detected through the decay of I1, with a maximum absorbance at ca. 380 nm. From similar kinetic results and analyses, we propose that a second cysteinate adds to I1 to form I2. By plotting ln k1(RS-) and ln k2(RS-) for the first and second adduct formation steps, respectively, against the redox potentials of the NO+/NO couples, linear free energy plots are obtained, as previously observed with OH- as a nucleophile. The addition rates for both processes increase with the nitrosyl redox potentials, and this reflects a more positive charge at the electrophilic N atom. In a third step, the I2 adducts decay to form the corresponding Ru-aqua complexes, with the release of N2O and formation of cystine, implying a two-electron process for the overall nitrosyl reduction. This is in contrast with the behavior of nitroprusside ([Fe(CN)5NO]2-; NP), which always yields the one-electron reduction product, [Fe(CN)5NO]3-, either under substoichiometric or in excess-cysteine conditions.