Binding of cysteine and glutathione to Ru(II) and Ru(III) centers: Formation and products reactivities

摘要

The interactions Of L-cysteine (CysSH) and the tripeptide glutathione (gamma-L-glutamate-L-CySteine-glycine, GSH) molecules with the trans-[Ru(NH3)(4)(4-pic)(H2O)](2+) ion (4-pic = 4-picoline) have been investigated by cyclic voltammetry, UV-Vis, H-1 NMR, and EPR spectroscopies. Experimental data strongly suggest that the sulfur atom of the SH group, present in CysSH and in GSH molecule, is the binding site of these ligands to the trans-[Ru(NH3)(4)(4-pic)(H2O)](2+) species. The trans-[Ru-II(NH3)(4)(4-pic)(CysSH)](+3) and trans[Ru-II(NH3)(4)(4-pic)(GSH)](3+) ions showed a maximum absorption band at 346 nm (epsilon = 5.0 x 10(3) and 5.4 x 10(3) M-1 cm(-1), respectively) at pH 1.0 attributed to the transition 4d(pi) Ru-II -> pi*(4-pic). Solutions containing the CysSH and GSH complexes exhibited a reversible electrochemical behavior at pH 7.2 and 7.8 with values of E-1/2 equal to -0.378 V and -0.400 V (SCE), respectively, attributed to the Ru(III)/Ru(II) redox couple. The pK(a) values measured from changes in the electronic and voltammetric spectra of trans-[Ru-II(NH3)(4)(4-pic)(L)](3+/+2) ions as a function of changes in the hydrogen ion concentration of the solution are, respectively, 5.6 +/- 0.1 and 6.6 +/- 0.2 for L = CysSH and GSH. The second-order specific rate constants (k(1) and k(-1)) and equilibrium constants (K-eq) values for the reaction (1) (LH = CysSH or GSH) calculated from spectrophotometric data (25 +/- 0.2 degrees C, mu = 0.20 M NaCF3COO/CF3COOH) are k(1) = (4.7 +/- 0.2) x 10(-2) M-1 s(-1); k(-1) = (4.4 +/- 1.0) x 10(-4) s(-1) and K-eq(II) = (1.2 +/- 0.2) x 10(2) M-1 to CysSH system and k(1) = (5.6 +/- 0.2) x 10(-2) M-1 s(-1), k(-1) = (5.3 +/- 1.4) x 10(-4) s(-1) and K-eq(II) = (1.1 +/- 0.4) x 10(2) M-1 to GSH system.