Dynamic studies of transnitrosation of thiols of biological importance by the nitrosated 4,4′,4″,4′′′-tetrasulfophthalocyaninecobaltate(III) anion in aqueous solution

The kinetics of interaction of Co(III)TSPcNO (TSPc = 4,4′,4″,4′′′-tetrasulfophthalocyanine) with various thiols of biological relevance, e.g., reduced glutathione (GSH), captopril (CapSH), N-acetyl-L-cysteine (NALC), and L-cysteine ethyl ester (LCEE) have been investigated spectrophotometrically. The observed rate constants for transnitrosation are all first-order with respect to the respective thiols. The second-order rate constants which were determined at physiological temperature, 37 °C are 258 ± 8, 159 ± 3, 66.7 ± 1.3 and 37.4 ± 0.6 M− 1 s− 1, respectively. The second-order rate constants decreased according to the sequence LCEE > CapSH > GSH > NALC. The activation parameters (ΔH≠ and ΔS≠) were derived from the Eyring's equation. The experimental activation parameters were then correlated by an isokinetic plot, for the reduction of [Co(III)TSPc(NO−)]4− by the thiols, making use of the expression: ΔH‡ = ΔG0‡ + β0ΔS‡ where ΔGo‡ is the intrinsic free energy of activation, and βo the isokinetic temperature. The plot which showed very good linearity (R2 = 0.997), gave values of ΔGo‡ (61 ± 1 J K− 1 mol− 1) from the intercept, and βo (260 ± 11 K) from the slope. It is concluded that a common mechanism is adhered to in the reduction of Co(III)TSPcNO, irrespective of the type of thiol being used, to give the corresponding S-nitrosothiol, which is further confirmed by high performance liquid chromatography with mass spectrometric detector.