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 degrees 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 (Delta H(not equal) and Delta S(not equal)) 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: Delta H(double dagger) =Delta G(0)(double dagger)+beta(0)Delta S(double dagger) where Delta G(0)(double dagger) is the intrinsic free energy of activation. and beta(0) the isokinetic temperature. The plot which showed very good linearity (R(2)=0.997), gave values of Delta G(0)(double dagger) (61 +/- 1 JK(-1) mol(-1)) from the intercept and beta(0) (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. (C) 2009 Elsevier B.V. All rights reserved.