Effect of ionic strength and temperature on the protonation of oxidized glutathione

The protonation constants for oxidized glutathione, H_i-1L ^(4-i+1)-, K_i^H =[H_iL ^(4-i)-]/[H_i-1L^(4-i+1)-][H⁺] i=1,2,...,6 have been measured at 5, 25 and 45°C as a function of the ionic strength (0.1 to 5.4 mol·[kg(H₂O)]^-1) in NaCl solutions. The effect of ionic strength on the measured protonation constants has been used to determine the thermodynamic values (K_i^H0 ) and the enthalpy (ΔH_i ) for the dissociation reaction using the SIT model and Pitzer equations. The SIT (ε) and Pitzer parameters (β⁽⁰⁾, β⁽¹⁾ and C) for the dissociation products (L^4-, HL^3-, H₂L^2-, H ₃L^-, H₄L, H₅L⁺, H ₆L²⁺) have been determined as a function of temperature. These results can be used to examine the effect of ionic strength and temperature on glutathione in aqueous solutions with NaCl as the major component (body fluids, seawater and brines).