Dissociation constants of protonated methionine species in NaCl media

The sulfur-containing amino acid, methionine, has a role in the physiological environment because of its strong interactions with metals. To understand these interactions of metals with methionine, one needs reliable dissociation constants for the protonated methionine species (NH ₃⁺CH(CH₂CH₂SCH₃)COOH; H₂B⁺). The values of stoichiometric dissociation constants, pK_i*, for protonated methionine species (H ₂B⁺⇔H⁺+HB, K₁; HB⇔H ⁺+B^-, K₂) were determined from potentiometric measurements in NaCl solutions as a function of ionic strength, 0.25-6.0 mol (kg H₂O)^-1 and temperature (5-45 °C). The results were extrapolated to pure water using the Pitzer equations to estimate the activity of H⁺, H₂B⁺, HB and B^- as a function of ionic strength and temperature. The resulting thermodynamic values of K₁ and K₂ were fit to the equations (T/K): ln K₁=69.0013-3496.58/(T/K)-10.9153 ln (T/K); ln K ₂=116.4162-10638.02/(T/K)-18.0553 ln (T/K) with standard errors of 0.003 and 0.033, respectively, for ln K₁* and ln K ₂*. Pitzer interaction parameters (λ_HB-Na and ζ_HB-Na-Cl) for the neutral HB were determined from literature data. The Pitzer parameters (β_H2BCl⁰, β_H2BCl¹ and C_H2BCl^φ) for the interactions of H₂B⁺ with Cl^- and Na ⁺ with and B^2- (β_NaB⁰, β_NaB¹ and C_NaB^φ) were also determined. These coefficients can be used to make reasonable estimates of the activity coefficients of methionine species and the pK_i* for the dissociation of methionine in physiological solutions, composed mostly of NaCl over a wide range of temperature and ionic strength.