Dissociation constants of protonated methionine species in NaCl media Original Research Article

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 (NH3+CH(CH2CH2SCH3)COOH; H2B+). The values of stoichiometric dissociation constants, pKi*, for protonated methionine species (H2B+⇔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 K2=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 K2*. Pitzer interaction parameters (λHB–Na and ζHB–Na–Cl) for the neutral HB were determined from literature data. The Pitzer parameters (β0H2BCl, β1H2BCl and CφH2BCl) for the interactions of H2B+ with Cl− and Na+ with and B2− (β0NaB, β1NaB and CφNaB) were also determined. These coefficients can be used to make reasonable estimates of the activity coefficients of methionine species and the pKi* for the dissociation of methionine in physiological solutions, composed mostly of NaCl over a wide range of temperature and ionic strength.