Dissociation constants of protonated cysteine species in seawater media

The dissociation constants (pK₁, pK₂ and pK₃) for cysteine have been measured in seawater as a function of temperature (5 to 45 °C) and salinity (S = 5 to 35). The seawater values were lower than the values in NaCl at the same ionic strength. In an attempt to understand these differences, we have made measurements of the constants in Na–Mg–Cl solutions at 25 °C. The measured values have been compared to those calculated from the Pitzer ionic interaction model. The lower values of pK₃ in the Na–Mg–Cl solutions have been attributed to the formation of Mg²⁺ complexes with Cys²⁻ anions

Mg²⁺ + Cys²⁻ = MgCys

The stability constants have been fitted to

after corrections are made for the interaction of Mg²⁺ with H⁺.The pK₁ seawater measurements indicate that H₃Cys⁺ interacts with SO₄²⁻. The Pitzer parameters β⁰(H₃CysSO₄), β¹(H₃CysSO₄) and C(H₃CysSO₄) have been determined for this interaction. The formation of CaCys as well as MgCys are needed to account for the values of pK₂ and pK₃ in seawater.The consideration of the formation of MgCys and CaCys in seawater yields model calculated values of pK₁, pK₂ and pK₃ that agree with the measured values to within the experimental error of the measurements. This study shows that it is important to consider all of the ionic interactions in natural waters when examining the dissociation of organic acids.