Dissociation Constants of Protonated Cysteine Species in NaCl Media

Abstract

The sulfur-containing biomolecule, cysteine has a role in physiological and natural environment because of its strong interactions with metals. To understand these interactions of metals with cysteine, one needs reliable dissociation constants for the protonated cysteine species [${NH}_{3}^+$CH(CH₂SH)COOH; H₃B⁺]. The values of dissociated constants, p$K_1^*$, for protonated cysteine species (H₃B⁺ ⇄ H⁺ + H₂B, K ₁; H₂B ⇄ H⁺ + HB⁻,K ₂; HB⁻ ⇄ H⁺ + B²⁻,K ₃) were determined from potentiometric measurements in NaCl solutions as a function of ionic strength, 0.5–6.0 mol-(kgH₂O)⁻¹ and between 5, and 45°C. The equations

$$\begin{gathered} {p}K_1^* = 1.378 + 228.4/T - 0.4044I^{0.5} + 0.2472I \hfill \\ {p}K_2^* = 2.031 + 1833.4/T - 0.1847I^{0.5} + 0.2190I \hfill \\ {p}K_3^* = 2.861 + 2191.3/T - 0.2170I^{0.5} + 0.2217I \hfill \\ \end{gathered} $$

were fitted to the results with a standard errors of the fits of 0.116, 0.057, and 0.093 for ${p}K_1^*$, ${p}K_2^*$, and ${p}K_3^*$, respectively. The ${p}K_i^*$ results were used to determine new Pitzer parameters (β⁰, β¹, and C) for the interactions of Na⁺ and Cl⁻ with cysteine species. These coefficients can be used to make reasonable estimates of the activity coefficients of the cysteine species and ${p}K_i^*$ for the dissociation of cysteine in physiological and natural waters containing mostly NaCl.