Ionization of cysteine in aqueous solutions. Part 2.—Specific-ionization constants

Abstract

Using the net-ionization constants previously determined together with some new spectro-photometric data, the four specific-ionization constants for cysteine have been obtained as a function of ionic strength and temperature. The carboxyl (1) group was always ionized under the conditions employed and the data for the sulphydryl (2) and protonated amino (3) groups are given with respect to the order of ionization by the appropriate pK values. The variations with temperature (K) at zero ionic strength are given by: pK^T₁₂= 8.799–1.793 × 10³/T+ 5.105 × 10⁵/T², pK^T₁₃= 2.321 + 1.615 × 10³/T+ 1.010 × 10⁵/T², pK^T₁₂₃= 7.152–2.980 × 10³/T+ 3.096 × 10⁵/T², pK^T₁₃₂= 13.631–3.438 × 10³/T+ 7.190 × 10⁵/T², while the corresponding enthalpy (kcal mol^–1) and entropy (cal mol^–1 deg.^–1) values at 25 and 50°C are ΔH((1, 2) 7.5, 6.0), ((1, 3) 10.5, 10.2), ((1, 2, 3) 9.4, 8.5), ((1, 3, 2) 6.4, 4.3) and –ΔS((1, 2) 13.9, 18.6), ((1, 3) 5.4, 6.3), ((1, 2, 3) 16.8, 19.6), ((1, 3, 2) 25.4, 31.8) respectively. Ionic strength variations follow closely (pK ± 0.01) those accounted for by an equation of the form of the extended Debye-Hückel equation for ionic strengths up to 0.8.