Ionization of cysteine in aqueous solutions. Part 1.—Net-ionization constants

Abstract

The net-ionization constants (pK^T₂, pK^T₃) for cysteine in aqueous solution at zero ionic strength have been determined over the temperature range 5–95°C. The constants have been obtained by calculation from pH titration data and can be represented by the equations, pK^T₂= 4.202 + 6.473 × 10²/T+ 1.775 × 10⁵/T², pK^T₃= 11.299–21.91 × 10²/T+ 5.996 × 10⁵/T², where T is the temperature in °K. Good agreement has been obtained between the heats and entropies of ionization, implicit in these results, and the values obtained by direct calorimetry at 25°C. The large discrepancies between corresponding pK values quoted in the literature are discussed and the derivation of specific-ionization constants is considered.