Protonation of DL-4-amino-3-hydroxybutanoic acid and its complexing capacity with copper(II) ions in aqueous solution

Abstract

The protonation constants of DL-4-amino-3-hydroxybutanoic acid [H₃L⁺= H₃[graphic omitted]·CH₂·CH(OH)·CH₂CO₂H], as determined potentiometrically at 25 °C and I= 0·1M-KCl, are log k₁= 13·0 ± 0·1, log K₂= 9·487(11), and log K₃= 3·834(12) for the hydroxy-, amine, and carboxy-groups, respectively. From measurements of log K at different temperatures between 5 and 35 °C, values of Δ;H₂, ΔS₂, ΔH₃, and ΔS₃ at 25 °C have been calculated and their sensitivity to changes in structure have been tested by comparison with appropriate compounds. With copper(II) the complexes [CuL], [Cu₂L₂], and [CuL₂^2–] are formed. In the first two complexes the ligand is probably tridentate. forming both five- and six-membered chelate rings, but in [CuL₂^2–] the second ligand molecule is bidentate through the amine and 3-hydroxyl groups.