Acid–base and metal ion-binding properties of diaminopropyl D-glucopyranoside and diaminopropyl D-mannopyranoside compounds in aqueous solution

Abstract

For three sugar-appended diamine compounds (1,3-diamino-2-propyl β-D-glucopyranoside (2-β-D-Glc-pn), (2S)-2,3-diaminopropyl β-D-glucopyranoside (1-β-D-Glc-pn) and 1,3-diamino-2-propyl α-D-mannopyranoside (2-α-D-Man-pn)), acidity constants and stability constants with Ni²⁺, Cu²⁺ and Zn²⁺ have been measured (I = 0.16 M NaCl, 25 °C). The two acidity constants of each of the three sugar-diamines differ by 10^1.65 to 10^3.09, indicating that removal of the proton from HL⁺ species is more difficult than deprotonation from the fully protonated dication H₂L²⁺. Statistical and polar effects, as well as the formation of an intramolecular hydrogen bond, may cause this increased stability of the HL⁺ species. The strength of the hydrogen bond and the degree of its formation (percentage) were estimated. The sugar ring has only a small influence on the intramolecular hydrogen bond formation. For the different metal ion–ligand systems, the predominating species in solution are quite different. In the Cu²⁺–1-β-D-Glc-pn system, the dominant species are always CuL²⁺ and CuL₂²⁺ in the pH range 4 to 10, where the total ligand concentration is larger than total metal ion concentration. For Ni²⁺, NiL₃²⁺ is also important under these same conditions; however, for Zn²⁺, the hydrolysis species ZnL₂(OH)⁺ and ZnL₂(OH)₂ predominate in the high pH region. All possible species in the system were included during the calculations, and the corresponding stability constants were determined. The hydrolysis of the metal ions themselves is important in some cases and all possible hydrolysis species were included in the fitting calculation. The stability constant plots log Kversus pK yielded straight reference lines for 1,3-diamine or 1,2-diamine ligands, reflecting the complete absence of sugar oxygen atoms in the metal ion coordination. The linkage between the metal ion and the diamine residue depends solely on the basicity of the ligand.