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

For three sugar-appended diamine compounds (1,3-diamino-2-propyl beta-D-glu copyranoside (2-beta-D-Glc-pn), (2S)-2,3-diaminopropyl beta-D-glucopyranosi de (1-beta-D-Glc-pn) and 1,3-diamino-2-propyl alpha-D-mannopyranoside (2-al pha-D-Man-pn)), acidity constants and stability constants with Ni2+, Cu2+ a nd Zn2+ have been measured (I = 0.16 M NaCl, 25 degrees C). The two acidity constants of each of the three sugar-diamines differ by 10(1.65) to 10(3.0 9), indicating that removal of the proton from HL+ species is more difficul t than deprotonation from the fully protonated dication H2L2+. Statistical and polar effects, as well as the formation of an intramolecular hydrogen b ond, may cause this increased stability of the HL+ species. The strength of the hydrogen bond and the degree of its formation (percentage) were estima ted. The sugar ring has only a small influence on the intramolecular hydrog en bond formation. For the different metal ion-ligand systems, the predomin ating species in solution are quite different. In the Cu2+-1-beta-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 conditio ns; however, for Zn2+, the hydrolysis species ZnL2(OH)(+) and ZnL2(OH)(2) p redominate in the high pH region. All possible species in the system were i ncluded during the calculations, and the corresponding stability constants were determined. The hydrolysis of the metal ions themselves is important i n some cases and all possible hydrolysis species were included in the fitti ng calculation. The stability constant plots log K versus pK yielded straig ht reference lines for 1,3-diamine or 1,2-diamine ligands, reflecting the c omplete absence of sugar oxygen atoms in the metal ion coordination. The li nkage between the metal ion and the diamine residue depends solely on the b asicity of the ligand.