The stability constants of the complexes of three ligands, DO3A (1,4,7,10-tetrazacyclododecane-1,4,7-triacetic acid), HP-DO3A (10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid), and DO3MA (1R,4R,7R)-alpha,alpha’,alpha "-trimethyl-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid), with Ca2+, Cu2+, and Zn2+ were measured at 25.0 +/- 0.1 degrees C and mu = 0.1 (TMACl). Calcium and gadolinium complexes which have five-membered chelate rings follow linear log K-ML vs Sigma pK(a) and log K-GdL vs log K-CaL correlations. suggesting Ca2+ and Gd3+ have similar coordination behavior. A linear correlation between log K-ML and Z/r (charge density on the metal ion, for HP-DO3A complexes is consistent with the lower stability constant of the Ca2+ vs Gd3+ complex. The overall selectivity constants for Gd3+ over other metal ions were calculated. The crystal structures of three hydrated forms of [Ca(HP-DO3A)](2) were determined by X-ray single-crystal analysis : a C-2 symmetric trinuclear structure. Ca(H2O)(4)[Ca(HP-DO3A)](2) . 10H(2)O, a centrosymmetric hexanuclear structure. {Ca(H2O);[Ca(HP-DO34A)](2) . 3H(2)O}(2), and a C-2 symmetric trinuclear structure, Ca(H2O)(4)[Ca(HP-DO3A)](2) . 2H(2)O, which is formed through topotactic dehydration of the first structure at 22 degrees C. The conformations of the macrocyclic ligands, [3333], and their coordination to the embedded calcium ions are virtually identical in all forms. The embedded calcium is sandwiched between two nearly parallel planes of four nitrogens and oxygens. The macrocyclic ring in the crystal structure of HCu(DO3A) adopts a quadrangular [2334] conformation in which the four nitrogens and two opposite carboxylate arms are coordinated to the Cu(II) atom.