The Gd3+-DO3A arylsulphonamide (DO3A-SA) complex is a promising pH-sensitive MRI agent. The stability constants of the DO3A-SA and DO3A complexes formed with Mg2+, Ca2+, Mn2+, Zn2+, and Cu2+ ions are similar, whereas the logicK(LnL) values of Ln(DO3A-SA) complexes are 2 orders of magnitude higher than those of the Ln(DO3A) complexes. The protonation constant (log K-MHL) of the sulphonamide nitrogen in the Mg2+, Ca2+, Mn2+, Zn2+, and Cu2+ complexes is very similar to that of the free ligand, whereas the logK(LnHL) values of the Ln(DO3A-SA) complexes are lower by about 4 logK units, indicating a strong interaction between the Ln(3+) ions and the sulphonamide N atom. The Ln(HDO3A-SA) complexes are formed via triprotonated *Ln(H(3)DO3A-SA) intermediates which rearrange to the final complex in an OH--assisted deprotonation process. The transmetalation reaction of Gd(HDO3A-SA) with Cu2+ is very slow (t(1/2) = 5.6 X 10(3) h at pH = 7.4), and it mainly occurs through proton-assisted dissociation of the complex. The H-1 and C-13 NMR spectra of the La-, Eu-, and Lu(DO3A-SA) complexes have been assigned using 2D correlation spectroscopy (COSY, EXSY, HSQC). Two sets of signals are observed for Eu-, Y-, and Lu(DO3A, SA), showing two coordination isomers in solution, that is, square antiprismatic (SAP) and twisted square antiprismatic (TSAP) geometries with ratios of 86-14, 93 7, and 94-6%, respectively. Line shape analysis of the C-13 NMR spectra of La-, Y-, and Lu(DO3A-SA) gives higher rates and lower activation entropy values compared to Ln(DOTA) for the arm rotation, which indicates that the Ln(DO3A-SA) complexes are less rigid due to the larger flexibility of the ethylene group in the sulphonamide pendant arm. The fast isomerization and the lower activation parameters of Ln(DO3A-SA) have been confirmed by theoretical calculations in vacuo and by using the polarizable continuum model. The solid state X-ray structure of Cu(H(2)DO3A-SA) shows distorted octahedral coordination. The coordination sites of Cu2+ are occupied by two ring N- and two carboxylate O-atoms in equatorial position. The other two ring N-atoms complete the coordination sphere in axial positions. The solid state structure also indicates that a carboxylate O atom and the sulphonamide nitrogen are protonated and noncoordinated.