Solution Structures, Stabilities, Kinetics, and Dynamics of DO3A and DO3A−Sulphonamide Complexes

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 logKLnL values of Ln(DO3A–SA) complexes are 2 orders of magnitude higher than those of the Ln(DO3A) complexes. The protonation constant (log KMHL) of the sulphonamide nitrogen in the Mg2+, Ca2+, Mn2+, Zn2+, and Cu2+ complexes is very similar to that of the free ligand, whereas the logKLnHL values of the Ln(DO3A–SA) complexes are lower by about 4 logK units, indicating a strong interaction between the Ln3+ ions and the sulphonamide N atom. The Ln(HDO3A–SA) complexes are formed via triprotonated *Ln(H3DO3A–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 (t1/2 = 5.6 × 103 h at pH = 7.4), and it mainly occurs through proton-assisted dissociation of the complex. The 1H and 13C N...