Lanthanide(III)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid complexes in acidic medium: significant decrease in water exchange rate

UV-Vis and lanthanide-induced O-17 shift measurements on the complex [Eu(DO TA)](-) (H(4)DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) have shown that the inner co-ordination sphere of the Ln(3+) ion is not af fected on protonation which suggests that the proton is attached to a non-c o-ordinated oxygen atom of a carboxylate group. Proton NMR measurements per formed as a function of the H+ concentration revealed that the protonation slightly accelerates the intramolecular dynamic processes: the enantiomeriz ation for [La(DOTA)](-) and the enantiomerization and interconversion betwe en the major and minor isomer for [Eu(DOTA)](-). Contrary to first glance e xpectations, the water exchange rate on [Gd(DOTA)(H2O)](-) decreases signif icantly with increasing extent of protonation, and at 1.0 M H+ concentratio n is about ten times lower than in neutral media. In 1.0 M acidic solution the proton relaxivities were found to be higher than expected solely on the basis of the water exchange rates. This finding is interpreted with a fast er proton exchange in acidic solutions which is the consequence of the cata lytic effect of H+ ions.