Solution Dynamics and Stability of Lanthanide(III) (S)-2-(p-Nitrobenzyl)DOTA Complexes

Addition of a benzyl substituent to the macrocyclic ring of DOTA has a substantial impact on the conformational ring flipping motion of the macrocycle in the resulting LnDOTA complexes. The p-NO₂-benzyl substituent in the Ln(p-NO₂-Bn-DOTA)^- complexes lies in an equatorial position and effectively "locks" the conformation of the ring into the δδδδ configuration. The presence of the p-NO ₂-benzyl group also increases the population of the square antiprismatic (SAP) coordination isomer for all Ln(p-NO ₂-Bn-DOTA)^- complexes relative to that seen for the respective LnDOTA^- complexes. Despite this increase in SAP isomer population, the rate of water exchange in these complexes remains comparatively fast. The kinetic and thermodynamic stabilities of the Ln(p-NO ₂-Bn-DOTA)^- complexes are also slightly lower than the corresponding LnDOTA^- complexes but appear to be sufficiently high for in vivo use.