The confluence of structure and dynamics in lanthanide(iii) chelates: How dynamics help define structure in solution

Coordination exchange processes tend to dominate the solution state behaviour of lanthanide chelates and generally prohibit the study of small conformational changes. In this article we take advantage of coordinatively rigid Eu³⁺ chelates to examine the small conformational changes that occur in these chelates as water dissociatively exchanges in and out of the inner coordination sphere. The results show that the time-averaged conformation of the chelate alters as the water exchange rate increases. This conformational change reflects a change in the hydration state (q/r_LnH⁶) of the chelate. The hydration state has recently come to be expressed as two separate parameters q and r_LnH. However, these two parameters simultaneously describe the same structural considerations which in solution are indistinguishable and intrinsically related to, and dependent upon, the dissociative water exchange rate. This realization leads to the broader understanding that a solution state structure can only be appreciated with reference to the dynamics of the system.