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

Abstract

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/rLnH6) of the chelate. The hydration state has recently come to be expressed as two separate parameters q and rLnH. 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.

On Water and its Effect on the Performance of T1‐Shortening Contrast Agents

“…Both kosmotropes (NaCl and CaCl 2 ) have the effect of increasing the relaxivity of both the SAP and the TSAP isomer. This increase is markedly larger in the case of the SAP isomer – an effect that can be directly attributed to the reduced inner‐sphere hydration of the TSAP isomer ,. Assuming that the second‐ and outer‐sphere contributions to relaxivity are about the same in TETA and NB‐DOTMA chelates, the relative increase in inner‐sphere relaxivity for the TSAP isomer is just 80 % of that obtained for the SAP isomer upon addition of CaCl 2 .…”

“…To address the former question it is necessary to consider the τ M value of each chelate. The exchange kinetics of the TSAP isomer are somewhat faster than optimal for high relaxivity ,,. The hydration parameter ( q / r 6 ) of the TSAP isomer is decreased by the extremely rapid water exchange and this limits the possible gains in relaxivity achievable from increasing τ R .…”

Crystal Structures of DOTMA Chelates from Ce3+ to Yb3+: Evidence for a Continuum of Metal Ion Hydration States

“…These 23 chelates present ap ictureo facontinuum of partial hydration states spanning q = 1t oq = 0. [28] Experimental Section DOTMA and its chelates were prepared, X-ray quality crystals grown and crystal structures determined by previously described methods. This series of structures affords an insight into the changes that occur during dissociative exchange.…”

“…Other interactions between the water and the lattice influence both its position and the extent of its interaction with the metal. [28] An umber of metrics have been used to describe the changes to the ligand structure associated with changei nh ydration:t he O-Ln-O angle,t he area of the O 4 plane, and the d/ cr atio. Given that the ionic radii are not accurately determined (the ionic radii of nitrogen and oxygen may not be identical in each chelate), such good correlationi sp owerful evidence that it is appropriate to consider the variation in the LnÀOH 2 distance in terms of changes in CN, and therefore also q.…”

“…This change in the metal position affects the structure of the chelating ligand, an effect that provided the basis for our own earlier work examiningt he effect of exchange on hydration. [28] An umber of metrics have been used to describe the changes to the ligand structure associated with changei nh ydration:t he O-Ln-O angle,t he area of the O 4 plane, and the d/ cr atio. These parameters are ultimately ar eflection of the position of the Ln 3 + within the coordination cage (Figure 7).…”