Tetraamide derivatives, of DOTA

In some cases the pH dependence of the relaxivity is associated with changes in the structure of the second hydration shell. Two such systems have been reported by Sherry s group. The first case deals with a macrocyclic tetraamide derivative of DOTA (DOTA-4AmP, Chart 12) that possesses an unusual ri vs. pH dependence (131). In fact, the relaxivity of this complex increases from pH 4 to pH 6, decreases up to pH 8.5, remains constant up to pH 10.5 and then increases again. The authors suggested that this behavior is related to the formation/disruption of the hydrogen bond network between... 

Fig. 13. DOTA tetraamide derivative, the Gd(III) complex of which is a pH sensitive MRI contrast agent...

A sizeable contribution of the term rf has also been evidenced in the study of the pH-dependence of the relaxivity in the case of the Gd(III) complexes with tetraamide DOTA derivatives. As previously discussed, these cationic monoaqua complexes are characterized by a slow rate of exchange of the coordinated water molecule, which strongly limits the inner sphere... 

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