The ther
modyna
mic selectivity for Gd
3+ relative to Ca
2+, Zn
2+, and Fe
3+ of two ligands of potential interest as
magnetic resonance i
maging (MRI) contrast agents has been deter
mined by NMR spectroscopy and potentio
metricand spectrophoto
metric titration. The two hexadentate ligands TREN-6-Me-3,2-HOPO (H
3L2) and TREN-bisHOPO-TAM-EA (H
4L3) incorporate 2,3-dihydroxypyridonate and 2,3-dihydroxyterephthala
mide
moieties. They were chosento span a range of basicity while
maintaining a structural
motif si
milar to that of the parent ligand, TREN-1-Me-3,2-HOPO (H
3L1), in order to investigate the effect of the ligand basicity on its selectivity. The 1:1 stability constants(
![](/i<font color=)
mages/gifchars/beta2.gif" BORDER=0 ALIGN="
middle">
110) at 25
![](/i<font color=)
mages/entities/deg.gif">C and 0.1 M KCl are as follows.
L2: Gd
3+, 20.3; Ca
2+, 7.4; Zn
2+, 11.9; Fe
3+, 27.9.
L3: Gd
3+, 24.3;Ca
2+, 5.2; Zn
2+, 14.6; Fe
3+, 35.1. At physiological pH, the selectivity of the ligand for Gd
3+ over Ca
2+ increases withthe basicity of the ligand and decreases for Gd
3+ over Fe
3+. These trends are consistent with the relative aciditiesof the various
metal ions;-
more basic ligands favor harder
metals with a higher charge-to-radius ratio. Thestabilities of the Zn
2+ co
mplexes do not correlate with basicity and are thought to be
more influenced by geo
metricfactors. The selectivities of these ligands are superior to those of the octadentate poly(a
minocarboxylate) ligandsthat are currently used as MRI contrast agents in diagnostic
medicine.