COORDINATION PROPERITIES OF ETHYLENDINITRILO-$N,N'$-BIS (2-HYDROXYPHENYL) -$N,N'$ -DIACETATE WITH TERBIUM (III) IN AQUEOUS SOLUTION. AN ANALOG OF LANTHANIDE ION BINDING IN PROTEINS

Author Institution:The title ligand, also called N, N-ethylenebis (2(o-hydroxyphenyl)) - glycine (EHPG), contains two carboxylate, two amino and two phenolate donor groups and forms a fully hexadentate chelate with $Tb^{3+}$ by pH 8. Coordination of phenolate oxygens and exitation at 290 nm produces strong green luminescence from $Tb^{3+}$. The utility of the slopes of a plot of luminescence decay constants versus mole fraction of $H_{2}O$ in $H_{2}O/D_{2}O$ mixtures is investigated for determining the number of water molecules bound to $Tb^{3+}$ in the Tb (EHPG) complex at various pH values. for the fully formed EHPG complex at pH 8, the coordinatin number of $Tb^{3+}$ is 8, as two water molecules are also bound to the $Tb^{3+}$. The analysis includes an allowance for a contribution to radiation less deactivation from N-H oscillators of coordinated amino nitrogens in EHPG. In proteins, where binding of amino groups to $Tb^{3+}$ is unlikely, the number of bound water molecules, W, is given simply by W = 9 r, where r is the ratio of slopes for mole fraction of $H_{2}O$ in $H_{2}O/D_{2}O$ mixtures for the terbiumprotein complex to aqueous $Tb^{3+}$