Abstract
Two new ligand-containing histidine based on
N,
N′,
N″-tris(N-benzyl-
l-histidinyl)tri(2-aminoethyl)amine,
L
1
, namely
N,
N′,
N″-tris[(1S)-2-methoxy-2-oxy-1-(1-benzylimidazol-4-ylmethyl)]nitrilotriacetamide
L
2
and
N,
N′,
N″-tris{N-benzyl-N-[N-benzyl-N-(N-benzyl-
l-histidinyl)-
l-histidinyl]-
l-histidinyl}tri(2-aminoethyl)amine
L
3
were prepared. Zinc(II) binding studies by these ligand systems were analyzed by means of potentiometric and
1H NMR titrations in aqueous methanol (33 %
v/
v). Subsequently their zinc(II) complexes [
L
1
Zn(H
2O)](ClO
4)
2·HClO
4 (
1), [
L
2
Zn(OH
2)](ClO
4)
2·H
2O (
2), and ([
L
3
Zn
3(H
2O)
3](ClO
4)
6·3HClO
4·5H
2O (
3), respectively were synthesized and characterized. The reactivity of the trinuclear complex (
3) toward the hydrolysis of the toxic organophosphate parathion was investigated and compared with that of the mononuclear reference complex (
1). From the pH dependence of the apparent rate constants, and the deprotonation constant (p
K
a) of the coordinated water molecules in (
1), the active species were confirmed to be {[H
L
1
Zn(OH)]
2+/[
L
1
Zn(H
2O)]
2+} at pH 8.5. The trizinc complex (
3) effects hydrolysis of parathion, with three times rate enhancement over the mononuclear (
1), indicating that cooperative action of the three zinc centers is limited.
Kinetic studies reveal that the trinuclear complex [H
3
L
3
Zn
3(H
2O)
3]
6+ (
3) affects hydrolysis of parathion, with three times rate enhancement over the mononuclear reference [H
L
1
Zn(H
2O)]
3+ (
1), indicating that cooperative action of the three zinc centers is limited.
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