Chemistry Reference
In-Depth Information
Figure 13.11
Bimetallic ligand for Fe(III) reported by Schnaith and Que (HPTB), and Liu
(DTPB)
quantitatively religated, thus confi rming the occurrence of a hydrolytic mechanism
with the sole cleavage of the P-O3
′
bond. The complex shows a remarkable affi nity
for plasmid DNA (K
a
= 1
10
5
M
− 1
), probably related to the high positive charge of
its bimetallic structure. The comparison with other bimetallic complexes indicates
that a m-oxo bridge between the two metal ions and the presence of a labile
BF
4
−
ligand play a key role in determining the reactivity: the fi rst by ensuring the proper
intermetallic distance and the second by allowing a facile substitution to yield free
coordination sites for interaction with the substrate.
Surprisingly, examples of bimetallic systems based on Cu(II) or Zn(II) are
scarce and show a disappointingly low reactivity.
34
The only important exception is
a binuclear Zn(II)-binding heptapeptide (Figure 13.12), described by Scrimin and
coworkers,
35
which cleaves plasmid DNA with a fi rst - order
×
rate
constant
of
1.0
10
− 5
s
− 1
at pH 7.0, 37 ° C and 3.6 mM complex concentration. Due to the presence
of several a-disubstituted amino acids, the peptide is folded to a relevant extent in
a 3
10
-helical conformation, and the two metal chelating TACN moieties face each
other at a distance of about 6 Å (the pitch of the 3
10
-helix). Even if such distance is
larger than that found in bimetallic hydrolytic enzymes, the system is about 20 times
more reactive than its monometallic counterpart and the cooperativity between the
two metal centres has been clearly demonstrated with Zn
2+
- concentration - depend-
ent experiments. The high cooperativity observed has been justifi ed by assuming the
formation of a sandwich-like complex in which the two Zn(II)-TACN complexes
interact with a single DNA phosphate group, taking full advantage of their comple-
mentary roles for the hydrolytic cleavage.
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