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metal complexes contained two coplanar
Triazole
ligands. This ligand arrangement was
in agreement with an earlier prediction of the geometry of the metal complex within a
DNA duplex based on DFT calculations [43]. The calculations showed that, although in
the lowest energy configuration the duplex contains a complex in which the two ligands
coordinated to the metal ion are not coplanar, the energy of the duplex containing a com-
plex with coplanar ligands is close to the lowest energy one. Hence, when incorporated in
the duplex, the complex could adopt a planar geometry due to stacking interactions. The
overall structure of the duplex in solution showed only small deviations from a B DNA
structure. Specifically, the twist angle of the Ag-containing alternative base pairs is lower
than that of nucleobase pairs in B DNA, making this part of the DNA a little less twisted
than “regular” B DNA. The base pair rise of the metal-containing alternative base pairs
was larger than that of the nucleobase pairs in B DNA and than that expected in the case
of Ag-Ag bonds between Ag
þ
ions in adjacent [
Triazole
.Ag.
Triazole
] complexes.
Shionoya and Carell demonstrated the synthesis of DNA duplexes containing
stretches of up to ten, adjacent [M
L
2
] complexes flanked by stretches of GC base
pairs [15d,f,h,73]. The stretch of metal complexes were either homometallic, namely
up to five [Cu
H
2
] complexes or up to ten [Cu
Salen
]or[Mn
Salen
] compexes, or
heterometallic, namely a combination of [Cu
H
2
]and[Hg
Py
2
] complexes or of
[Cu
Salen
]and[Hg
T
2
] complexes. The [M
L
2
] stoichiometry of the complexes was
confirmed by UV titrations and ESI mass spectrometry. The stoichiometry studies of
DNA duplexes containing a mixture of ligands confirmed also the metal selectivity
of the pairs of different ligands coexistent in the DNA [15f]. The CD spectra
revealed that the right-handed, helical B DNA structure adopted by the salicylalde-
hyde-containing DNA duplexes in solution issignificantlyaffectedbytheformation
of
[M
Salen
]
complexes;
identification
of
the
structure
of
the
metal-containing
duplexes based on CD spectroscopy only was not possible [15d].
The possibilities that exchange interactions between paramagnetic transition metal ions
in nucleic acid duplexes could be rationally controlled and that interesting magnetic prop-
erties such as single molecule magnetism could be attained are appealing. Information
about the interactions between the spins of the Cu
2þ
ions in adjacent complexes was
obtained by EPR spectroscopy. These studies showed that the spins of adjacent [Cu
H
2
]
complexes were ferromagnetically coupled [73]. Based on the magnitude of the dipolar
coupling between the adjacent Cu
2þ
ions, the Cu
2þ
-Cu
2þ
distance was estimated to be
3.2 A
. The electronic structure of the DNA duplex containing five [Cu
H
2
] complexes
has been studied using spin polarized DFT calculations [74]. The backbone of the DNA
was not included in the calculation which considered stacks of [Cu
H
2
] complexes. These
calculations could not determine if the ferromagnetic or the antiferromagnetic state of the
array of adjacent Cu
2þ
at the core of the DNA is lower in energy but addressed issues of
charge localization in the ferromagnetic state of the array. Specifically, the studies showed
significant charge delocalization on the ligands. The spin density had a s antibonding
character and was distributed over the d orbital of Cu and p orbitals of the four coordi-
nated oxygens. The highest occupied energy levels were discrete and a conduction band
was not formed. A more recent theoretical study of [Cu
H
2
] complexes in which the back-
bone was included showed that the lowest energy structure of the complex has a plane
reflection symmetry [75]. A stack of [Cu
H
2
] complexes with this symmetry would behave
as an insulating ferromagnet, which is in agreement with the experimental findings.
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