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the presence of excess ethylenediamine and Cu
2þ
or Mn
2þ
showed the features character-
istic for a B-DNA structure [15a,56]. The small shifts in the position and intensity of CD
bands upon the formation of the metal-Salen complexes can be interpreted as indicative
of some structural differences induced by the metal complex in the duplex.
The substitution in a 20-bp DNA duplex of one AT base pair with a pair of salicylalde-
hydes lowered the stability of the duplex (Table 10.1, entry 9) [15a]. At pH 9.0, the stabil-
ity of the salicylaldehyde-containing DNA duplex increased in the presence of Cu
2þ
or of
Cu
2þ
and methyl amine, a gain that was attributed to the formation of an interstrand
square-planar [Cu(
Sal
)
2
] complex or [Cu
2þ
-bis(hydroxo-imine)] complex, respectively.
Comparison
Cu
2þ
of
the
stabilization
effect
exerted
on
the
duplex
by
either
15
C) or ethylenediamine (D
T
m
6
C) with the effect of the combination of
(D
T
m
Cu
2þ
and ethylenediamine (D
T
m
>
40
C) indicated a cooperative stabilization effect
exerted by the metal coordination and ethylene crosslinking [15a]. The stabilization of
the duplex upon formation of [Cu(
Salen
)] complexes depended on the identity of the
metal ion. A stoichiometric amount of Mn
2þ
produced a large duplex stabilization, simi-
lar to that of Cu
2þ
, but Zn
2þ
and Ni
2þ
caused a relatively small stabilization and only if
added in excess to duplex solutions containing ethylenediamine.
The mechanical properties of the mixed-sequence, 20-bp DNA duplex containing a
central pair of salicylaldehyde ligands have significantly been affected by the coordina-
tion of Cu
2þ
to the two ligands [15g]. The rupture force of the ds DNA was significantly
higher in the presence of Cu
2þ
. The analysis of the rate-dependent separation of the two
strands led to the conclusion that thermal off-rates for the non-modified and salicylalde-
hyde-modified DNA was about 10
13
s
1
, while the on- and off- rates for the Cu
2þ
-con-
taining salicylaldehyde-modified DNA were about 10
4
M
1
s
1
and 10
3
s
1
,
respectively. The mechanism underlying the mechanical properties of the metal-contain-
ing DNA duplex depended on the rate with which the mechanical stress was applied. Spe-
cifically, the mechanism was dominated by the Watson-Crick part of the duplex at low
loading rates and by the metal complex at high loading rates.
Two DNA hairpins containing a
Salen-M
complex have been also reported. The melt-
ing temperature of the hairpin in which
Salen-M
played the role of loop was only 2-3
C
higher than the melting temperature of the control hairpins that had a loop of three nucle-
obases, GAA, TAA or TTT [56]. In contrast, the melting temperature of the hairpin in
which
Salen-M
played the role of an alternative base pair in the stem of the hairpin was
higher by several tens of degrees than that of the hairpin which contained two
Sal
ligands.
This difference between the effect of the
Salen-M
complex on the thermal stability of
the hairpin depending on its position is similar to the observed effect of the position of
a metal complex alternative base pair on the thermal stability of the duplex in which
the metal complex is incorporated (see above p. 351, for bipyridine-based alternative
base pairs).
O,O-Coordinating Ligands: Hydroxypyridone. Hydroxypyridone (
H
) can form square
planar [M
L
2
] complexes with metal ions (Figure 10.7) [57]. Tanaka
et al.
[12a] and
Schlegel
et al.
[53] synthesized
H
-containing phosphoramidites and used them to incor-
porate
H
ligands into DNA and GNA oligomers, respectively (Table 10.1, entry 8). UV
titrations [12a,57,58] and mass spectrometry [12a] confirmed the formation of a [Cu
H
2
]
complex in solutions of the hydroxypyridone DNA ligandoside with Cu
2þ
. As observed
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