Chemistry Reference
In-Depth Information
Figure 4.41
(a) Stepwise synthesis of a self-complementary adeninyl-thymine containing
thioether, A-SS-T. (b) Scheme illustrating the structure of the adeninyl-thymine-containing
complexes [MCl(
N3
-A-SS-T)]
+
(M = Pd(II) or Pt(II)). (c) Scheme illustrating the Hoogsteen
base pairing observed in the crystal structure of [MCl(
N3
-A-SS-T)]
+
research in recent years.
63
However, the direct electrical characterization of DNA
has been controversial
64
since this molecule has been described variously as a con-
ductor
65
and an insulator.
66
Currently it is widely accepted that it behaves as an
insulator from 30 nm lengths up.
67
Several approaches have been attempted to try
to modify the electronic structure of DNA in order to increase conductivity.
63
A new
molecular wire candidate based on DNA is so-called M-DNA.
68
This type of DNA
consists of a complex formed between divalent metal ions (Zn
2+
, Ni
2+
and Co
2+
) and
duplex DNA (Figures 4.42 and 4.43). M-DNA is formed at high pH (7.5- 9.0) and
the model proposed consists of a structure in which the metal cations replace the
imino protons involved in H - bonds between A - T and G - C base - pairs (Figure 4.43 ).
As a result, the metal ions are located in the interior of the helix resulting in
metal-metal distances close to 3.4 Å, corresponding to the value of the typical p -
stacking between nucleobases in B-DNA. However, despite preliminary NMR data
consistent with liberation of the protons inside the strand, the exact position of the
metals remains unclear. Interestingly, M-DNA converts back to B-DNA either when
the pH of the solution decreases or when chelating agents such as EDTA are
added.
Among several interesting physical features, Lee
et al.
have reported that DNA
strands modifi ed with fl uorescent probes, fl uorescein and rhodamine, show a strong
quenching of the fl uorescence upon M-DNA formation. They suggested that this
effect could be caused by a fast electron transfer.
69
However, the electrical charac-
