Chemistry Reference
In-Depth Information
NMR chemical shifts and signal widths (n
1/2
) of seven types of methylmercury
II
-
nucleobase complexes in DMSO solution.
66
Among them, there were
199
Hg NMR
data for the methylmercury
II
complex with uridine or thymine at N3 (type-V), and
those with guanosine at N1. Next, in 1984, they reported NMR studies on a U-Hg
II
-
U pair with a C5-Hg
II
- N3 linkage, and a U - Hg
II
- 6 - thioguanosine pair with a C5 -
Hg
II
- S6 linkage
67
In 1983, Buchanan and Bell reported a
199
Hg NMR chemical shift
of the inosine-Hg
II
complex.
63
Other sets of
199
Hg NMR data of nucleobase com-
plexes were reported by Lippert and coworkers.
21 - 25,27
One is from the Hg
II
- guanine
derivative complexes,
22
and another is from Hg
II
,Pt
II
bimetallic complexes with
cytidine.
21,27
From their studies, one-bond
199
Hg -
195
Pt
J
- coupling (2783 Hz), fi ve - bond
199
Hg -
1
H
J
- coupling (20 Hz for
199
Hg - N4 - C4 - C5 - C6 -
1
H6),
21
four - bond
199
Hg -
1
H
J
-
coupling (78Hz for
199
Hg - N3 - C4 - C5 -
1
H5)
27
and three-bond
199
Hg -
1
H
J
- coupling
(107 - 222 Hz for
199
Hg - C5 - C6 -
1
H6)
23 - 25
were detected. These multinuclear NMR data
provide important structural information for investigating more complicated
systems.
16.4.2 Oligonucleotide - Metal Systems
The fi rst NMR data on Hg
II
-DNA complexes were reported by Young and cowork-
ers in 1982.
68
In their report, they recorded
1
H NMR spectra of poly-d(AT) in the
presence of various concentrations of Hg
II
, and obtained the following data: (i) as
the concentration of Hg
II
was increased, imino proton resonances were decreased
and disappeared at a molar ratio of 0.25 for [Hg
II
]/[per - nucleotide concentration];
(ii) average chemical shifts of nonexchangeable protons (H8 and H2 of adenine (A),
and H6 and 5-CH
3
protons of T) showed sigmoidal transitions against the molar
ratio of [Hg
II
]/[per-nucleotide concentration]; (iii) obtained data supported a strand-
slippage model with the formation of T-Hg
II
- T base pairs.
Studies on the Hg
II
-oligonucleotide systems were then reported by Sletten and
coworkers (Table 16.2).
69 - 72
They studied two self-complementary DNA duplexes,
d(CGCGAATTCGCG)
2
and d(GCCGATATCGGC)
2
. It was found that Hg
II
bound
to AT-rich regions of both duplexes. In the fi rst sequence,
1
H and
15
N NMR spec-
troscopic techniques were employed to elucidate the Hg
II
binding mode in DNA
duplexes. The most striking data are natural abundance
1
H -
15
N HMQC spectra
69,71
in which cross peak detection is extremely diffi cult because of the low sensitivity of
15
N nuclei. Sletten and coworkers overcame this issue by using a high concentration
sample (4.8 mM duplex: d(CGCGAATTCGCG)
2
) with
1
H -
15
N HMQC spectra using
a two - bond
1
H -
15
N
J
-coupling, and so made it possible to detect
15
N resonances of
N1 of cytidine, N7 and N9 of purine residues (adenosine and guanosine) and N1
and N3 of A.
69,71
This technique has also been applied to other metal-DNA
systems.
73,74
From the data, it was found that very large lower-fi eld shifts were observed at
the N1 resonances of A5 and A6 residues (15.3 ppm and 9.4 ppm, respectively)
(Table 16.2 ).
69,71
It should be noted that Sletten and coworkers concluded that the
observed lower - fi eld shift would be attributed to Hg
II
binding to amino nitrogen
(N6) with proton-Hg
II
exchange, after careful consideration. This is because for the
