Chemistry Reference
In-Depth Information
In 1978, Marzilli and coworkers monitored the Hg
II
binding to cytidine using
13
C NMR spectroscopy. In their report, higher-fi eld shifts of carbon atoms (C2, C4)
were reported (Table 16.1).
53
Next, they reported
1
H and
13
C NMR spectra of com-
plexes between Hg
II
and nucleosides (uridine, cytosine and adenosine) in dimeth-
ylsulfoxide (DMSO).
54,55
In that study, they found that cytidine bound to Hg
II
,
probably at N3, but that uridine did not bind to Hg
II
through its N3 atom in
DMSO.
54,55
This observation was somewhat controversial, since proton-Hg
II
exchange reactions in H
2
O had already been proposed for uridine. In 1982, however,
they found that proton-Hg
II
exchange reactions in DMSO can occur at the N1 of
guanosine 'in the presence of triethylamine base' (Table 16.1).
56
Through the
13
C
NMR experiments, moderate but signifi cant chemical shift perturbations were
observed for the resonances from the carbon atoms adjacent to the metal-binding
nitrogen atoms (Table 16.1 ).
For the Hg
II
-thymidine system, Buncel and coworkers in 1981, 1985 and 1986
reported
13
C NMR spectra of the complexes (Table 16.1).
57 - 59
In these reports, they
isolated the complexes of Hg
II
- thymidine and Hg
II
- guanosine, and dissolved these
complexes in DMSO.
57 - 61
The derived solutions were used for NMR measurements.
According to these data, signifi cant lower - fi eld shifts of the
13
C nuclei adjacent to
Hg
II
-binding sites (N3) were observed for C2 and C4 nuclei (Table 16.1) (see refer-
ences for details).
58,59
These data strongly supported the Hg
II
- T complexation through
N3, as proposed by Katz.
8
Interestingly, Buncel and coworkers reported that proton-
Hg
II
exchange reaction occur at guanine to give a Hg
II
- guanosine covalent complex
through N1. This is because they observed the lower-fi eld shifts of the
13
C chemical
shifts of C2 and C6 (N1-neighbours) (see references for details).
57 - 59
In 1982, 1983 and 1986, Buchanan and coworkers monitored Hg
II
binding proc-
esses to guanosine, cytosine, adenosine and inosine in DMSO by using
15
N NMR
spectroscopy (Table 16.2).
62 - 64
From their experiments, very large chemical shift
perturbations were observed for N7 of guanosine (
20 ppm higher fi eld), which
meant that the Hg
II
binding site in guanosine was not N1 but N7 (Table 16.2)
62
Under their conditions, proton-Hg
II
exchange did not occur. However, this appeared
to be because NMR spectra were recorded in the DMSO solution without any base,
which is indispensable for a deprotonative metallization of guanosine at N1 in
DMSO, based on the observations by Marzilli and coworkers.
56
Next, Buchanan and
coworkers studied the methylmercury
II
complexes with 5
∼
- CMP using
15
N NMR spectroscopy.
64
It was pointed out that at pH 8 in H
2
O, Hg
II
binding sites
became the N1 of 5
′
- GMP and 3
′
- GMP with proton - Hg
II
exchange processes (Table 16.2).
64
In another water solution system, Polak and Plavec, in 1999, reported
5
N NMR
spectra of Hg
II
complexes with methyl esters of 5
′
′
- GMP and 3
′
- GMP (guanosine -
- monophosphate) (Table 16.2 ).
65
In their report, both compounds were seen to
bind to Hg
II
through their N7 atoms. They postulated that chelation of Hg
II
with N7
and the phosphate group makes Hg
II
bind to N7. With these data, however, Hg
II
binding sites of various nucleosides were determined with
15
N NMR spectroscopy.
In a later section (Section 16.4.2),
15
N NMR data on the proton-Hg
II
exchange
systems for a DNA duplex are described.
In respect of another interacting point,
199
Hg NMR spectra of Hg
II
- nucleoside
complexes were recorded by Norris and Kumar, in 1984.
66,67
They reported
199
Hg
3
′
