Chemistry Reference
In-Depth Information
Table 5.1
Conformer percentages of Me
2
dab-PtG
2
complexes at different pH values measured
from the H8 signal intensities. Increase of pH from
∼
3 to
∼
7 highlights the effect of SSC
(stabilization of
D
HT in the case of 3
′
-GMP adducts and stabilization of
Λ
HT in the case of
5
10 highlights the effect of guanine
O6/NH
cis
amine H-bond interaction (stabilization of
D
HT in the case of
S,R,R,S
confi guration
of the diamine and stabilization of
′
-GMP adducts), while further increase of pH from
∼
7 to
∼
Λ
HT in the case of
R,S,S,R
confi guration of the diamine).
The latter stabilization (
HT favoured at high pH for
R,S,S,R
confi guration of the diamine) is
not observed in the case of 5
Λ
′
-GMP because of simultaneous stabilization of the
D
HT conformer
stemming from 5
′
-phosphate/NH
cis
amine H-bond interaction as outlined in Section 5.3.3.
(
S,R,R,S
)
(
R,S,S,R
)
G
pH
% HH
pH
% HH
%
L
HT
%
D
HT
%
L
HT
%
D
HT
3.4
a
b
3
′
-GMP
80
13
7
3.3
16
84
7.2
a
60
35
5
6.9
4
96
b
9.6
a
37
60
3
9.3
86
14
b
3.2
92
2
6
3.1
7
71
22
5
′
-GMP
7.0
98
1
1
7.3
12
70
18
10
c
10.4
20
72
8
10.0
69
21
a
At 5 °C; all others were at room temperature.
b
Not determined.
c
The percentage of this conformer (
Λ
HT) does not increase at the expenses of the other HT conformer (
D
HT) when the
pH is raised to 10.
the G O6 and the N-H of the
cis
amine are on the same side of the platinum coor-
dination plane (Figure 5.7). We believe that the nonincrease of the L HT concentra-
tion at basic pH is due to a contemporary increase in stability of the D HT conformer
with the consequence that, on moving from neutral to basic pH, the conformer
concentrations remain practically unchanged. The stabilization at basic pH of the
DHT conformer most probably stems from a phosphate/NH
cis
amine interaction
(as depicted in Figure 5.7). Indeed, for the DHT conformer of (
R,S,S,R
) - Me
2
dab -
Pt(5
- phosphate protruding towards
the
cis
amine is on the same side of the platinum coordination plane as the N-H of
the
cis
amine. In Me
2
dab - Pt(3
′
- GMP)
2
we match the situation in which the 5
′
- GMP)
2
adducts, conformers cannot be affected by
phosphate/NH
cis
amine interaction for stereochemical reasons because the 3
′
′
-
phosphate is too remote from the
cis
amine N-H.
Because the 5
- phosphate/NH
cis
amine hydrogen bonding involves one part
of the ligand close to the metal (the NH) and one part far from the metal (the
phosphate), we have called this interaction 'fi rst - to - second sphere communication '
(FSC). This is not the case for the previous two types of guanine/
cis
amine interac-
tions which involve parts of ligands close to the metal and can be referred to as
' fi rst - to - fi rst sphere communication' (FFC).
′
5.4 Solid - State Structures of Dynamic Nucleotides
In the solid state, numerous examples of DHT complexes of 5
′
- GMP and several
related 5
′
-phosphate derivatives have been found, not only for Pt with various
