Chemistry Reference
In-Depth Information
is only 2.5-7-fold smaller than
k
az
/
k
s
for the same ions.
13,14
Based on the known
values of
k
s
for these ions,
k
d
G
is 1.9
10
9
M
1
s
1
for
64n
and
64o
, and
.
13,14
Other purine nucleosides including guanosine (G), 8-methylguanosine (8-MeG),
adenosine (A), inosine (I), and xanthosine (X) also trap
10
8
M
1
s
1
6.2
for
64h
.
14
All show pH-
independent trapping in the pH range 3.5-7.5, except X. The plot of observed
k
X
/
k
s
versus pH is consistent with trapping by X and its conjugate base X
.
14
The more
basic purines with p
K
a
N
7
-H
รพ
64n
and
64o
2.0 (dG, G, 8-MeG, and X
) generate a C-8 adduct
10
9
M
1
s
1
. It was suggested that this
is the diffusion controlled limit for reaction of these ions with purine nucleosides.
14
The 8-MeG adduct
exclusively with
64n
and
64o
with
k
nuc
2
130
(Scheme 4.36) is the reduction product of the initial C-8
that was detected, but not isolated.
14
Less basic purines generate different
adducts. I generates both a C-8 adduct and an O-6 adduct
adduct
129
, while the exclusive
product of the reaction with A is the azabicycloheptadiene (benzene imine)
131
.
14
These purines also exhibit significantly lower selectivity (about 1-10% of dG) for
trapping
132
. The pyrimidines thymidine, uridine, and cytosine showed no
evidence of reaction with
64n
and
64o
.
14
McClelland and coworkers verified the magnitude of
k
d
G
for
64n
and
64o
64h
,
64n
, and
64o
from LFP experiments.
15-17
They also provided data for
and other
distal ring-substituted biphenylyl nitrenium ions. Their results confirm that
k
d
G
reaches an apparent diffusion controlled limit of about 2.0
64g
,
64p
,
64q,
10
9
M
-1
s
1
for substi-
10
6
s
1
.
16
Even
tuted 4-biphenylyl ions with
k
s
64g
,
64h
, and
64q
that do not react
10
8
M
1
s
1
.
16
Only
at the diffusion controlled limit have
k
d
G
>
that has a
lifetime of ca. 0.6 ms and does not react with N
3
at the diffusion controlled limit,
has a relatively small
k
d
G
of 3.6
5
64p
10
7
M
1
s
1
.
16
Since
64p
reacts slowly in water, it
10
4
M
1
. Carbocations with
has a high dG/solvent selectivity,
k
d
G
/
k
s
, of 2.3
64g
64h
64n
64o
lifetimes similar to
do not have detectable reactions with
dG in aqueous solution.
16
The high selectivity of the 2-fluorenyl and 4-biphenylyl
,
,
, and
SCHEME 4.36.
Unusual purine adducts of
64n
and
64o
.
