Synthesis of 8-halogenated-7-deaza-2′-deoxyguanosine as an 8-oxo-2′-deoxyguanosine analogue and evaluation of its base pairing properties

Article abstract of DOI:10.1016/j.tet.2014.01.047

8-Halogenated-7-deaza-2′-deoxyguanosines (8-halo-7-deaza-dG) were designed to structurally mimic 8-oxo-2′-deoxyguanosine (8-oxo-dG), which is representative of an oxidized nucleoside. It has been shown by NMR that the conformation around the N-glycosidic bond of (8-halo-7-deaza-dG) is preferably syn, similar to 8-oxo-dG. The base pairing properties of 8-halo-7-deaza-dG were studied by measuring the thermal denaturation temperature of the duplexes, showing that their base pair with dC is destabilized compared with natural dG. These results also support their preference for syn conformation. Unlike 8-oxo-dG, 8-halo-7-deaza-dG did not form a stable base pair with dA, most likely due to the lack of N7-H hydrogen bonding with dA. In conclusion, the newly-designed 8-halo-7-deaza-dG analogs resemble 8-oxo-dG in its shape and preference for syn conformation, but they do not form Hoogsteen base pair with the opposing dA.

Full text of DOI:10.1016/j.tet.2014.01.047

Tetrahedron 70 (2014) 2040e2047
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Tetrahedron
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Synthesis of 8-halogenated-7-deaza-20-deoxyguanosine as an 8-
oxo-20-deoxyguanosine analogue and evaluation of its base pairing
properties
*
*
Yizhen Yin, Yosuke Taniguchi , Shigeki Sasaki
Graduate School of Pharmaceutical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan
a r t i c l e i n f o
a b s t r a c t
8-Halogenated-7-deaza-20-deoxyguanosines (8-halo-7-deaza-dG) were designed to structurally mimic
8-oxo-20-deoxyguanosine (8-oxo-dG), which is representative of an oxidized nucleoside. It has been
shown by NMR that the conformation around the N-glycosidic bond of (8-halo-7-deaza-dG) is preferably
syn, similar to 8-oxo-dG. The base pairing properties of 8-halo-7-deaza-dG were studied by measuring
the thermal denaturation temperature of the duplexes, showing that their base pair with dC is desta-
bilized compared with natural dG. These results also support their preference for syn conformation.
Unlike 8-oxo-dG, 8-halo-7-deaza-dG did not form a stable base pair with dA, most likely due to the lack
of N7-H hydrogen bonding with dA. In conclusion, the newly-designed 8-halo-7-deaza-dG analogs
resemble 8-oxo-dG in its shape and preference for syn conformation, but they do not form Hoogsteen
base pair with the opposing dA.
Article history:
Received 16 December 2013
Received in revised form 18 January 2014
Accepted 21 January 2014
Available online 30 January 2014
Keywords:
Oxidized nucleoside
8-Oxo-20-deoxyguanosine
8-Halogenated-7-deaza-20-deoxyguanosine
Nucleoside mimic
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
dA and dATP to opposing 8-oxo-dG, causing AT to CG and GC to
TA transversion mutations.15,16
Cellular DNA is continuously exposed to various types of
damage, e.g., irradiation, oxidation, etc.1e4 8-Oxo-20-deoxy-
guanosine (8-oxo-dG) is representative of a damaged nucleoside;
it is formed by the reaction of 20-deoxyguanosine (dG) with re-
active oxygen species (ROS). Abundant 8-oxo-dG has been linked
to aging and diseases, such as cancer.5e7 8-Oxo-dG forms stable
base pairs with 20-deoxycytidine (dC) and 20-deoxyadenosine
(dA), while dG forms a stable base pair with only dC.8e13 8-Oxo-
dG and dC form a standard WatsoneCrick base pair (Fig. 1A).
Because 8-oxo-dG exists in the 8-keto form preferentially at
physiological pH,14 it forms a Hoogsteen base pair with dA
(Fig. 1B). 8-Oxo-dG adopts an anti conformation in its Wat-
soneCrick base pair, while occupying a syn conformation in its
Hoogsteen base pair. Because of the steric effect of the 8-oxygen
atom, the anti conformation of 8-oxo-dG is less stable than that
of dG, decreasing the thermal stability of the 8-oxo-dG:dC base
pair. In contrast, as the syn conformation of 8-oxo-dG is relatively
stable, and the 8-oxo-dG:dA Hoogsteen base pair displays com-
parable thermal stability. Due to the base pairing properties of 8-
oxo-dG, DNA polymerases incorporate 8-oxo-dGTP to opposing
Specific repair enzymes suppress the promutagenicity and
genotoxicity of 8-oxo-dG by excising 8-oxo-dG or dA from the 8-
oxo-dG:dC or 8-oxo-dG:dA pairs, respectively. In a repair system,
A
B
* Corresponding authors. Tel./fax: þ81092 642 6615; e-mail addresses: taniguch@
phar.kyushu-u.ac.jp (Y. Taniguchi), sasaki@phar.kyushu-u.ac.jp (S. Sasaki).
Fig. 1. Base paired structures of (A) 8-oxo-dG:anti-dC and (B) 8-oxo-dG:syn-dA.
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.01.047

Y. Yin et al. / Tetrahedron 70 (2014) 2040e2047
2041
8-oxo-dGTP is enzymatically hydrolyzed to 8-oxo-dGMP to avoid
its incorporation into DNA.17,18 Preference in dCTP or dATP in-
corporation to opposing 8-oxo-dG at a replication step depends on
the polymerase, most likely because syn or anti conformation in the
base-paring with 8-oxo-dG is determined at the enzyme active site.
To investigate the mechanism of DNA polymerases and repair en-
zymes, Hamm’s group investigated effects of 7-deaza-dG, 9-deaza-
dG, 8-thio-dG, and 8-halogenated-dG as mimics for 8-oxo-dG on
their efficiency19e24 It was revealed that Fpg (for-
mamidopyrimidine [fapy]-DNA glycosylase) repairs 7-deaza-dG
approximately equivalent to 8-oxo-dG.24 However, 7-deaza-dG is
an inferior substrate to 8-oxo-dG for hOGG1. Seela’s group has re-
ported 7-halogenated-7-deaza-dG and 8-methyl-7-deaza-dG de-
rivatives. The former is not a mimic for the structural shape of 8-
oxo-dG,25 nor is the latter, in the sense that it adopted the anti
conformation in X-ray and NMR studies.26 However the octamer
duplex constructed of continuing 8-methyl-7-deaza-dG and dC
showed Z-DNA structure, suggesting that 8-methyl-7-deaza-dG
might adopt a syn conformation in oligonucleotide.27 It is clear from
these studies that the 7-NH of 8-oxo-dG mimics is necessary for
both hydrogen bonding and shape complementarity in its pairing
with dA and that the 8-hetero atom is necessary for a preference of
the syn- over the anti-conformation. To better imitate 8-oxo-dG by
taking into account both the 7-NH and 8-oxygen atom, we designed
8-halogenated-7-deaza-dG derivatives (Fig. 2). It was expected that
the 8-halogen atom might mimic the C8-oxygen in terms of the
dipole interaction, that is, important for lesion discrimination28e30
and that the 7-CH might resemble the shape of the 7-NH in 8-oxo-
dG. Here, we describe the synthesis of 8-halogenated-7-deaza-20-
deoxyguanosines, the investigation of their conformation in solu-
tion, and the evaluation of their base pairing properties.
acetylated 7-deaza-guanine (5) were performed using N-haloge-
nated-succinimides in DMF to furnish the corresponding haloge-
nated nucleoside derivatives.32,33 The use of N-chlorosuccinimide
(NCS) produced 8-chlorinated 7-deaza-dG (6) in a good yield. 8-
Brominated 7-deaza-dG (7) was obtained in a satisfactory yield
using N-bromosuccinimide (NBS), however, iodination with N-
iodosuccinimide (NIS) gave 8-iodinated 7-deaza-dG (8) in low yield
due to the formation of biproducts. A number of synthetic attempts
in which we changed reaction times, temperatures, reagent stoi-
chiometry, etc. were unsuccessful. After removing the acetyl groups
in 7 N ammonium in methanol at room temperature, the haloge-
nated positions of the resulting diol compounds (2, 3, and 4) were
confirmed by NMR spectroscopy, including 2D-COSY, NOESY,
HMBC, and HMQC spectra in DMSO-d6. Typically, the 1H and 13C
signals of the 8CeH of the deazaguanine base disappeared and
showed upfield shifts, respectively, due to the halogenation re-
actions. The hydroxyl groups were transiently protected with
TMSCl and the 2-amino group was efficiently protected using
phenoxyacetic anhydride to produce 2NH-Pac-protected diol
compounds (9, 10, and 11) in good yields. These diol compounds
were converted into the corresponding phosphoramidite de-
rivatives (12, 13, and 14) using conventional methods, and in-
corporated into the middle position of the 13-mer-oligonucleotides
(ODN1 and ODN2, where X or Y were Cl-deaza-dG (2), Br-deaza-dG
(3) or I-deaza-dG (4)). The synthesized ODNs were cleaved from the
CPG and the Pac groups were removed with 50 mM K2CO3 in MeOH
at room temperature. Subsequently, the products were purified
using reverse-phase HPLC and carefully treated with 1e5% acetic
acid to remove the DMTr protective group while retaining the
halogen atom at the 8-position. Finally, the structure and purity of
all synthesized ODNs were confirmed by MALDI-TOF mass
measurements.
Because the conformation around the N-glycosidic bond of the
Fig. 2. Structures of oxo-dG and 8-halogenated-7-deaza-dG derivatives.
2. Results and discussion
Scheme 1. Reagents and conditions: (a) acetic anhydride, pyridine, 88%. (b) NRS (R¼Cl,
Br, and I), DMF, 86%, 51%, and 21%, respectively. (c) 7 N ammonia in MeOH, 60e70%. (d)
TMSCl, pyridine, followed by phenoxyacetic anhydride, pyridine, 70e80%. (e) (1)
DMTrCl, pyridine. (2) 2-cyanoethyl-N,N0-diisopropyl-chlorophosphoroamidite, DIPEA,
CH2Cl2, 39e61% over two steps.
The syntheses of the 8-halogenated-7-deaza-dG derivatives are
presented in Scheme 1. The 50- and 30-hydroxyl groups of 7-deaza-
dG (1) were acetylated using acetic anhydride in pyridine.31 The
regioselective halogenation reactions at the 8-position of the

2042
Y. Yin et al. / Tetrahedron 70 (2014) 2040e2047
8-halogenated-deaza-dG nucleosides are reflected in the chemical
shifts if their 1H and 13C NMR spectra,21 the NMR spectra of dG, oxo-
dG, deaza-dG (1), Cl-deaza-dG (2), Br-deaza-dG (3), and I-deaza-dG
(4) were measured in DMSO-d6. The selected chemical shifts are
summarized in Table 1. The differences in the chemical shifts of the
signals corresponding to those of dG are summarized in Fig. 3A. 8-
Remarkably, the large upfield shifts of the C20 were commonly
observed for all 8-halogenated-deaza-dG. The downfield shifts of
the H20 signal increased in the order of Cl, Br, and I, most likely
reflecting the electron withdrawing nature of the C8-halogen atom.
The signals of H10 and H20 did not show large chemical shift
changes.
To obtain insight into preference in syn- or anti conformation
around the N-glycosidic bond of 8-halogenated-7-deaza-dG, each
conformer was estimated by quantum mechanical calculation in
vacuo. The reported crystal structures were utilized as the initial
structures.35 The calculated energies are summarized in Table 2,
and the optimized structures are displayed in Fig. 4. For dG and
deaza-dG, the relative energies indicated that the anti conforma-
Table 1
Selected chemical shift (ppm) of corresponding diol compoundsa
Base dG
8-Oxo-dG deazadG Cl-deazadG Br-deazadG I-deazadG
C10
C20
C3
C40
C50
H200
H20
H10
C7
82.5
81.2
35.7
71.4
87.3
62.4
2.96
1.91
6.02
db
82.2
39.2
71.0
86.9
62.0
2.30
2.05
6.29
102.2
116.7
6.25
6.89
83.0
37.2
71.0
87.2
62.1
2.99
2.03
6.31
101.4
115.0
6.32
db
84.2
37.3
71.1
87.3
62.2
3.04
2.02
6.28
105.4
101.9
6.41
db
86.8
37.5
71.3
87.4
62.3
3.12
1.97
6.18
112.9
71.3
6.54
db
39.5
70.7
87.5
61.7
2.49
2.18
6.10
db
tion is favored (
spectively). In contrast, the anti and syn conformations of oxo-dG
are relatively isoenergetic ( E is 0.06 kcal/mol). Cl-deaza-dG
DE are ꢀ3.09 kcal/mol and ꢀ3.48 kcal/mol, re-
D
slightly prefers the anti conformation, albeit with a small ener-
getic difference. The anti conformation is favorable for Br-deaza-
dG (the relative energies of Cl-deaza-dG and Br-deaza-dG are
ꢀ0.54 and ꢀ1.75 kcal/mol, respectively). An intramolecular in-
teraction between the bromide atom at the 8-position and the
oxygen atom on the sugar may contribute to the stabilization of
anti Br-deaza-dG, because the distance of these atoms is within
C8
H7
H8
135.2 151.6
db
db
db
7.90
a
1H and 13C NMR shifts were recorded using 0.04 M solutions of the corre-
sponding diol compounds in DMSO-d6.
36
ꢀ
b
the sum of their van der Waals radii (3.29e3.32 A). Notably, I-
deaza-dG prefers the syn conformation ( E is 0.94 kcal/mol).
Lack of the corresponding atom.
D
These quantum chemical calculations indicate that the 8-
halogenated-7-deaza-dG derivatives are more likely to prefer the
syn conformation than is the non-substituted deaza-dG. The
chemical shifts were predicted by HF/6e311G(d,p)_DMSO or HF/
LanL2DZ/6e311G(d,p)_DMSO using the structures derived from
Oxo-dG showed medium and large upfield shifts of the C10 and C20
signals, respectively, and a small downfield shift of the H20 sig-
nals.21,34 These changes were interpreted as evidence of its syn
conformation. In contrast, 7-deaza-dG (1) displayed the same
chemical shifts as dG, except for the C8 signal, when its chemical
shifts were calculated by assuming 100% anti-conformer.
Table 2
DFT energies for the 8-halogenated-7-deaza-dG derivatives in vacuoa
Nucleoside
E (a.u.)
DE (kcal/mol)
Bond length
of R-C8 (A)
ꢀ
dG (anti)
dG (syn)
oxodG (anti)
oxodG (syn)
deazadG (anti)
deazadG (syn)
Cl-deazadG (anti)
Cl-deazadG (syn)
Br-deazadG (anti)
Br-deazadG (syn)
I-deazadG (anti)b
I-deazadG (syn)b
ꢀ1041.848521
ꢀ1041.843600
ꢀ1117.087880
ꢀ1117.087980
ꢀ1025.793500
ꢀ1025.787957
ꢀ1485.388305
ꢀ1485.387439
ꢀ3596.900728
ꢀ3596.897944
ꢀ1036.549415
ꢀ1036.550920
1.08271
1.21774
1.08050
1.73135
1.88166
ꢀ3.09
0.06
ꢀ3.48
ꢀ0.54
ꢀ1.75
0.94
2.11099
1.08271
a
A B3LYP/6-31G(d) geometric optimization and frequency analysis in vacuo.
A B3LYP/LanL2DZ/6e31G(d) geometric optimization and frequency analysis in
b
vacuo. Relative energies (DE) were determined as the difference between the anti
and syn conformations (antiesyn); a positive difference indicates that the syn con-
formation is favored.
Fig. 3. The differences in the chemical shifts between dG and 8-oxo-dG, 7-deaza-dG,
and 8-halogenated-7-deaza-dG derivatives. NMR data are shown in Table 1, and the
simulated data were calculated using HF/6e311G(d,p)_DMSO or HF/LanL2DZ/
6e311G(d,p)_DMSO, which were modified with the optimized anti- or syn-dG struc-
ture using B3LYP/6e31G(d)_DMSO. In order to match to the observed values, the
simulated chemical shifts were calculated by the weighted average assuming 75% syn-
conformer and 25% anti-conformer except for 7-deaza-dG, which was calculated as-
suming 100% anti-conformer.
Fig. 4. Optimized structures of the 8-chloro-7-deaza-dG derivatives generated using
B3LYP/6e31G(d) in vacuo.

Y. Yin et al. / Tetrahedron 70 (2014) 2040e2047
2043
modification of the syn- and anti-conformers of dG, and were
corrected with the difference from the observed values for dG.
When the chemical shifts were calculated by assuming weighted
average of 75% syn- and 25% anti-conformer, the predicted
and green bars). These results suggest that the syn preference of 8-
halogenated-7-deaza-dG decreases the stability of base pair
with dC.
Unlike deaza-dG, 8-halogenated-7-deaza-dG showed destabi-
Table 3
Melting temperatures and thermodynamic parameters for the duplexes containing 8-halogenated-7-deaza-dGa
ODN1: 50 d(CTTTCTXCTCCTT) 30
ODN2: 50 d(AAGGAGYAGAAAG) 30
X
Y
Tm
(ꢁC at 2
D
Hꢁ
D
Sꢁ
D
Gꢁ
X
Y
Tm
(ꢁC at 2
D
Hꢁ
D
Sꢁ
D
Gꢁ
310 K
(kcal/mol)
310 K
m
M)
(kcal/mol)
(cal Kꢀ1 molꢀ1
)
(kcal/mol)
m
M)
(kcal/mol)
(cal Kꢀ1 molꢀ1
)
dG
dG
dG
dG
oxodG
oxodG
oxodG
oxodG
deazadG
deazadG
deazadG
deazadG
Cl-deazadG
Cl-deazadG
Cl-deazadG
Cl-deazadG
Br-deazadG
Br-deazadG
Br-deazadG
Br-deazadG
I-deazadG
I-deazadG
I-deazadG
I-deazadG
dC
dA
dG
T
dC
dA
dG
T
dC
dA
dG
T
dC
dA
dG
T
dC
dA
dG
T
dC
dA
dG
T
48.2
33.1
35.7
36.5
44.3
39.8
32.7
33.0
46.9
31.9
34.1
37.2
41.9
30.0
30.7
34.5
41.0
28.2
30.0
34.1
39.7
29.2
30.5
34.7
ꢀ115ꢂ18
ꢀ94.6ꢂ4
ꢀ78.8ꢂ4
ꢀ76.3ꢂ1
ꢀ98.3ꢂ8
ꢀ91.4ꢂ3
ꢀ45.9ꢂ5
ꢀ95.4ꢂ5
ꢀ78.7ꢂ5
ꢀ68.2ꢂ6
ꢀ66.6ꢂ10
ꢀ117ꢂ20
ꢀ77.8ꢂ5
ꢀ121ꢂ20
ꢀ77.5ꢂ6
ꢀ71.0ꢂ6
ꢀ85.7ꢂ5
ꢀ99.6ꢂ12
ꢀ90.3ꢂ13
ꢀ81.0ꢂ2
ꢀ113ꢂ8
ꢀ328ꢂ55
ꢀ280ꢂ13
ꢀ226ꢂ12
ꢀ218ꢂ4
ꢀ13ꢂ0.8
ꢀ7.8ꢂ0.04
ꢀ8.6ꢂ0.04
ꢀ8.8ꢂ0.01
ꢀ11ꢂ0.8
dC
dA
dG
T
dC
dA
dG
T
dC
dA
dG
T
dC
dA
dG
T
dC
dA
dG
T
dC
dA
dG
T
dG
dG
dG
dG
oxodG
oxodG
oxodG
oxodG
deazadG
deazadG
deazadG
deazadG
Cl-deazadG
Cl-deazadG
Cl-deazadG
Cl-deazadG
Br-deazadG
Br-deazadG
Br-deazadG
Br-deazadG
I-deazadG
I-deazadG
I-deazadG
I-deazadG
44.1
35.4
35.7
33.7
42.3
39.2
33.4
32.2
41.6
35.4
32.0
33.1
38.0
30.3
31.4
30.8
37.0
30.1
31.0
30.5
35.3
29.4
31.4
29.3
ꢀ101ꢂ6
ꢀ71.1ꢂ8
ꢀ78.8ꢂ4
ꢀ68.4ꢂ1
ꢀ96.8ꢂ8
ꢀ96.1ꢂ7
ꢀ80.0ꢂ16
ꢀ73.2ꢂ5
ꢀ72.6ꢂ1
ꢀ106ꢂ11
ꢀ68.2ꢂ5
ꢀ85.9ꢂ5
ꢀ88.2ꢂ13
ꢀ74.2ꢂ8
ꢀ294ꢂ58
ꢀ85.4ꢂ11
ꢀ73.4ꢂ7
ꢀ202ꢂ25
ꢀ141ꢂ8
ꢀ90.1ꢂ6
ꢀ73.3ꢂ4
ꢀ108ꢂ14
ꢀ107ꢂ8
ꢀ137ꢂ6
ꢀ288ꢂ20
ꢀ201ꢂ25
ꢀ226ꢂ12
ꢀ194ꢂ2
ꢀ11ꢂ0.2
ꢀ8.7ꢂ0.2
ꢀ8.6ꢂ0.04
ꢀ8.2ꢂ0.01
ꢀ10.6ꢂ0.2
ꢀ9.6ꢂ0.1
ꢀ281ꢂ13
ꢀ263ꢂ9
ꢀ278ꢂ25
ꢀ279ꢂ21
ꢀ233ꢂ52
ꢀ252ꢂ17
ꢀ202ꢂ4
ꢀ9.7ꢂ0.05
ꢀ8.2ꢂ0.16
ꢀ7.8ꢂ0.06
ꢀ11ꢂ0.2
ꢀ122ꢂ15
ꢀ283ꢂ15
ꢀ217ꢂ15
ꢀ195ꢂ20
ꢀ189ꢂ35
ꢀ347ꢂ65
ꢀ218ꢂ15
ꢀ370ꢂ65
ꢀ226ꢂ20
ꢀ202ꢂ18
ꢀ244ꢂ16
ꢀ301ꢂ40
ꢀ268ꢂ42
ꢀ235ꢂ8
ꢀ7.9ꢂ0.6
ꢀ7.8ꢂ0.07
ꢀ10.0ꢂ0.1
ꢀ8.4ꢂ0.15
ꢀ7.8ꢂ0.12
ꢀ7.8ꢂ0.07
ꢀ9.3ꢂ0.34
ꢀ7.2ꢂ0.24
ꢀ3.3ꢂ0.11
ꢀ7.3ꢂ0.27
ꢀ8.9ꢂ0.13
ꢀ4.5ꢂ0.56
ꢀ6.1ꢂ0.16
ꢀ7.0ꢂ0.14
ꢀ8.6ꢂ0.1
ꢀ7.8ꢂ0.14
ꢀ8.1ꢂ0.38
ꢀ9.2ꢂ0.46
ꢀ10.2ꢂ0.1
ꢀ6.4ꢂ0.51
ꢀ7.2ꢂ0.14
ꢀ8.3ꢂ0.01
ꢀ10.2ꢂ0.1
ꢀ6.1ꢂ0.37
ꢀ7.0ꢂ0.36
ꢀ8.2ꢂ0.02
ꢀ10.0ꢂ0.1
ꢀ6.9ꢂ0.25
ꢀ7.6ꢂ0.25
ꢀ8.0ꢂ0.30
ꢀ314ꢂ34
ꢀ195ꢂ18
ꢀ252ꢂ17
ꢀ255ꢂ42
ꢀ216ꢂ28
ꢀ936ꢂ188-
ꢀ252ꢂ35
ꢀ208ꢂ22
ꢀ637ꢂ83
ꢀ435ꢂ27
ꢀ268ꢂ20
ꢀ209ꢂ13
ꢀ328ꢂ46
ꢀ322ꢂ25
ꢀ424ꢂ19
ꢀ333ꢂ26
ꢀ235ꢂ31
ꢀ189ꢂ27
ꢀ295ꢂ45
ꢀ79.9ꢂ10
ꢀ66.1ꢂ8
ꢀ99.4ꢂ14
ꢀ6.3ꢂ0.38
ꢀ7.1ꢂ0.14
ꢀ5.5ꢂ0.13
a
Conditions: 1e8
mM duplex, 100 mM NaCl, and 10 mM sodium phosphate buffer, pH 7.0. The data were analyzed with the melt curve processing program, MeltWin 3.5. Tm
values are for 2
m
M oligonucleotides, and experimental errors are less than 0.5 ꢁC.
chemical shift changes were similar to the observed values (Fig. 3A
vs Fig. 3B). These results suggest that 8-halogenated-deaza-dG are
similar to 8-oxo-dG with respect to their preference for the syn- or
anti-conformation around the N-glycosidic bond.
To elucidate the base pairing properties of duplex DNA (ODN1
and 2) containing the 8-halogenated-7-deaza-dG derivatives, UV
melting experiments were performed in a buffer containing
100 mM NaCl and 10 mM sodium phosphate at pH 7.0. The melting
temperatures (Tm values) for 2 mM oligonucleotides and the ther-
modynamic parameters of the experiment are summarized in
Table 3. The Tm values for each base pair were compared with the
corresponding base pair with dG according to the equation
D
Tm(Z)¼Tm(Z)ꢀTm(dG), where Z represents 8-oxo-dG, deaza-dG or 8-
halogenated-7-deaza-dG. The Tm values for the base pairs formed
with dC and dA are displayed in Fig. 5. The Tm values for the 8-oxo-
D
D
dG:dC base pair are negative (Fig. 5A and C, red bars), indicating
destabilization compared with the natural dG:dC base pair. In
contrast, the DTm value for 8-oxo-dG:dA are positive (Fig. 5B and D,
red bars), representing stabilization compared with the dG:dA base
pair. These results have been interpreted as syn 8-oxo-dG dimin-
ishing the stability of the base pair with dC and enhancing the
stability with dA as shown in Fig. 1. The base pair of syn 8-oxo-dG
with dA produces more stability than the non-canonical base pair
between anti-dG and dA (Fig. 6A)37,38 As discussed in the confor-
mational analysis, the syn preference of 7-deaza-dG is similar to dG.
Therefore, it is reasonable that the base pairs of 7-deaza-dG with dC
and dA did not affect the duplex stability compared to the natural
base pairs of dG:dC and dG:dA (blue bars in Fig. 5A and B). Although
8-halogenated-7-deaza-dG forms a Watson-Crick base pair with dC
(Fig. 6B), the base pair of dC with Cl-deaza-dG, Br-deaza-dG, and I-
deaza-dG were strongly destabilized (Fig. 5A and C, orange, yellow,
Fig. 5. The bar graphs for the
DTm values between the corresponding dG analogues
and dG in 2 M solutions of duplex DNA (
m
DTm(Z)¼Tm(Z)ꢀTm(dG), Z represents modified
dG). ODN1/ODN2 duplexes were used, where X and Y represent 8-oxo-dG, 7-deaza-dG,
and 8-halogenated-7-deaza-dG. ODN1: 50-d(CTTTCTXCTCCTT). ODN2: 30-
d(GAAAGAYGAGGAA).
lizing effects for the base pair with dA (Fig. 5B and D, orange, yel-
low, and green bars). The non-canonical base pair with anti dA most
likely destabilized by the syn preference. In addition, although the

2044
Y. Yin et al. / Tetrahedron 70 (2014) 2040e2047
phase C18 columns. The Tm measurements were taken with a UV
A
B
spectrometer and the thermodynamic parameters were analyzed
using the curve-fitting program, MeltWin (V3.5).
4.1.1. 20,50-Di-O-acetyl-8-chloro-7-deaza-20-deoxyguanosine
(6). 20,50-Di-O-acetyl-7-deaza-20-deoxyguanosine
5
(500 mg,
1.43 mmol)was dissolved in anhydrous DMF (10 mL). The flask was
covered before 210 mg (1.57 mmol) of N-chlorosuccinimide was
added to the reaction mixture. The resulting solution was stirred at
room temperature for 30 min. The reaction mixture was diluted
with 20 mL of methanol and stirred for 40 min. The organic solvent
was concentrated, and the residue was purified by column chro-
matography (silica gel, CHCl3/MeOH¼20:1) to give 6 (470 mg, 86%)
as a white powder. Mp 177e179 ꢁC. IR (cmꢀ1): 3175, 1743, 1667,
C
D
1624, 1527, 1431, 1367, 1238, 1043. 1H NMR (DMSO-d6)
d 10.61 (br s,
1H), 6.35e6.29 (m, 4H), 5.39e5.35 (m, 1H), 4.40e4.36 (m, 1H),
4.18e4.10 (m, 2H), 3.39e3.31 (m, 1H), 2.29e2.35 (m, 1H), 2.07 (s,
3H), 1.99 (s, 3H). 13C NMR (DMSO-d6):
150.4, 114.9, 101.6, 99.7, 82.8, 80.9, 74.2, 63.5, 34.1, 20.5, 20.4. HR-
ESI-MS m/z [MþH]þ calcd for C15H18ClN4O6 385.0909, 387.0886,
found 385.0941, 387.0935.
d 170.1, 170.0, 157.5, 152.6,
Fig. 6. Schematic structure of base pair formation (A) anti-dG or deazadG with dA, (B)
syn-8-halogenated derivatives with anti-dC, (B) syn-8-halogenated derivatives with
anti dA and computational models, (D) Cl-deazaG:A, calculated using Gaussian09 at
the PM6 level.
4.1.2. 20,50-Di-O-acetyl-8-bromo-7-deaza-20-deoxyguanosine
shape of syn 8-halogenated-7-deaza-dG may fit with the Wat-
soneCrick face of dA, a lack of hydrogen bonding with the 1 N of dA
results in insufficient stabilization. Thus, the destabilizing effect
exerted by 8-halogenated-deaza-dG on the base pair formation
with dA might imply that 8-halogenated-7-deaza-dG is preferable
in the syn conformation in DNA. CD spectra for duplexes formed
with the 8-halogenated-7-deaza-dG derivatives indicate that these
nucleoside derivatives did not disturb the canonical B-type duplex
DNA formation (data not shown).
(7). 20,50-Di-O-acetyl-7-deaza-20-deoxyguanosine
5
(500 mg,
1.43 mmol) was dissolved in anhydrous DMF (24 mL). The flask was
covered and then 280 mg (1.62 mmol) of N-bromosuccinimide was
added in several portions over 1 h. The reaction mixture was di-
luted with 20 mL of methanol and stirred for 40 min. The organic
solvent was concentrated, and the residue was purified by column
chromatography (silica gel, CHCl3/MeOH¼30:1) to provide
7
(313 mg, 51%) as a white powder. Mp 150e152 ꢁC. IR (cmꢀ1): 3168,
1741, 1669, 1625, 1547, 1430, 1365, 1235, 1049. 1H NMR (DMSO-d6)
d
10.60 (br s, 1H), 6.44 (s, 1H), 6.32 (s, 2H), 6.27 (t, 1H, J¼7.4 Hz),
5.42e5.37 (m, 1H), 4.43e4.38 (m, 1H), 4.20e4.10 (m, 2H),
3.49e3.42 (m, 1H), 2.35e2.29 (m, 1H), 2.07 (s, 3H), 1.99 (s, 3H). 13
3. Conclusion
C
In summary, 8-halogenated-7-deaza-dG derivatives were
designed to be 8-oxo-dG mimics in shape, and their syntheses were
achieved via the reaction between 7-deaza-dG and N-halogenated-
succinimides. These compounds were incorporated into 13-mer-
oligonucleotides. It has been shown by NMR that the conformation
around the N-glycosidic bond of 8-halogenated-7-deaza-dG is
preferably in the syn conformation similar to 8-oxo-dG. The base-
paring properties of 8-halogenated-7-deaza-dG in DNA were
investigated by measuring the denaturation temperature, and it
was shown that its base pair with dC destabilizes duplexes. These
results imply that 8-halogenated-7-deaza-dG also prefer syn con-
formation when incorporated in DNA. The base pair between syn 8-
halogenated-deaza-dG and anti dA did not stabilize the duplexes,
most likely due to a lack of hydrogen bonding with 7CeH. Thus, 8-
halogenated-7-deaza-dG analogs have been shown to be suitable
candidates to investigate the effects on efficiency in the 8-oxo-dG
repair system. Further study is now on-going along this line.
NMR (DMSO-d6) d 170.1, 170.0, 157.4, 152.4, 151.1, 105.6, 101.4, 101.2,
84.2, 80.9, 74.3, 63.6, 34.1, 20.7, 20.5. HR-ESI-MS m/z [MþH]þ calcd
for C15H18BrN4O6 429.0404, 431.0386, found 429.0382, 431.0416.
4.1.3. 20,50-Di-O-acetyl-8-iodo-7-deaza-20-deoxyguanosine (8). Two
hundred milligrams (0.57 mmol) of 20,50-di-O-acetyl-7-deaza-20-
deoxyguanosine 5 was dissolved in anhydrous DMF (10 mL). The
flask was covered and then 198 mg (0.88 mmol) of N-iodosuccini-
mide was added in several portions over 1 h. The reaction mixture
was diluted with 10 mL of methanol and stirred for 40 min. The
organic solvent was concentrated, and the crude product was pu-
rified using short silica gel column (CHCl3/MeOH¼5:1). The residue
was separated by column chromatography (silica gel, CHCl3/
MeOH¼50:1 to 40:1) to give 8 (58 mg, 21%) as a white powder. Mp
174e176 ꢁC. IR (cmꢀ1): 3305, 1733, 1674, 1630, 1597, 1557, 1429,
1362, 1247, 1030. 1H NMR (DMSO-d6)
d 10.56 (br s, 1H), 6.56 (s, 1H),
6.27 (s, 2H), 6.18 (t, 1H, J¼7.4 Hz), 5.44e5.40 (m, 1H), 4.46e4.42 (m,
1H), 4.22e4.10 (m, 2H), 3.54e3.47 (m, 1H), 2.33e2.27 (m, 1H), 2.07
4. Experimental section
(s, 3H), 1.99 (s, 3H). 13C NMR (DMSO-d6)
d 170.1, 170.0, 157.2, 152.0,
151.5, 113.1,103.6, 86.7, 80.9, 74.4, 71.4, 63.7, 34.3, 20.7, 20.5. HR-ESI-
4.1. General conditions
MS m/z [MþH]þ calcd for C15H18IN4O6 477.0266, found 477.0297.
The 1H NMR (400 MHz, 500 MHz) and 13C NMR (125 MHz)
4.1.4. 8-Chloro-7-deaza-20-deoxyguanosine
(2). Compound
6
spectra were recorded using solutions in CDCl3 or DMSO-d6. The 31
P
(470 mg,1.22 mmol) was suspended in 7 N ammonium in methanol
solution (15 mL). The resulting solution was stirred at room tem-
perature overnight. The organic solvent was concentrated, and the
residue was purified by column chromatography (silica gel, CHCl3/
MeOH¼10:1) to give 2 (258 mg, 70%) as a white powder. Mp
188e190 ꢁC. IR (cmꢀ1): 3232, 1675, 1646, 1608, 1553, 1519, 1437,
NMR (161 MHz) spectra were recorded using 10% phosphoric acid
in D2O as an internal standard (0 ppm). The IR spectra were
obtained with an FTIR spectrometer. The high-resolution mass
spectra were recorded on the positive mode with an ESI-TOF mass
spectrometer using bradykinin and angiotensin as the internal
standards. The MALDI-TOF mass spectra were recorded on the
negative mode. HPLC purification was carried out with reverse-
1368, 1100. 1H NMR (DMSO-d6)
2H), 6.25 (s, 2H), 5.18 (d, 1H, J¼4.0 Hz), 4.86 (m, 1H), 4.36e4.30 (m,
d 10.45 (br s, 1H), 6.35e6.28 (m,

Y. Yin et al. / Tetrahedron 70 (2014) 2040e2047
2045
1H), 3.77e3.71 (m, 1H), 3.63e3.42 (m, 2H), 3.03e2.94 (m, 1H),
J¼7.8 Hz), 5.24 (d, 1H, J¼4.0 Hz), 4.85 (s, 2H), 4.74 (t, 1H, J¼5.8 Hz),
4.40e4.39 (m, 1H), 3.77e3.75 (m, 1H), 3.66e3.48 (m, 2H),
2.06e1.99 (m, 1H). 13C NMR (DMSO-d6)
d 157.5, 152.5, 150.3, 115.0,
101.4, 99.6, 87.2, 83.0, 71.0, 62.1, 37.2. HR-ESI-MS m/z [MþH]þ calcd
3.22e3.12 (m, 1H), 2.12e2.07 (m, 1H). 13C NMR (DMSO-d6)
d 170.7,
for C11H14ClN4O4 301.0698, 303.0673, found 301.0719, 303.0658.
157.6, 155.5, 147.9, 145.5, 129.5, 121.3, 114.6, 106.1, 105.6, 105.3, 87.3,
84.2, 70.8, 66.3, 62.0, 37.0. HR-ESI-MS m/z [MþH]þ calcd for
4.1.5. 8-Bromo-7-deaza-20-deoxyguanosine
(3). Compound
7
C19H20BrN4O6 479.0561, 481.0543, found 479.0592, 481.0593.
(278 mg, 0.65 mmol) was suspended in 7 N ammonium in methanol
solution (15 mL). The resulting solution was stirred at room tem-
perature overnight. The organic solvent was concentrated, and the
residue was purified by column chromatography (silica gel, CHCl3/
MeOH¼10:1) to give 3 (133 mg, 60%) as a white powder. Mp
197e199 ꢁC. IR (cmꢀ1): 3230, 1651, 1606, 1504, 1434, 1368, 1099. 1H
4.2.3. 2-N-Phenoxylacetyl-8-iodo-7-deaza-20-deoxyguanosine
(11). This compound (119 mg, 70%) was isolated as a white powder.
Mp 204e206 ꢁC. IR (cmꢀ1): 3196, 1682, 1611, 1547, 1498, 1433, 1243,
1058, 756. 1H NMR (DMSO-d6)
d 11.62 (br s, 1H), 11.36 (br s, 1H),
7.33e7.29 (m, 2H), 7.00e6.96 (m, 3H), 6.78 (s, 1H), 6.29 (t, 1H,
J¼7.6 Hz), 5.22 (d, 1H, J¼4.0 Hz), 4.85 (s, 2H), 4.74 (t, 1H, J¼5.8 Hz),
4.45e4.39 (m, 1H), 3.78e3.75 (m, 1H), 3.70e3.50 (m, 2H),
NMR (DMSO-d6)
5.18 (d,1H, J¼4.0 Hz), 4.94 (m,1H), 4.37e4.32 (m,1H), 3.77e3.71 (m,
1H), 3.64e3.44 (m, 2H), 3.07e3.01 (m, 1H), 2.06e1.98 (m, 1H). 13
d 10.66 (br s, 1H), 6.41 (s, 1H), 6.29e6.21 (m, 3H),
C
3.08e2.99 (m, 1H), 2.12e2.03 (m, 1H). 13C NMR (DMSO-d6)
d 170.7,
NMR (DMSO-d6)
d
157.3, 152.3, 150.8, 105.4, 101.9, 101.3, 87.3, 84.2,
157.6, 155.3, 148.4, 145.1, 129.5, 121.3, 114.6, 113.5, 107.7, 87.3, 86.5,
71.1, 62.2, 37.3. HR-ESI-MS m/z [MþH]þ calcd for C11H14BrN4O4
76.1, 70.9, 66.3, 62.0, 37.1. HR-ESI-MS m/z [MþH]þ calcd for
345.0193, 347.0174, found 345.0183, 347.0193.
C19H20IN4O6 527.0422, found 527.0421.
4.1.6. 8-Iodo-7-deaza-20-deoxyguanosine (4). Compound 8 (235 mg,
0.50 mmol) was suspended in 7 N ammonium in methanol solution
(15 mL). The resulting solution was stirred at room temperature for
overnight. The organic solvent was concentrated, and the residue
was purified by column chromatography (silica gel, CHCl3/
MeOH¼10:1) to give 4 (126 mg, 65%) as a white powder. Mp
171e173 ꢁC. IR (cmꢀ1): 3106, 1673, 1630, 1533, 1474, 1435, 1391,
4.3. General procedure of synthesis for amidite compounds
To a solution of the appropriate Pac-protected compound in
pyridine was added 4,40-dimethoxytrityl chloride (1.5 equiv) at
room temperature. After stirring for 1 h, the reaction mixture was
quenched with saturated aqueous NaHCO3 and extracted with
EtOAc three times. The organic layer was washed with brine, dried
over Na2SO4, and evaporated under vacuum. The residue was puri-
fied using column chromatography. The resulting DMTr-protected
compound was dissolved in CH2Cl2. To this solution was added dii-
sopropylethylamine (6 equiv) and 2-cyanoethyl-N,N0-diisopropyl-
chlorophosphorodiamidite (3 equiv) at 0 ꢁC. After stirring for 2 h, the
reaction mixture was quenched with saturated aqueous NaHCO3
and extracted with EtOAc. The organic layer was washed with brine,
dried over Na2SO4, and evaporated under vacuum. The residue was
purified using column chromatography (silica gel, Hexane/EtOAc).
1102. 1H NMR (DMSO-d6)
(m, 3H), 5.16 (d, 1H), 4.91 (m, 1H), 4.38e4.32 (m, 1H), 3.77e3.72 (m,
1H), 3.66e3.46 (m, 2H), 3.16e3.08 (m, 1H), 2.01e1.93 (m, 1H). 13
d 10.51 (br s, 1H), 6.54 (s, 1H), 6.21e6.09
C
NMR (DMSO-d6)
d 157.1, 151.9, 151.2, 112.9, 103.7, 87.4, 86.8, 71.3,
62.3, 37.5. HR-ESI-MS m/z [MþH]þ calcd for C11H14IN4O4 393.0054,
found 393.0075.
4.2. General procedure for synthesis of Pac-protected
compounds
To a solution of 2, 3 or 4 (0.33 mmol) in pyridine (10 mL) was
4.3.1. The b-phosphoroamidite derivative of 2-N-phenoxylacetyl-8-
added TMSCl (500
mL, 3.96 mmol). The reaction mixture was stirred
chloro-7-deaza-20-deoxyguanosine (12). Compound 12 (39% for
for 1 h at room temperature, and phenoxyacetic anhydride (143 mg,
0.50 mmol) was added. After stirring overnight at room tempera-
ture, the reaction was quenched with water (10 mL) and extracted
with CHCl3. The organic layer was evaporated, and the residue was
purified by column chromatography (silica gel, CHCl3/MeOH¼50:1
to 40:1 to 30:1).
two steps) was isolated as a white powder. 1H NMR (CDCl3)
d
7.41e7.25 (m, 8H), 7.20e7.13 (m, 3H), 7.09e7.05 (m, 1H),
6.95e6.92 (m, 2H), 6.72e6.67 (m, 4H), 6.59 (s, 1H), 6.50 (t, 1H,
J¼7.0 Hz), 4.51 (t, 2H, J¼3.0 Hz), 4.20e4.15 (m, 1H), 3.90e3.75 (m,
1H), 3.73 (s, 3H), 3.72 (s, 3H), 3.64e3.52 (m, 3H), 3.42e3.06 (m, 4H),
2.60e2.33 (m, 3H). 1.30e1.06 (m, 12H). 31P NMR (CDCl3)
149.05. ESI-MS (m/z): 959.3, 961.4 [MþNa]þ.
d 149.23,
4.2.1. 2-N-Phenoxylacetyl-8-chloro-7-deaza-20-deoxyguanosine
(9). This compound (115 mg, 80%) was isolated as a white powder.
Mp 236e238 ꢁC. IR (cmꢀ1): 3215, 1681, 1611, 1550, 1499, 1441, 1244,
4.3.2. The b-phosphoroamidite derivative of 2-N-phenoxylacetyl-8-
bromo-7-deaza-20-deoxyguanosine (13). Compound 13 (45% for
1053, 1009. 1H NMR (DMSO-d6)
d
11.66 (br s, 1H), 11.52 (br s, 1H),
two steps) was isolated as a white powder. 1H NMR (CDCl3)
7.33e7.25 (m, 2H), 6.99e6.88 (m, 3H), 6.59 (s, 1H), 6.42 (t, 1H,
J¼7.6 Hz), 5.25 (s, 1H), 4.83 (s, 2H), 4.75 (s, 1H), 4.38 (m, 1H),
3.76e3.74 (m, 1H), 3.59e3.49 (m, 2H), 3.11e3.04 (m, 1H), 2.12e2.09
d 7.41e7.25 (m, 8H), 7.18e7.12 (m, 3H), 7.09e7.05 (m,1H), 6.94e6.91
(m, 2H), 6.73 (s, 1H), 6.71e6.66 (m, 4H), 6.48e6.46 (m, 1H), 4.49 (t,
2H, J¼4.6 Hz), 4.20e4.17 (m, 1H), 3.80e3.76 (m, 1H), 3.73 (s, 3H),
3.72 (s, 3H), 3.70e3.52 (m, 3H), 3.44e3.10 (m, 4H), 2.60e2.32 (m,
(m, 1H). 13C NMR (DMSO-d6)
d 170.7, 157.7, 155.6, 147.2, 145.8, 129.5,
121.3, 118.6, 114.6, 104.1, 102.2, 87.2, 82.9, 70.7, 66.3, 61.9, 36.9. HR-
ESI-MS m/z [MþH]þ calcd for C19H20ClN4O6 435.1066, 437.1045,
found 435.1113, 437.1036.
3H). 1.30e1.06 (m, 12H). 31P NMR (CDCl3)
(m/z): 1004.3, 1006.8 [MþNa]þ.
d 149.03, 148.88. ESI-MS
4.3.3. The
iodo-7-deaza-20-deoxyguanosine (14). Compound 14 (61% for two
steps) was isolated as a white powder. 1H NMR (CDCl3)
7.41e7.25
(m, 8H), 7.17e7.12 (m, 3H), 7.09e7.05 (m,1H), 6.95 (s,1H), 6.93e6.90
(m, 2H), 6.70e6.65 (m, 4H), 6.38 (t, 1H), 4.46 (t, 2H), 4.21e4.17 (m,
1H), 3.80e3.76 (m, 1H), 3.72 (s, 3H), 3.71 (s, 3H), 3.70e3.52 (m, 3H),
b-phosphoroamidite derivative of 2-N-phenoxylacetyl-8-
4.2.2. 2-N-Phenoxylacetyl-8-bromo-7-deaza-20-deoxyguanosine
(10). This compound (115 mg, 73%) was isolated as a white powder.
Mp 194e196 ꢁC. IR (cmꢀ1): 3207, 1678, 1611, 1548, 1498, 1438, 1245,
d
1056, 1009, 754. 1H NMR (DMSO-d6)
d
11.66 (br s, 1H), 11.45 (br s,
1H), 7.33e7.29 (m, 2H), 7.00e6.96 (m, 3H), 6.68 (s, 1H), 6.39 (t, 1H,

2046
Y. Yin et al. / Tetrahedron 70 (2014) 2040e2047
3.47e3.12 (m, 4H), 2.60e2.31 (m, 3H). 1.30e1.06 (m, 12H). 31P NMR
nucleoside analogues with 9-methyl-adenine were calculated us-
ing the Gaussian09 program at PM6.
(CDCl3)
d
149.27, 149.13. ESI-MS (m/z): 1051.3 [MþNa]þ
4.8. NMR study
4.4. DNA synthesis
Assignments of the anomeric stereochemistry and conformation
about glycosyl bonds for the corresponding diol compounds were
confirmedbyNMRspectroscopyusinga0.04 MsolutioninDMSO-d6.
Oligonucleotides (ODNs) were synthesized using standard DNA
synthesis procedures with an NTS-H6 DNA/RNA synthesizer
(NIHON TECHNO SERVICE CO., Ltd). The synthesized ODNs were
cleaved from the resin and deprotected at the nucleobase using
50 mM K2CO3 in methanol at room temperature for 4 h. The DMTr-
ODN was purified by HPLC (nacalai tesque COSMOSIL C18-ARII)
using a linear gradient (A: 0.1 M TEAA buffer, B: CH3CN, B concd 10
to 40%/20 min). The DMTr-group on the ODNs was removed with
1e5% aqueous acetic acid at room temperature; the resulting DMTr-
off ODNs were washed with ether. The purities and structures were
confirmed with MALDI-TOF Mass measurements. 8-Oxo-dG and 7-
deaza-dG containing ODNs were purchased from Gene Design Inc.
or Genenet.
Acknowledgements
This study was supported by a Grant-in-Aid for Scientific Re-
search (S) (Grant Number 21229002) and Challenging Exploratory
Research (Grant Number 24659047 for Y.T.) from the Japan Society
for the Promotion of Science (JSPS). Y.Y. was supported by the China
Scholarship Council (Grant Number 20116220540).
Supplementary data
Supplementary data of NMR spectra of the new products are
available. Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.tet.2014.01.047.
MALDI-TOF mass results of synthesized ODNs (m/z)
ODNs
Calcd
Found
ODN1 50 d(CTTTCT X CTCCTT)
X¼8-chloro-7-deaza-dG (2)
X¼8-bromo-7-deaza-dG (3)
X¼8-iodo-7-deaza-dG (4)
ODN2 50 d(AAGGAG Y AGAAAG)
Y¼8-chloro-7-deaza-dG (2)
Y¼8-bromo-7-deaza-dG (3)
Y¼8-iodo-7-deaza-dG (4)
3873.56
3917.51
3965.50
3872.61
3916.70
3964.46
References and notes
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4180.70
4228.69
4135.65
4179.86
4227.74
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