Angewandte
Chemie
two times higher than that of IdUd. The enhanced deiodina-
tion of IUd by compound 4 can be attributed to the favored
hydrogen bonding between the amine side chain of 4 and the
ribose sugar moiety of IUd. The initial rates for the
debromination of BrdUd by 1–7 were found to be remarkably
lower than that of the deiodination under identical conditions.
A comparison of the initial rates indicates that 1 exhibits the
À2
À1
highest activity (12.7 Æ 0.4 10 mm min ) of the series and,
quite unexpectedly, there is a decrease in the activity when the
À2
À1
selenium compounds 2 (9.9 Æ 0.1 10 mm min ) and 3
10.9 Æ 0.2 10 mm min ) are used for the debromination
reactions (Figure 2B; Table S1). The initial rates observed for
À2
À1
(
À2
À1
molecules 4 (10.8 Æ 0.5 10 mm min ) and 5 (11.1 Æ 0.3
À2
À1
1
0
mm min ) were also similar to or even lower than that
of 1. It should be noted that the deribosylation generally
observable during the debromination of BrdUd with an
[10]
enzyme nucleophile was not observed with 1–7.
To understand the effect of the deoxyribose and ribose
moieties in the dehalogenation, we treated compounds 1–7
with 5-iodouracil (IU) and 5-bromouracil (BrU). Compared
to IdUd or IUd, IU undergoes much slower deiodination by
1–7 (Figures 2B and 3A), suggesting that the presence of the
deoxyribose or ribose moiety in IdUd and IUd facilitates the
deiodination reactions. Similar to the debromination of
BrdUd, debromination of BrU were found to be 3–24 times
slower than the deiodination of IU (Figure 2B; Table S1).
Furthermore, 4 was found to be about two times less active
than 3, indicating that the introduction of a basic amino group
decreases the activity, which is in contrast to the deiodination
activity of these compounds. Although the reactivity of BrU/
BrdUd toward sulfur and selenium nucleophiles is expected
to be different from that of IU/IdUd, the remarkable
difference in the reaction rates suggests that the debromina-
tion and deiodination may follow different pathways. It
Figure 2. A) Structures of compounds 1–7 and different nucleosides
used in this study. B) Comparison of initial rates for the dehalogena-
tions of IdUd, IUd, and BrdUd by 1–7.
selenium compounds can mediate the dehalogenation of
halogenated nucleobases and nucleosides in aqueous media
under physiological conditions. We also show that the
mechanism of debromination by sulfur and selenium
compounds is different from that of deiodination.
The dehalogenation reactions were carried out at 378C in
phosphate buffer (100 mm; pH 7.0) and were monitored by
HPLC and mass spectrometry. When IdUd was treated with
should be noted that a common S 2 mechanism has been
N
proposed for both deiodination and debromination of 5-halo-
[
6]
2’-deoxyuridines by cysteine. Dehalogenation of IdUd or
5
10
BrdUd by TSase does not involve the cofactor N ,N -
methylene-THF (Figure 3B). Interestingly, the formation of
5-thioalkyl-dUMP (8) was only observed with BrdUMP,
which is unusual as the intermediate INT-2 is expected to be
a common intermediate for both BrdUMP and IdUMP.
These observations also suggest that the deiodination and
debromination may follow different mechanistic pathways.
Although the dehalogenation of BrU or IU or the
corresponding nucleosides by cysteine and selenocysteine
follows the mechanism proposed for the TSase, the mecha-
nism by which 1–5 mediate the dehalogenation depends on
the nature of halogen atom. When the reaction of 2 with BrU
1
having two thiol moieties, the reaction afforded the
expected deiodinated product 2’-deoxyuridine (dUd). The
initial rate for this reaction was found to be 16.8 Æ 0.5
À2
À1
1
0
mm min (Figure 2B) and a significant enhancement in
the reaction rate was observed when 2 with a thiol/selenol pair
À2
À1
was used for the deiodination (27.4 Æ 1.3 10 mm min ).
Similar to our earlier observations on the deiodination of
[9b,c]
T4,
the activity of 3 having two selenol moieties (321.0 Æ
À2 À1
1
3.6 10 mm min ) was found to be an order of magnitude
higher than that of 2 (Figure 2B). A further enhancement
77
in the activity was observed when the substituted
was followed by Se NMR spectroscopy at different temper-
atures (Figure 3C), a new signal was observed at d = 222 ppm,
which is shifted downfield relative to that of 2 (206 ppm),
indicating the formation of 10 (Figure 3D). A similar
reactivity was observed for 3. For both 2 and 3, the reactions
led to the formation of uracil and the corresponding
dichalcogenides (Figure 3D). Therefore, the mechanism of
debromination of BrU by 1–3 follows an addition–elimina-
tion pathway (Figure 3D). In contrast, the deiodination of IU
proceeds through a XB-mediated pathway (XB = halogen
À2
À1
compounds 4 (431.3 Æ 6.5 10 mm min ) and 5 (349.1 Æ
À2
À1
4
.4 10 mm min ) were employed. The activity of cysteine
(
6) and selenocysteine (7) was comparable to that of 1.
Interestingly, the deiodination of 5-iodouridine (IUd) was
found to be more facile than that of IdUd by 1–7 (Figure 2B;
see also Table S1 in the Supporting Information), indicating
that the nature of the sugar moiety plays an important role in
the deiodination. The rate of deiodination of IUd by 4
À2
À1
(
1058.7 Æ 39.6 10 mm min ) was found to be more than
Angew. Chem. Int. Ed. 2015, 54, 9298 –9302
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