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Z.-S. Li et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3723–3727
halogen is in the position of R1 will be synthesized in the future
to see if enhanced activity can be observed.
Table 2
Ki values of 3e and 3f against wild type AtAHAS and mutant type AtAHAS W547L
Quite a few nonsymmetrical disulfides display significant herbi-
cidal activities for the rape root growth bioassay at 100 mg/L.
Amongst them, 3e and 3f exhibit 92.6% and 87.8% inhibition val-
ues, comparable to the 88.5% inhibition data of MSE. Compounds
3a, 3c, 3g, 5k and 5m show >70% inhibitions, while compounds
3b, 3h, 3i, 3k, 3o, 4g, 5b, 5d, 5i, 5l and 5n possess >50% inhibitions.
For compounds 3 (3a–3i, 3k and 3o) and 5 (5d–5o), there exist a
general correlation between their in vitro AtAHAS inhibition values
and their in vivo rape root growth inhibition data at 100 mg/L con-
centration. At the same conditions, compounds 4 show very weak
rape root growth inhibition and there is not a good correlation be-
tween the in vitro and in vivo biological activities except 4e.
Although we have shown that the nonsymmetrical aromatic
disulfides display AHAS inhibition and rape root growth inhibition,
it is not clear whether the herbicidal activity of the disulfides is due
to AHAS inhibition. For this aim we set up an assay that the BCAAs
were added to the solution where the rape seeds germinate and 3e
was chosen for the assay.3 The biological activity of 3e was mea-
sured at concentrations of 50 and 25 mg/L. For comparison, MSE
was used as a control for this assay, at concentrations of 1 and
0.5 mg/L. From Table 3, we can see that the rape root is inhibited
by 12.7% only in the presence of BCAAs. The percent inhibition data
of 3e are 61.9% and 39.5% in the absence of the BCAAs at the tested
concentrations, whereas the corresponding inhibition values are
52.4% and 25.8% in the presence of the BCAAs. Considering that
the BCAAs alone inhibit the rape root growth inhibition, the herbi-
cidal activity of 3e is actually weakened to 20% and 25%. For MSE,
the reversal inhibition data are about 47% at 1 mg/L and 55% at
0.5 mg/L. It should be pointed out, that the inhibition cannot be to-
tally reversed at both concentrations even for the commercial MSE.
The reversal inhibition result implies that the rape root growth
inhibition of 3e has relationship with the BCAA biosynthesis
interruption.
Compound
Ki (
l
M) for AtAHAS
W574L
Resistance factor
Wild type
3e
3f
4.78 0.31
3.28 0.35
0.26 0.03
3.0 0.1
30.0 8.0
19.0 4.5
85.3 1.92
6.3
5.8
320
157
MSE
Imazaquina
470
2
a
Data from Ref. 28.
Although compounds 3e and 3f show desirable rape root
growth inhibition at 100 mg/L, it is more valuable to see whether
these disulfides can exhibit greenhouse herbicidal activity. Hence
these two disulfides were evaluated both their pre-emergence
and post-emergence herbicidal activities against monocotyledon
and dicotyledon species using pot bioassay (Table 4). For compar-
ison, compound 6 was also subjected to the pot assay since its
greenhouse activity has not been evaluated yet. MSE was used as
a control commercial sulfonylurea herbicide. The dosages for the
disulfides are 1500 g/ha and 750 g/ha, and for the control herbicide
Figure 1. Three-dimensional contour maps for the CoMFA model. In map A,
sterically favored and disfavored regions are shown in green and yellow, respec-
tively. In map B, electrostatic favored and disfavored regions are shown in blue and
red, respectively.
the compounds containing the [1,3,4]thiadiazole groups, one H
atom in R1 gives a general stronger AtAHAS inhibition at 100 mg/
L than other disulfides where R1 is a bulky group; when R1 is the
same group, however, the variation of substituent in R2 does not
influence the AHAS inhibition so obviously. For the series of disul-
fides with [1,3,4]oxadiazole groups, as we have not obtained the
compounds where R1 is H atom, their in vitro data are a bit lower,
and only 5n show 90% inhibition at 100 mg/L. Based on the AtAHAS
inhibition data, CoMFA 3D-contour maps for all the disulfides were
built. The leave-one-out q2 is 0.538 when the optimum number of
components is 7, and the non-cross-validated r2 is 0.913, with a
standard error of estimate of 0.137 and an F value of 47.875. The
steric and electrostatic contributions are 54.3% and 45.7%, respec-
tively. As depicted in Figure 1, compound 3e was used as the tem-
plate to illustrate the quantitative structure–activity relationships.
For the steric contour map (Fig. 1A), a bulky group is favorable for
better inhibition in the green contour region and such a group is
likely to decrease the activity in the yellow region. For the electro-
static contour map (Fig. 1B), a positive charged substituent is
favorable in the blue region for AHAS inhibition and a negative
charged group in the red region will decrease the activity. There-
fore we will design new compounds that fulfill this requirement.
For instance, nonsymmetrical disulfides that –CF3, –NO2 or any
Table 3
The effect of exogenous BCAAs supplement on rape root growth inhibitiona
Rape root length
(mm)
Rape root growth
inhibition (%)
Blank control
Blank control with
BCAAs
33.6
29.3
—
12.7
3e
25 mg LÀ1
25 mg LÀ1
(+BCAAs)
50 mg LÀ1
50 mg LÀ1
(+BCAAsÀ) 1
20.3
24.9
39.5
25.8
12.8
16.0
61.9
52.4
MSE 0.5 mg L
0.5 mg LÀ1
(+BCAAs)
11.0
25.1
67.3
25.2
1 mg LÀ1
9.9
21.9
70.4
34.9
1 mg LÀ1
(+BCAAs)
a
Valine, leucine and iso-leucine were added to the compound emulsion at final
concentrations of 0.5 mM each.