Inhibitory Potency Against Plant PDHc

As described in Chaps. 2-6, most of l-(substimted phenoxyacetoxy)alkyl-phosphonates with 2,4-Cl2 as on the phenoxy-benzene ring exhibited much better herbicidal activity against dicotyledons than monocotyledons. Therefore, some alkylphosphonates and the corresponding sodium salts with 2,4-Cl2 as were chosen to examine their inhibition against PDHc from both dicotyledons and monocotyledons plants in vitro. Mung bean (V. radiata) and pea (Pisum sativum) as representative dicotyledonous plants, rice Oryza sativa) as a representative monocotyledonous plant were used in enzyme assays. The IC50 values of the tested compounds against plant PDHc are listed in Table 7.3. 

1 The Effect of Compounds Against PDHc from Dicotyledons 

As shown in Table 7.3, all tested compounds except IC-5 and IJ-1 exhibited significant inhibition against PDHc from both mung bean and pea. IA-4, IC-1, IC-22, IG-21, and IID-10 exhibited powerful inhibitory potency against PDHc from the tested dicotyledonous plants (IC50 30 pM). The most powerful inhibitor against 

3 Selective Inhibition Against PDHc from Different Plants 

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The action mechanism of herbicidal activity of the alkylphosphonates lo was explored on the basis of biochemical experiments. Representative alkylphosphonates lo were selected to examine their inhibition against plant PDHc in vitro including enzyme activity assays, kinetic experiments, and enzyme selective inhibition. Correlation between the inhibitory potency against plant PDHc and the herbicidal activity of the alkylphosphonates lo was summarized on the basis of bioassay. 

R and R in the phosphonates were also essential for inhibitory potency against plant PDHc. When 2,4-dichloro as Y on the phenoxy-benzene ring was kept constant, inhibitory potency against plant PDHc could be greatly enhanced by the modifications of R and R. ... 

The analyses for structure-inhibitory potency relationships indicated that the improvement of inhibitory potency against plant PDHc required an optimal combination of R R, R, and Y . As shown in Scheme 7.4, when 2,4-Cl2 as Y are kept constant, inhibitory potency of the compound could be greatly enhanced by the chemical modification of R, R and R in the phosphoms-containing moiety. Especially the conversion of phosphonate esters to their salts could make great improvement in inhibitory potency against plant PDHc. Thus IIB-2 was found to exhibit the best inhibitory potency against rice PDHc. 

Relationships between Inhibitory Potency Against Plant PDHc and Herbicidal Activity... 

Comparing the inhibitory potency against plant PDHc with the herbicidal activity of tested compounds, it could be found that their herbicidal activity correlated well with their enzyme inhibition including selectivity and SAR. Based on the general stmcrnre of phosphonates lo, the relationship between enzyme inhibition and herbicidal activity is summarized as follows ... 

Table 7.6 Herbicidal activity and inhibitory potency against plant PDHc of some alkylphosphonates...

The above results indicate that the improvement of both inhibitory potency against plant PDHc and herbicidal activity requires a reasonable combination of R, R, R and Y . 

Scheme 7.6 Possible metabolic pathway of IID-10 in plant and inhibitory potency against pea PDHc of possible metabolic products...

All tested compounds, such as IC-22, IG-21, and HD-10 showed much higher inhibitory potency against PDHc from dicotyledons (pea or mung bean) than their inhibitory potency against PDHc from monocotyledon (rice). They also exhibited higher herbicidal activity against dicotyledonous plants (such as pea, common amaranth, rape) than monocotyledonous plants (such as barnyard grass and rice). The data are listed in Table 7.6. 

IC-22 (HW02, clacyfos), IG-21 (HWS) and their sodium salts were demonstrated to be effective inhibitors against plant PDHc. IC-22 and IG-21 exhibited potent selective inhibition against dicot PDHc, but lower inhibitory potency against monocotyledonous rice PDHc. It showed that both compounds could selectively inhibit the growth of dicotyledonous plants due to their selective inhibition against dicot PDHc El. 

HWS exhibited a potent inhibitory potency against dicot plant (pea) PDHc, which was consistent with its good selectivity against dicotyledonous weeds. HWS has a very weak inhibitoiy potency against PDHc fi om mammal (pig heart) (Chap. 7). HWS also has a low acute toxicity profile against rat and other tested animals. It is safe to non-target species, such as honey bee, quad, zebra fish, and silkworm. 

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