| Emamectin Benzoate, synthesized from abamectin B1by substitution of an epi-methylamino (NHCH3) group for a hydroxyl (OH) group at the4"-position on the disaccharide and produced as a benzoate salt, is widely used as a new type of high effective antibiotic insecticide and acaricide and is developed for the control of lepidopterous pests on crops e.g. vegetables, cottons, tobaccos, tea leaves, and flowers. In this paper, we optimized the preparation method of Emamectin Benzoate, studied the photodegradation of Emamectin Benzoate at various concentration under UV light at various exposure time and its toxicities to Chilo suppressalis (Walker), in order to provide scientific basis for the cause of degradation of Emamectin Benzoate, formulation producing and proper application in field.As to the experiment design, the Emamectin Benzoate solution treated by UV light exposure was used for quantitative determination of the active ingredient (MAB1a), analysis of the photolytic products and indoor toxicity test, which was more reasonable than either determining the content or analyzing the photolytic products in published literature. The result illustrated the rules of photodegradation of Emamectin Benzoate from two aspects, the variation of the content of MAB1a and the toxicity decrease to insecticides. One of the photolytic products, MAB1a-8,9-epoxide, had not been reported in the literature yet.Emamectin Benzoate was synthesized from abamectin B1in lab, by substituting an epi-methylamino (NHCH3) group for a hydroxyl (OH) group at the4"-position on the disaccharide and then produced as a benzoate salt. There were six steps of reactions:5-hydroxyl group protection,4"-hydroxyl group transformed to carbonyl group,4"-carbonyl group transformed to methylamino group,5-hydroxyl group deprotection, benzoate salt formation of4"-methylamino group, and recyclization. The synthesized product was confirmed as Emamectin Benzoate by comparing1H-NMR,13C-NMR, IR, UV, MS spectrums consecutively with literature. The total yield was approximately62%.The standard curve of MAB1a HPLC peak area to concentration was established, and there was a good linear correlation between peak area and concentration at the range from4mg/L to472mg/L. The analytical method had a high precision and showed that the average assay of Emamectin Benzoate AI was95.56%. The analytical method was used to test the photodegradation rate of MAB1a at various concentration (1mg/L,10mg/L,100mg/L,1000mg/L) under UV light at different exposure time (3h,6h,12h,24h,36h,48h,72h,96h,120h) separately. The result indicated that photodegradation was an important influence factor of the stability of Emamectin Benzoate. Among the experimental data, the photodegradation rate of MAB1a at1mg/L increased fast, which reached70.02%at3h exposure and100%at48h exposure; the photodegradation of MAB1a at10mg/L was slightly slow than the former, which reached66.54%at3h exposure and100%at120h; the photodegradation of MAB1a at100mg/L significantly slowed down than the two former, which reached40.92%at3h exposure and93.67%at120h exposure; not only the photodegradation of MAB1a at1000mg/L was slowed down significantly than the three former, but also the photodegradation rate decreased significantly, which reached9.00%at3h exposure and only58.27%at120h exposure. The result also indicated that the photodegradation rate was relative to the concentration and exposure time. In the range of tested concentration, the lower the concentration of MAB1a was, the faster the photodegradation rate increased as the exposure time prolonged; on the contrary, the higher the concentration of MAB1a was, the slower the photodegradation rate increased as the exposure time prolonged, and the photodegradation rate decreased significantly. The active ingredient of Emamectin Benzoate was photodegradated under UV light, and then the photolytic products were separated and accumulated by HPLC. Three photolytic products were accumulated and their structures were elucidated by ESI-MS and NMR as8,9-Z-photoisomer, MAB1a-8,9-epoxide, AB1a respectively, in which MAB1a-8,9-epoxide had not been reported in the literature yet.To study the activity of Emamectin Benzoate under UV light exposure, we determined its toxicities to Chilo suppressalis (Walker), which was one of the target insects, at various exposure time. Result:For the MAB1a at concentration of1mg/L, the activity was totally lost at3h UV light exposure due to the photolysis; for the MAB1a at concentration of10mg/L, the value of LD50to four-age Chilo suppressalis was one third of the untreated samples after3-12h UV light exposure, but after24h and36-120h, none activity was found; for the MAB1a at concentration of100mg/L, the activities of the samples disposed separately at3～24h,48-72h and96～120h were respectively1/2,1/5, and1/15～1/16of the untreated sample; for the MABla at concentration of1000mg/L, the activities of the samples disposed at48-72h and96-120h were respectively1/2,2/7～1/3of the untreated sample, but the activity of sample disposed at3-24h were nearly equal to the untreated sample. The result indicated that the toxicity of Emamectin Benzoate to Chilo suppressalis (Walker) under UV light exposure was not only relative to the hours of light exposure but also the concentration of Emamectin Benzoate.From the above study on photodegradation and indoor toxicity test, it indicated that photodegradation under UV light was an important cause for influencing the stability of Emamectin Benzoate and its insecticidal activities; it also revealed that if Emamectin Benzoate was produced as a formulation of low concentration and the applied concentration in field was at a low level, its persistent period would be shor after usage, and the control effect would be bad. However, it did not mean the higher of the concentration of Emamectin Benzoate formulation and the higher applied concentration in field, the more effective it would be, because it would cause unnecessary waste due to the increased photolysis of the active ingredients. |
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