Emamectin
Emamectin is a semi-synthetic macrocyclic lactone insecticide derived from avermectins, a group of natural compounds produced by the soil bacterium Streptomyces avermitilis, and is most commonly formulated as emamectin benzoate for practical applications. It targets the nervous systems of insects, particularly lepidopteran larvae, causing paralysis and death, and is valued for its translaminar activity that allows penetration into plant tissues.[1][2] Developed by Merck & Co., Inc. as a second-generation avermectin, emamectin was first marketed in 1997 in countries including Israel and Japan, with U.S. Environmental Protection Agency (EPA) registration following in 1999 under product codes such as Proclaim and Denim.[2] Its chemical structure consists primarily of a 9:1 mixture of the benzoate salts of 4"-epi-methylamino-4"-deoxyavermectin B1a and B1b, with a molecular formula of C56H81NO15 and low water solubility (approximately 0.024 g/L at pH 7).[2] Emamectin exhibits moderate acute toxicity to mammals (oral LD50 of 50-76 mg/kg in rats) but is classified as highly toxic to bees (contact LD50 of 3.5 ng/bee) and aquatic invertebrates (EC50 of 0.00004 mg/L).[2] The compound's mode of action involves binding to glutamate-gated chloride channels in invertebrate nerve and muscle cells, increasing chloride ion influx, hyperpolarizing the cell membrane, and disrupting nerve signal transmission, which results in feeding cessation, paralysis, and eventual insect mortality within hours to days.[2][1] In the environment, emamectin benzoate demonstrates low mobility in soil (Koc values of 25,363–730,000) and moderate persistence, with aerobic soil half-lives ranging from days to 193 days and photodegradation half-lives of 3.6–10.9 days in water.[2] Emamectin is registered for use on a wide range of crops, including cotton, peanuts, tobacco, leafy vegetables (e.g., cabbage, lettuce), fruiting vegetables (e.g., peppers, tomatoes), and pome fruits (e.g., apples, pears), primarily against lepidopteran pests such as bollworms, diamondback moths, and leafrollers.[3] In forestry, it is applied via trunk injection (e.g., under the brand Tree-äge) to protect ash trees from emerald ash borer larvae, providing systemic control for up to two years with minimal non-target impact when used appropriately.[2] Its efficacy, combined with reduced application rates compared to earlier avermectins like abamectin, has made it a key tool in integrated pest management programs, though regulatory tolerances limit residues in harvested commodities to ensure consumer safety.[3]Chemical Identity
Molecular Structure
Emamectin is a semi-synthetic derivative of avermectin B1, also known as abamectin, specifically identified as 4"-deoxy-4"-epi-methylaminoavermectin B1.[4] This modification involves the introduction of a methylamino group at the 4" position of the terminal sugar moiety, enhancing its insecticidal potency compared to the parent compound.[4] Emamectin exists primarily as a mixture of two homologues, emamectin B1a (>90%) and emamectin B1b (<10%), differing only in the alkyl substituent at the C-25 position (ethyl for B1a and methyl for B1b).[4] It is commonly formulated as the benzoate salt, emamectin benzoate, for stability and application. The core structure of emamectin features a 16-membered macrocyclic lactone ring system characteristic of the avermectin family, comprising a polycyclic aglycone core in the southern hemisphere fused to a spiroketal ring and bearing an allylic hydroxyl group at C-5.[5] Attached at the C-13 position is a disaccharide moiety in the northern hemisphere, consisting of two α-L-oleandrose units linked by a glycosidic bond, with the terminal oleandrose modified at the 4" position.[4] Key functional groups include the lactone carbonyl, multiple hydroxyl groups (e.g., at C-3, C-5, and C-7), a trisubstituted double bond at C-22/23, and the distinctive 4"-methylamino group, which imparts basicity to the molecule.[5] The molecular formula of the emamectin free base (B1a) is C49H75NO13, with a molecular weight of 886.1 g/mol, while the B1b homologue is C48H73NO13 (872.1 g/mol).[5] For the benzoate salt form, the formula is C56H81NO15 (B1a benzoate, 1008.3 g/mol) or C55H79NO15 (B1b benzoate, 994.2 g/mol).[4] Emamectin possesses multiple chiral centers, with the specified stereochemistry as (10E,14E,16E,22Z)-(1R,4S,5′S,6S,6′R,8R,12S,13S,20R,21R,24S), contributing to its diastereomeric and enantiomeric complexity.[4] In comparison to abamectin, which has a hydroxyl group at the 4" position of the terminal oleandrose, emamectin features the epi-methylamino substitution (NHCH3 in place of OH), resulting in improved systemic activity and potency against lepidopteran pests.[4] This alteration maintains the overall avermectin scaffold but shifts the electron density and hydrogen-bonding potential at the sugar terminus, optimizing interactions with target biological sites.[5]Physical and Chemical Properties
Emamectin benzoate is typically obtained as a white to off-white crystalline powder, which facilitates its handling in solid formulations.[4] This physical form contributes to its stability during storage and transport under ambient conditions.[4] The compound exhibits a melting point range of 141–146 °C, indicating moderate thermal stability before decomposition occurs above 148 °C in air.[6] Its solubility in water is low and pH-dependent, measuring approximately 24 mg/L at pH 7 and 25 °C, which limits its mobility in aqueous environments but enhances its persistence on treated surfaces.[4] In contrast, it shows high solubility in organic solvents, such as acetone at 140 g/L and methanol at 270 g/L (both at 25 °C), reflecting its lipophilic nature.[6] Key physicochemical parameters include an octanol-water partition coefficient (logP) of 5.0–5.7 at pH 7 and 20–25 °C, underscoring its strong affinity for lipids and potential for bioaccumulation in non-aqueous phases.[7][6] The pKa values are 4.2 for the benzoic acid moiety and 7.6 for the amine group, influencing its ionization and solubility behavior across pH ranges.[6] Emamectin benzoate demonstrates good stability under neutral to slightly acidic conditions (pH 5–8 at 25 °C), with hydrolysis half-life exceeding 19 weeks at pH 9; however, it degrades more readily in strong alkaline environments.[4] Photolytic stability in aqueous solution yields a half-life of 0.5–65 days at pH 7 and 25 °C, depending on irradiation intensity.[4] Spectroscopic characteristics aid in its identification and quality control. Ultraviolet-visible absorption shows a maximum at 245 nm (molar absorptivity approximately 37,000 L mol⁻¹ cm⁻¹ in neutral conditions), useful for analytical detection.[6] Infrared (IR) and nuclear magnetic resonance (NMR) spectra, including ¹H-NMR and ¹³C-NMR, confirm the molecular structure through characteristic peaks for avermectin-derived macrolide rings and the benzoate ester.[4] Due to its inherent low aqueous solubility, emamectin is commonly formulated as the benzoate salt to enhance dispersibility in pesticide products, such as emulsifiable concentrates or water-dispersible granules, improving efficacy in agricultural applications.[8]| Property | Value/Details | Conditions | Source |
|---|---|---|---|
| Melting Point | 141–146 °C | - | [6] |
| Water Solubility | 24 mg/L | pH 7, 25 °C | [4] |
| Acetone Solubility | 140 g/L | 25 °C | [6] |
| logP | 5.0–5.7 | pH 7, 20–25 °C | [7][6] |
| pKa (amine) | 7.6 | 25 °C | [6] |
| UV λ_max | 245 nm | Neutral solution | [6] |