Selective Polarity- and Adsorption-Guided Extraction/Purification of Annona sp. Polar Acetogenins and Biological Assay Against Agricultural Pests

Abstract

Annonaceae acetogenins (AG) comprise a family of natural chemical modifications of long-chain fatty acids (C_35-37) bearing one to several hydroxyls (less often oxo), middle-chain tetrahydrofuran rings, and a 7-lactonized, α/β-unsaturated carboxyl group. Acetogenins’ strong biological activity as larvicides, pesticides, and antitumorals is dependent on these structural variations. The hydroxylation degree is particularly important for these etfects. Seeds, albeit rich in fats (mostly triacylglycerols, [TAG]), are a nonpredatory source of these drugs as compared to other botanical parts such as roots and stems. Conventional lipid extractions lead to quantitative lipid recovery and then the unfavorable natural ratio of TAG:AG in the range >90:<0.1 These extracts thus require, for instance, partitions and extensive sílica gel column Chromatographic steps, in order to enrich or purify the AG fraction(s). Great operational difficulties result from the similar polarity and mol. wt. range of TAG and AG when carrying out these purification steps. An alternative fast two-step procedure to obtain polar acetogenin (pAG)-enriched preparations was developed. The extraction procedure for Annona spp. seeds pAG was carried out with acetonitrile (E° = 0.65; log P = - 0.33) as a polar organosolvent, followed by the adsorption of the solvent-free extract on activated charcoal, then washed with hexane and/or chloroform (E° = 0.0 and 0.40: log P = 3.5 and 2.0) for most of the contaminating TAG removal, and then with acetone (E° = 0.56; log P = - 0.23) to the desorption of an enriched pAG fraction. An alternative procedure for pAG extraction was supercritical fluid extraction (SFE) at moderate thermopressurization conditions (65-82°C; 120-130 atm) using CO₂ with 10% acetonitrile as the polarity modifier. The pAG fractions’ bioactivity was evaluated with the brine-shrimp test (BST), and for feed deterrance, growth inhibition, and lethality against the high-impact agricultural pests Anticarsia gemmatalis and Pseudaletia sequax caterpillars feeding on soya or grass leaves sprayed with a 10% alcohol-stabilized emulsion of pAG.

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