Native Kenyan plants as possible alternatives to methyl bromide in soil fumigation.

Methyl bromide (CH3Br) is a biocidal fumigant used widely in crop production and commodity preservation worldwide. CH3Br escapes to the stratosphere and releases bromine atom (Br), which contributes to significant destruction of the ozone (O3). It is therefore necessary to explore alternatives to CH3Br that are environmentally safe and suitable for resource-poor African farmers. We present here the results of a study on the inhibitory activity of crude extracts from Kenyan medicinal plants against three soil pathogens, Fusarium oxysporum, Alternaria passiflorae, and Aspergillus niger. Crude organic extracts of Warburgia ugandensis Sprague, Azadirachta indica A. Juss, Tagetes minuta and Urtica massaica were active against all soil pathogens, while those from U. massaica were not. Chromatographic purification of the crude extract of W. ugandensis provided two pure compounds, muzigadial (1) and muzigadiolide (5). The minimum inhibitory concentration (MIC value) for muzigadial (1) ranged from 5 to 100mug/ml. Muzigadiolide (5) was not active. Greenhouse tests of W. ugandensis extracts against F. oxysporium pathogen showed the most effective inhibitory concentration to be at least 5 mg/ml. Quantitative structure-activity relationship (QSAR) models were used to rationalize the variation in biological activities of muzigadial (1), warburganal (2), polygodial (3), ugandensidial (4), muzigadiolide (5), azadirachtin (6), and CH3Br. The models were based on several molecular descriptors including LogP, van der Waals surface area (VDW(A)), van der Waals volume (VDW(v)), dipole moment, total energy, polarizability, and differences between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO gap).

Limonoids from Nigerian Harrisonia abyssinica and their stimulatory activity against Striga hermonthica seeds.

Deoxyobacunone (1), a new limonoid with a double bond in ring D, has been isolated from the root bark of Harrisonia abyssinica collected in Nigeria. Also, the known limonoids obacunone (2), harrisonin (3), 12beta-acetoxyharrisonin (4), and pedonin (5) have been isolated. The structure of 1 was assigned unambiguously by spectral data analysis. Under laboratory conditions, 10(-3)-10(-5) M concentrations of compounds 1-5 exhibited significant stimulatory activity (12-98%) against conditioned Striga hermonthica seeds. This study provided useful insight regarding the functionalities required for activity of limonoids against Striga seeds. The variation in activity was rationalized through quantitative structure-activity relationship (QSAR) models based on several molecular descriptors including van der Waals volume (VDW(v)), molecular polarizability (alpha), dipole moment (mu), log P, and the differences between the highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO-LUMO gap).

GR 24 enantiomers: synthesis, NMR spectroscopy, X-ray crystallography, and separation by chiral electrokinetic capillary chromatography.

Chiral discrimination of enantiomers of 2-methyl-4-(2-oxo-2,3,3a,8b-tetrahydro-4H-indeno[1,2b]furan-3-yl-iden emethoxy)but-2-en-4-olide (commonly referred to as GR 24) by three polymeric chiral surfactants (PCS) is studied by use of chiral polymeric surfactant capillary electrophoresis (CPSCE). The CPSCE results indicate that the optical configurations of valine residues on the PCS backbone affect chiral resolution and elution order of GR24 stereoisomers. The L- and D-forms of poly(sodium N-undecanoyl valinate) provide baseline separation of all four enantiomers while the DL-form separates diastereomers of GR 24 (1). A model is presented rationalizing the migration behavior and chiral resolution of 1 in CPSCE. The actual configuration of the stereogenic centers of GR 24 and 3-[(2,5-dihydro-3-methyl-2-oxo-5-furanyl)oxo]-methylene-3,3a,6,6a-tetrah ydro-2H-cyclopenta[b]furan-2-one (GR 7) is established by a concerted application of high-resolution nuclear magnetic resonance spectroscopy and X-ray crystallography.

Capillary electrophoretic separation of binaphthyl enantiomers with two polymeric chiral surfactants: 1H-nuclear magnetic resonance and fluorescence spectroscopy study.

The use of the water-soluble polymeric chiral surfactants (PCS), sodium N-undecanoyl-L-valinate (poly-L-SUV) and sodium undecanoyl-L-isoleucinate (poly-L-SUI) as buffer additives in electrokinetic chromatography (EKC) afforded the separation of racemic mixtures of 2,2'-dihydroxy-1,1'-binaphthyl (BOH) and 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BNP). The apparent binding constants of the PCS to the enantiomers of BNP and BOH were obtained through 1H-nuclear magnetic resonance (1H-NMR) titrations and fluorescence spectroscopy, respectively. The 1H-NMR titration studies show that the BNP enantiomers are localized in the hydrophobic micellar pockets of PCS and form complexes of a 1:1 stoichiometry. The binding constants of PCS of BOH were determined from a Benesi-Hildebrand treatment of the fluorescence data. The EKC data corroborate those of the binding constants, supporting the formation of inclusion complexes. A model rationalizing the chiral discrimination of the enantiomers of BNP is proposed based on the intermolecular interactions observed in 1H-NMR data.

NMR and molecular mechanics study of pyrethrins I and II.

Bioassay-directed fractionation of the organic extract of the Kenyan pyrethrum flowers (Chrysanthemum cinerariaefolium Vissiani) resulted in the isolation of two natural pyrethrin esters, pyrethrin I (PI) and pyrethrin II (PII) as the major constituents. These esters elicited inhibition of the multiple drug resistant (MDR) Mycobacterium tuberculosis. The high-field (1)H and (13)C nuclear magnetic resonance (NMR) chemical shifts of PI and PII were unequivocally assigned using modern two-dimensional (2D) proton-detected heteronuclear multiple-quantum coherence (HMQC) and heteronuclear multiple-bond correlation (HMBC) experiments. The conformations of both esters were deduced from (1)H-(1)H vicinal coupling constants and confirmed by 2D nuclear Overhauser effect spectroscopy (NOESY). Computer molecular modeling (MM) studies revealed that PI and PII molecules adopt a "love-seat" conformation in chloroform (CDCl(3)) solution.

Activity of extracts from nonhost legumes on the germination of Striga hermonthica seeds.

Dichloromethane and water extracts from leaves, stems and roots of ten different legume cultivars were assayed for germination stimulation of moisture and temperature conditioned Striga hermonthica (Del.) Benth. seeds. Optimum concentrations for most active extracts were in the undiluted or dilution × 10 range. Unlike water extracts, all dichloromethane extracts induced some stimulatory activities at the various concentrations. A potentially high stimulant producing cultivar, V. unguiculata cv. it-84d-975 was identified. The nondestructive thin-layer chromatography (TLC) technique was employed in investigating the dichloromethane-soluble substances in legume cultivars. TLC data indicated that most legume species and cultivars produce four chromatographically different germination stimulants with retention factors of 0.10, 0.60, 0.70 and 0.77.