MST-Department of Chemistry

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Browsing MST-Department of Chemistry by Author "Ahmed Hassanali"

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    Bioevaluation of insecticidal and repellent plants from central region of Kenya and chemical identification of bioactive derivatives
    (2012-01-26) Simiyu, Silas Khamala; Ndiege Isaiah O.; Ahmed Hassanali
    Despite considerable control efforts, malaria still remains the most prevalent and devastating disease in the tropics. With more than 40% of the world population at risk, malaria undermines the welfare of several South American, Asian and African states, endangering the survival of children (killing one child every 30 seconds) and straining scarce resources. It is estimated that US $ 2 billion is spent on malaria control and treatment programmes in Africa annually. The problem is becoming increasingly difficult to manage because of the continuous intensification and spread of resistance to anti-malarial drugs by the parasites. This poses a serious threat in increased severity of disease and health. Vector resistance to insecticides is a recurring theme and a major problem in malaria control programmes. The safety and efficacy of N N-diethyl-mtoluamide (DEET), the most potent of the modern synthetic repellent, is questionable. Other problems associated with repellents include vector resistance, avoidance and frequent repetitive application. The recent discovery of trans-cis-nepetalactone and p-mentane-3,8-diol that are more effective and environmentally friendly than DEET justifies bioprospecting from plants. In our search for new repellents we have continued with bioprospecting activities of the Kenyan flora. The essential oils from leaves of Artemisia afra, Senecio moorei, Cineraria grandifolia (Asteraceae), Nepeta azurea, Satureja pseudomensis (Labitae), Clausena anisata (Rutaceae) and Pseudocarum eminii (Umbelliferae) from Mt. Kenya region were evaluated for their repellency and insecticidal activity against adult female An. gambiae mosquitoes. The essential oil of Nepeta azurea was the most effective repellent (RD50 6.5x10-7mg/cm2). The essential oils from other species studied also showed significant repellency effect. The order of repellency was: P. eminii > S. moorei > C. anisata > S. pseudomensis > C. grandifolia > A. afra with RD50 = 4.39x104 , 1.27x10-3 , 7.86x10-3, 1.15x10-2, 2.2x10-2, 3.07x10-2 mg/cm2, respectively. The essential oil from A. afra was insecticidal at 1% w/v (LD50= 2.26x10-2 mg/cm2). The constituent compounds were identified through GC, GC-MS and GC co-injection and bio-assayed for repellent and insecticidal properties.
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    Repellence of essential oil of nigella sativa l. seeds against anopheles Gambiae and identification of the active blend
    (Kenyatta University, 2015-04) Ndirangu, Githui Ephantus; Margaret Mwihaki Ng’ang’a; Ahmed Hassanali
    Anopheles gambiae mosquitoes are vectors of malaria because of their ability to transmit Plasmodium falciparum parasites. The major impact of malaria is in subSaharan Africa where at least 90% of the deaths from malaria occur. In Kenya, malaria accounts for 30% of all outpatients and 19% of all admissions to health facilities. Malaria can also affect the quality of labour negatively and also can lead to low productivity through absenteeism. Anopheles gambiae is the vector associated with stable malaria transmission in Africa because it is strongly anthropophilic, feeding exclusively on humans. One of the greatest challenges facing malaria control is the spread and intensification of parasite resistance to treatment. P. falciparum has become resistant to almost all malaria drugs including artemisinin and its derivatives. This means that there is need to come up with effective methods to control mosquito populations as well as diversifying methods of malaria treatment. There is no single mosquito control method which is effective in all situations. Today, the most effective insect repellent is DEET but it has been associated with medical complications when used for a long time. In many parts of the world plant-derived natural products have been used to repel mosquitoes and other insects. The objective of this study was to evaluate the repellence of the essential oil on Nigella sativa L. seeds using An. gambiae and identify the active constituents and blend. Nigella sativa L. seeds were ground and hydro-distilled. Then bioassays of essential oil was conducted on human subjects against newly emerged female An. gambiae using DEET as the positive control. It was noted that the repellence (98.81±1.19 and 100.00±0.00 at concentration of 0.01g/ml and 0.1g/ml respectively) of the essential oil against An. gambiae was comparable to that of DEET (100.00±0.00 and 100.00±0.00 at concentration of 0.01g/ml and 0.1g/ml respectively) at higher doses; however, it showed lower repellence (36.97±1.81 and 50.41±2.87 against 51.11±13.32 and 86.22±4.51 of DEET at concentration of 0.0001g/ml and 0.0001g/ml respectively) at lower doses. GC-MS and GC-EAD (Gas Chromatography-linked Electro Antennography) analyses of the essential oil led to the identification of eight bioactive constituents namely α-thujene (19), longifolene (38), 1, 2, 3, 4, 5-pentamethylcyclopentane (18), α-pinene (20), βpinene (22), tetradecane (24), p-cymene (11), and α-longipinine (37). Subtractive bioassays to characterize the constituents that contributed most to the repellence of the oil was then carried out. The most repellent blend was found to contain (+)-β-pinene (41), (-)-β-pinene (42), (+)-α-pinene (39), (-)-α-pinene (40), α-longipinene (37), tetradecane (24) and 1,2,3,4,5 pentamethylcyclopentane (18) (RD75 = 3.763), though less repellent than DEET (RD75=1.630). Bioassay of pure (+)-α-pinene (39) and (-)-αpinene (40) showed that (+)-α-pinene (39) was a better repellent than (-)-α-pinene (40). More studies need to be undertaken on the essential oil of N. sativa seeds to determine the optical stereo-chemistry of the α-pinene (20) and β-pinene (22) and also establish whether α-thujene (19) and longifolene (38) contribute to repellency or not against An. gambiae. These results form the basis of downstream development of the appropriate blends for personal protection against An. gambiae.