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Authors: FATIMAH, T. I.
Keywords: Fruit tree
Theobroma cacao L
Antibacterial activity
Essential oils
Issue Date: Aug-2016
Abstract: Essential Oils (EOs) are volatile secondary metabolites characterised by a strong odour and widely used for pharmacological and industrial applications. There is dearth of information on chemical compositions and bioactivities of EOs of some fruit trees in Nigeria. This study was therefore designed to extract and characterise the EOs from selected fruit trees, screen the EOs for bioactivity as well as to isolate and characterise non-volatile constituents from Theobroma cacao L. pod-husk due to its availability. The plant samples (Carica papaya L., Theobroma cacao L., Persea americana M., Ananas comosus (L) Merr and Chrysophyllum albidum G. Don) were collected in Ibadan, identified and authenticated at the Herbarium of Forest Research Institute of Nigeria, Ibadan. Essential oils were extracted from the leaves, stem-barks, root-barks, fruits, peels, pod-husk and seeds of the plants using hydro-distillation method and analysed by Gas Chromatography (Flame Ionization Detector and Mass Spectrometry) techniques. The antibacterial activity of the EOs at 20 µg/mL was assayed on two Gram-positive and four Gram-negative bacteria using Microplate Alamar Blue Assay measured in UV/Visible spectrophotometer. The antioxidant activity of the EOs at 20 µg/mL was determined by radical scavenging procedure while insecticidal activity was evaluated by contact toxicity test using three grain pests. Pure compounds were isolated from methanol extract of T. cacao pod-husk by chromatographic techniques. The chemical structures of the compounds were elucidated using Infrared, Nuclear Magnetic Resonance and Mass Spectroscopic techniques. Data were analysed using descriptive statistics. Twenty-seven EOs were obtained and their yield ranged from 0.1 to 1.2% (v/w). The major components in P. americana EOs were β-caryophyllene (12.7%; leaf), tetradecanal (31.8%; root-bark), globulol (25.4%; peel), (Z,Z)-4,15-octadecadien-1-ol acetate (21.8%; seed), tetradecanal (24.9%; stem-bark) and p-xylene (40.5%; fruit). Carica papaya EOs mainly comprised benzylisothiocyanate (89.1%; seed), octadecanol (62.5%; root), octadecanol (71.1%; stem), m-xylene (35.1%; stem-bark), heptadecanol (25.2%; fruit), phytol (21.8%; leaf), benzylisothiocyanate (71.5%; root-bark) and 9-hexadecen-1-ol (16.9%; peel). P-xylene (62.4%; fruit), p-xylene (29.9%; shoot) and tetradecanoic acid (8.6%; peel) dominated A. comosus EOs. The principal constituents in T. cacao EOs were hexadecanoic acid (78.7%; leaf), o-xylene (53.3%; seed), ledol (33.6%; pod-husk) and β-bisabolol (17.3%; stem-bark). The dominant compounds in C. albidum EOs were m-xylene (66.7%; seed), p-xylene (21.4%; seed-bark), ethylhexadecanoate (19.9%; leaf), hexadecanoic acid (14.7%; stem-bark), m-xylene (53.1%; root-bark) and hexadecanoic acid (14.7%; fruit-bark). The chemical constituents for twenty-one of the EOs of the fruit plants were obtained for the first time ever. Theobroma cacao leaf EO exhibited the highest inhibition against Gram-negative bacteria at 78.6%, while P. americana fruit and peel EOs showed the highest inhibition against Gram-positive bacteria at 69.9%. Persea americana fruit and seed EOs displayed the highest and lowest radical scavenging activity at 42.1 and 1.2%, respectively. The EOs showed activity between 0 to 20% in insecticidal assay. Column chromatography of the methanol extract of T. cacao pod-husk yielded three known triterpenes: 24-hydroxy-9,19-cycloanost-25-en-3-one, stigmast-5-en-3β-ol and ergosta-5α,8α-epidioxy-6,22-dien-3β-ol. The EOs have antibacterial and antioxidant properties which is indicative of their potential as sources of pharmaceuticals.
Description: A Thesis in the Department of Chemistry, Submitted to the Faculty of Science in partial fulfillment of the requirements of the Degree of DOCTOR OF PHILOSOPHY of the UNIVERSITY OF IBADAN
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