The Volatiles of Acacia Howittii F. Muell.
By Brophy, Joseph J Goldsack, Robert J; Fookes, Christopher J R
Abstract The volatiles obtained from the phyllodes of Acacia howittii have been analyzed by GC and GC/MS and found to contain 1- hexen-3-one (8-21%) and 1-octen-3-one (36-46%) as principal components. The major terpenes present in the oil were alpha-pinene (0.6-7%) and beta-caryophyllene (2-5%). The yield of volatiles was less than 0.1%.
Key Word Index
Acacia howittii, Mimosaceae, phyllode volatiles, essential oil composition, 1-hexen-3-one, 1-octen-3-one.
The genus Acacia sens, strict., which belongs to the Mimosaceae, contains well in excess of 1,000 described species. Most of these are Australian. It should be noted that other plants currendy included in Acacia sens. lat. will be transferred to other genera following a decision by the Nomenclature Section of the XVII International Botanical Congress in Vienna in July 2005. The interested reader is referred to The World Wide Wattle web site for more information about Acacia taxonomy (1).
Acacia howittii F. Muell. (Howitt s Wattle, Sticky Wattle) is a smaU shrub or tree growing to 9 m in height, with a graceful drooping habit and small, pleasant-smelling flower heads, making it a useful garden specimen. It occurs in a small area of eastern Victoria from die upper Macalister River area near Mt Howitt south to near Yarram and east to near Tabberabbera. In spite of its popularity as an ornamental shrub, it is, in fact, a species on the list of rare Australian species. Further details can be found in the monograph by Maslin et al. (2).
When the atmosphere is stdl, especiady between sunset and sunrise, a faint, but not unpleasant acrylate-like odor, can be perceived near the plant. This was especiady noticeable during a recent heatwave (45[degrees]C) in Sydney. The odor comes from the plant s viscid phyllodes (laterally compressed petioles) (3). The viscid material is produced by small glandular trichomes that are scattered over the phyllode surfaces (3). We have noticed that although die sticky phyllodes were not attacked by jawed insects such as beedes, our specimen carried an active and apparently healthy population of an unidentified sap-sucking homopteran.
Little work has been reported on the phyllode or leaf ods of die genus Acacia. A. spirobis subsp. solandri (Bendi.) Pedley (syn. A. spirobis Benth) produced an od in which the main components were the sesquiterpenes beta- caryophyllene, viridiflorene and viridiflorol (4). Also reported in this paper is an unpublished work in which a lemon-scented oil, probably rich in citronellal being obtained from A. hilliana Maiden (I. Southwell, unpublished). AnotherAcacia widi sticky phyllodes, A. nuperrima ssp. cassitera Pedley, was found to exist in two chemical forms in which either the sesquiterpene etiier, kessane, (89%) or alpha-pinene (16%) were the major components (5, ,6). Acacia saligna (Labdl.) H. L.Wendl.has been reported to contain diirty-five components in its leaf oil, of which die major components were nonadecane, pentadecane, heptadecane, heptadecene and mediyl 14-mediylpentadecanoate (7).
Plant material: Leaf material, of unknown provenance, was obtained from plants cultivated in R.J.G s garden at Wahroonga, New South Wales. A voucher sample (RJG894) has been deposited in the J.T. Waterhouse Herbarium, University of New South Wales.
Isolation of oils: The leaf oils were isolated by hydrodistillation with cohobation as previously outlined (8). Analyses of the oils were carried out by gas chromatography and combined gas chromatography-mass spectrometry. The od yields quoted below are weight/weight and based on fresh material.
Identification of Components: Analytical gas chromatography (GC) was carried out on a Shimadzu GC17 gas Chromatograph. A WCOT DB-Wax [60 m x 0.5 mm, film thickness 1 [mu]m] was used, programmed from 50[degrees]-225[degrees]C at 3[degrees]C/min with He at 3.5 mL/min as carrier gas. GC integrations were performed on a SMAD electronic integrator without the use of correction factors. GC/MS was performed on bodi a VG Quattro mass spectrometer operating at 70 eV ionization energy; the column used was DB-Wax [60 m x 0.32 mm, film thickness 0.25 pm] programmed from 35[degrees]-220[degrees]C at 3[degrees]C/min, with He flowing at 35 cm/sec as carrier gas and a Shimadzu QP5000 instrument equipped with a DB-5 column [30 m x0.25 mm, film thickness 0.25 [mu]m]. The column was programmed from 35[degrees]-250[degrees]C at 5[degrees]C/min, helium carrier gas flow rate was 30 cm/sec. Compounds were identified by their identical GC retention times to known compounds and by comparison of their mass spectra with either known compounds or published spectra (9-13). FuU lists of the components identified are given in Table I.
Results and Discussion
The volatiles obtained by steam distillation of the phyllodes and twiglets of A. howittii were obtained in poor yield (
Table I. Compounds identified from the steam volatile oil of Acacia howittii
The principal terpenoid components identified in die od were alpha-pinene (0.6-7%), limonene (1-2%), 1,8-cineole (1-3%), linalool (0.4-0.6%), beta-caryophyllene (2-5%), (E,E)-alpha-farnesene (0.1- 0.5%) and caryophyllene oxide (2-A%).
Several aromatic compounds were identified in the mixture, the principal ones being styrene (3-4%), a phenyl butadiene (0.7-3%), a vinyl anisole (0.1-3%), benzaldehyde (2-4%) and a suspected amyl benzoate (0.2-2%).
Principal among the aliphatic components were a series of l- alken-3-ones. l-Hexen-3-one (8-21%), l-octen-3-one (36-46%) and l- decen-3-one (0.9-3%) were all identified in the steam distillate and there were lesser amounts of hexanal (0.1-0.6%), (Z)-3-hexenol (0.1- 0.6%) and nonanal (2-3%).
A headspace sample, obtained with an SPME fibre, was also taken during one of the steam distillations and, not surprisingly, there was a preponderance of the more volatile components than were found in the steam distdlate. alpha-Pinene (8.6%), styrene (30.8%) and beta-caryophyllene (27%) were the main contributors togetherwidi l- hexen-3-one (2.8%), l-octen-3-one (3.2%) and l-decen-3-one (0.8%). A more detadedlist is given in Table I. An SPME head-space sample from phyllodes, taken at room temperature and examined only by GC/MS showed that styrene was, by far, the major component.
The volatiles obtained from A. howittii present an unusual mixture of components, most of which are thought to originate in the ‘sticky’ coating on the surface of the phyllodes (3). The distinctive smell of the distdlate and, to a lesser extent the leaf itself, is thought to be mainly caused by the presence of styrene, l- hexen-3-one and l-octen-3-one. The major component, l-octen-3-one has been described as having the odor of mushroom (14), though in large amount it does not seem to possess this odor.
l-Hexen-3-one and l-octen-3-one have been identified previously in the steam volatdes of artichokes (15), both compounds have extremely low odor thresholds (15). 1-Octen-3-one and l-decen-3-one have been identified in die steam volatile concentrates from a fungus Aspergillus clavatus (16). l-Octen-3-one has also been foundin the musts of French and Romanian grape hybrids (14). l- Hexen-3-one has also been found in the vegetative bracts of 3 species of Chamaebatia (17). It has been synthesized from acetylenyl propyl ketone (18). It was also found in the head space of “fish oil enriched mayonnaise during stir up”, where it is described as having an earthy, metallic odor (19), and also in yellow passionfruit (20). The three homologues reported in our present investigation have LRI values in agreement widi those published above (14,17,19).
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Joseph J Brophy,* Robert J Goldsack,
School of Chemistry University of New South Wales UNSW Sydney NSW 2052, Australia
Christopher J R Fookes,
12 Ky ogle Place Grays Point NSW 2232, Australia
*Address for correspondence
Received: February 2006
Revised: May 2006
Accepted: May 2006
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