ORIGINAL RESEARCH published: 06 October 2015 doi: 10.3389/fchem.2015.00057
Structure-odor relationships of linalool, linalyl acetate and their corresponding oxygenated derivatives Shaimaa A. Elsharif 1 , Ashutosh Banerjee 2 and Andrea Buettner 1, 3* 1
Department of Chemistry and Pharmacy, Food Chemistry, Emil Fischer Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany, 2 Department of Chemistry and Pharmacy, Pharmaceutical Chemistry, Emil Fischer Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany, 3 Department of Sensory Analytics, Fraunhofer Institute for Process Engineering and Packaging, Freising, Germany
Edited by: Dejian Huang, National University of Singapore, Singapore Reviewed by: Muhammad Safder, University of Karachi, Pakistan Marcin Szymanski, Poznan University of Medical Sciences, Poland *Correspondence: Andrea Buettner, Erlangen, Germany
[email protected] Specialty section: This article was submitted to Food Chemistry, a section of the journal Frontiers in Chemistry Received: 31 July 2015 Accepted: 14 September 2015 Published: 06 October 2015 Citation: Elsharif SA, Banerjee A and Buettner A (2015) Structure-odor relationships of linalool, linalyl acetate and their corresponding oxygenated derivatives. Front. Chem. 3:57. doi: 10.3389/fchem.2015.00057
Frontiers in Chemistry | www.frontiersin.org
Linalool 1 is an odorant that is commonly perceived as having a pleasant odor, but is also known to elicit physiological effects such as inducing calmness and enhancing sleep. However, no comprehensive studies are at hand to show which structural features are responsible for these prominent effects. Therefore, a total of six oxygenated derivatives were synthesized from both 1 and linalyl acetate 2, and were tested for their odor qualities and relative odor thresholds (OTs) in air. Linalool was found to be the most potent odorant among the investigated compounds, with an average OT of 3.2 ng/L, while the 8-hydroxylinalool derivative was the least odorous compound with an OT of 160 ng/L; 8-carboxylinalool was found to be odorless. The odorant 8-oxolinalyl acetate, which has very similar odor properties to linalool, was the most potent odorant besides linalool, exhibiting an OT of 5.9 ng/L. By comparison, 8-carboxylinalyl acetate had a similar OT (6.1 ng/L) as its corresponding 8-oxo derivative but exhibited divergent odor properties (fatty, greasy, musty). Overall, oxygenation on carbon 8 had a substantial effect on the aroma profiles of structural derivatives of linalool and linalyl acetate. Keywords: Linalool, linalyl acetate, gas chromatography-olfactometry, odor threshold in air, 8-oxolinalyl acetate, 8-carboxylinalyl acetate, odor qualities, retention index
Introduction In folk medicines as well as aroma therapy, essential oils and fragrance compounds are being used as therapeutic agents for relieving pain, anxiety reduction and energy enhancement (Lahlou, 2004; Kako et al., 2008; Kiecolt-Glaser et al., 2008). Among them, due to their high volatility, the acyclic monoterpenes are a valuable class of compounds useful for the flavor and fragrance industries (King and Dickinson, 2003). One of the most important acyclic monoterpene substances is linalool 1 which represents about 70% of the terpenoids of floral scents (Stashenko and Martinez, 2008). In perfumery, linalool is a commonly used fragrant ingredient being a component of many perfumes top notes and being found in 60–90% of cosmetic products (Cal and Krzyzaniak, 2006). Its odor is described in literature as floral, citric, fresh and sweet (d’Acampora Zellner et al., 2007). It is also added to household cleaning agents, furniture care products, waxes, as well as to processed food and beverages, as a fragrance and flavor agent. Linalool is found in the essential oils of over 200 plant species, belonging to different families (Stashenko and Martinez, 2008). For example, linalool
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Structure-odor relationships of linalool and derivatives
metabolites are also excreted in urine as free forms or conjugates. Products of linalool reduction (dihydro-, tetrahydrolinalool) were also identified in rodent urine (Aprotosoaie et al., 2014). A significant proportion of orally administered linalool follows intermediary metabolic pathways as shown in Scheme 1 (scheme modified from Aprotosoaie et al., 2014). 8-Hydroxylinalool was not only found as a metabolite in mammalian species, but also as an oxidation product isolated from the grape berry mesocarp after linalool was applied to it (Luan et al., 2006). 8-Carboxylinalool was found to be among the constituents of the fruits of Euterpe oleracea (Chin et al., 2008) and the flower of Albizia julibrissin (Yahagi et al., 2012). Linalyl acetate metabolism was also studied in Pseudomonas incognita (Renganathan and Madyastha, 1983), where it was shown that the C-8-methyl moiety is subjected to selective oxidation, giving 8-hydroxylinalyl acetate which is then oxidized to 8-oxo and 8carboxylinalyl acetate, respectively. Apart from that, 8-oxolinalyl acetate was first isolated from lavandin oil and hence reported as a constituent of a natural product (Mookherjee and Trenkle, 1973). 8-Carboxylinalyl acetate was found in trace amounts (
