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Comprehensive structure-activity-relationship studies of sensory active compounds in licorice (Glycyrrhiza glabra)

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Abstract

Licorice saponins, the main constituents of Glycyrrhiza glabra L. roots, are highly appreciated by the consumer for their pleasant sweet and long lasting licorice taste. The objective of the present study was to understand the molecular features that contribute to bitter, sweet and licorice sensation of licorice roots, and whether individual compounds elicit more than one of these sensations. Therefore, a sensomics approach was conducted, followed by purification of the compounds with highest sensory impact, and by synthesis as well as full characterization via HRESIMS, ESIMS/MS and 1D/2D-NMR experiments. This led to the discovery and structure determination of 28 sweet, bitter and licorice tasting key phytochemicals, including two unknown compounds. A combination of sensorial, cell-based and computational analysis revealed distinct structural features, such as spatial arrangement of functional groups in the triterpenoid E-ring, driving to different taste sensations and sweet receptor hTAS1R2/R3 stimulation.

1. Introduction

Licorice roots (generally Glycyrrhiza glabra L., Leguminosae) are main ingredients of diverse licorice sweets which are highly appreciated by consumers all over the world due to their sweet and typical long-lasting licorice impression (Kitagawa, 2002). Licorice triterpenoid saponins elicit a high sweetness, which is utilized in various products, e.g., licorice confectioneries, food additives, cosmetics and flavor additives for tobacco. The quantitative predominating phytochemical in Glycyrrhiza spp. is the monodesmosidic saponin glycyrrhizin with an 18β-glycyrrhetinic acid skeletal structure linked with a disaccharide, consisting of two β-d-glucuronic acids (Fenwick et al., 1990Kitagawa, 2002). First sensory experiments showed that glycyrrhizin is 150 times sweeter than sucrose. Further triterpenoid saponins were isolated from licorice roots and semi-synthesized based on glycyrrhetinic acid with a relative sweetness of 70 for LS-A3 and of 941 for the β-d-glucuronic acid monosaccharide of 3-O-glycyrrhetinic acid (Kitagawa, 2002Kitagawa et al., 1993). Conversely, hydrolysis of the saponin revealed a complete elimination of the sweet taste (Esaki, Konishi, & Kamiya, 1978; Mizutani, Kuramoto, Tamura, Ohtake, Doi, Nakaura, et al., 1994).

Gustatory sense, especially sweet and bitter taste, plays a key role in food selection. The attractive sweet taste promises a carbohydrate-rich food, whereas unsavory bitter taste may cause a cautionary effect of possible poisonous food (Chandrashekar et al., 2000Nissim et al., 2017). The hTAS1R2/hTAS1R3 sweet taste receptor heterodimer possesses a broad receptive range, enabled by several different active sites, to detect diverse sweet tasting compounds, such as mono-, disaccharides, sweet d-amino acids, sweet proteinsglucosides as steviol glucosides and synthetic non-nutritive sweeteners (Behrens et al., 2011Hellfritsch et al., 2012Morini et al., 2005). In contrast to sweet taste, bitter taste in human is mediated by about 25 GPCRs, (G protein-coupled receptors), namely the hTAS2R receptor family. Members of this family often detect a wide range of bitter molecules, in order to recognize many bitter compounds with a small number of receptors (Chandrashekar et al., 2000Matsunami et al., 2000Meyerhof et al., 2010), as summarized in BitterDB database (Dagan-Wiener, Di Pizio, Nissim, Bahia, Dubovski, Margulis, et al., 2018). Some compounds such as certain artificial sweeteners can interact with both the sweet receptor heterodimer hTAS1R2/hTAS1R3 and with bitter taste receptors, inducing a sweet taste with a bitter off-taste (Hellfritsch et al., 2012Kuhn et al., 2004). Investigations of the signaling pathway of the human sweet taste receptor after stimulation with different artificial sweeteners and the licorice saponin glycyrrhizin showed an activation of different patterns of intracellular signals (Nakagawa et al., 2013Ohtsu et al., 2014). However, nothing is known about molecular mechanisms of licorice impression at receptor level.

As most publications are dealing with the pharmacological impact of the licorice root, systematic studies on the non-volatile sensometabolome of G. glabra are lacking. In particular, all investigations of sensory active compounds correspond to sweet taste, whereas the characteristic licorice impression has not been much of a topic in research so far (Fenwick et al., 1990Kitagawa, 2002). For decoding the key compounds responsible for the typical taste signature of a specific food or food product (e.g., thermally generated bitter compounds, bitter off-tastants of carrots, sensory active compounds in roasted cocoa nibs and asparagus) the so-called taste dilution analysis (TDA) was designed (Dawid and Hofmann, 2012bFrank et al., 2001).

The object of the present investigation is the isolation and structure determination by means of UV, ESIMS/MS, HRESIMS, 1D/2D-NMR as well as sensory evaluation of the compounds isolated from European licorice root (Glycyrrhiza glabra L.). Molecular features that contribute to sweet, bitter and licorice sensation are elucidated using a combination of instrumental analytics, psychophysical studies, cell-based receptor assays and computational docking experiments.

Reference:

https://www.sciencedirect.com/science/article/pii/S0308814621014266

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