Enantioselective Analysis of Methyl-Branched Alcohols and Acids in Rhubarb (Rheum rhabarbarum L.) Stalks

Abstract

The enantiomeric compositions of 2-methylbutanol (1), 4-methylhexanol (2), 2-methylbutanoic acid (3), and 4-methylhexanoic acid (4) present in rhubarb (Rheum rhabarbarum L.) stalks were determined. Enantiodifferentiation was achieved via multidimensional gas chromatography using heptakis(2,3,6-tri-O-ethyl)-beta-cyclodextrin as a chiral stationary phase. For all compounds the enantiomeric ratios were in favor of the (R)-enantiomers. The alcohols (1 and 2) exhibited generally high excesses of the (R)-enantiomers, the ratios varying slightly from batch to batch. For the acid (3) a rather narrow range averaging 65% (R):35% (S) was observed. The procedure applied to isolate the volatiles (vacuum headspace technique, simultaneous distillation--extraction, liquid--liquid extraction) had no significant impact on the enantiomeric ratios. The study describes for the first time a plant used as food material in which 2-methyl-branched volatiles are not nearly exclusively present as (S)-enantiomers. This information enlarges the current regulatory knowledge regarding the classification of these important flavor compounds as "natural" on the basis of their enantiomeric ratios.

Chirality: An Important Phenomenon Regarding Biosynthesis, Perception, and Authenticity of Flavor Compounds

“…This was confirmed, for example, for methyl and ethyl 2-methylbutanoate from apple varieties and from strawberries . However, enantioselective analysis also revealed other foods not being in accordance with this rule; for example, rhubarb stalks contain significant proportions of the ( R )-enantiomers of 2-methylbutanol, 2-methylbutanoic acid, and the chain-elongated 4-methylhexanol …”

“…34 However, enantioselective analysis also revealed other foods not being in accordance with this rule; for example, rhubarb stalks contain significant proportions of the (R)enantiomers of 2-methylbutanol, 2-methylbutanoic acid, and the chain-elongated 4-methylhexanol. 35 A few years ago, it has also been demonstrated that 2methylbranched aroma compounds present in fermented foods and expected to be formed via the well-known Ehrlich degradation of L-isoleucine do not consistently show (S)configurations. For 2-methylbutanol and the related 2methylbutyl acetate, nearly optically pure (S)-enantiomers were detected.…”

Separation methods for antibacterial and antirheumatism agents in plant medicines

“…Sybilska and Asztemborska compared the performance of three cyclodextrin columns (␤-DEX120, ␥-DEX120 and ␥-DEX255) for the chiral recognition of terpenoids in pharmaceuticals derived from natural sources [47]. Recently, some organic alcohols and acids in Rhubarb (Rheum rhabarbrum L.) were separated on a capillary column coated with heptakis(2,3,6-tri-O-ethyl)-cyclodextrin and OV-1701 [48].…”

Towards enantioselective ultrahigh performance liquid chromatography–mass spectrometry‐based metabolomics of branched‐chain fatty acids and anteiso‐fatty acids under reversed‐phase conditions using sub‐2‐μm amylose‐ and cellulose‐derived chiral stationary phases

“…In any case, a robust and reliable method for the chiral separation is the key to both structural characterization and clinical diagnostics. In the past, the chiral separation of BCFAs has been mostly accomplished by enantioselective GC with cyclodextrin‐based chiral columns and EI‐MS detection 15,19–21,28,29 . EI‐MS detection has the advantage that the resultant MS spectra provide information on the branching position.…”