Distributions of the Stereoisomers of ß-Mercaptoheptanones and ß-Mercaptoheptanols in Cooked Bell Pepper (Capsicum annuum).

2-Mercapto-4-heptanone, 4-mercapto-2-heptanone, and the corresponding mercaptoalcohols, previously identified in cooked red bell pepper (Capsicum annuum), were used as examples to determine the distributions of stereoisomers of naturally occurring polyfunctional thiols. The thiols were isolated using simultaneous distillation-extraction and enriched by affinity chromatography. Enantioselective analysis was performed via multidimensional gas chromatography. For the studied cultivar California Wonder, the investigation of different batches of cooked red bell pepper revealed consistent ratios of the stereoisomers independent of origin and date of purchase. Quantitative estimations showed that the stereoisomers were present in cooked red bell peppers at concentrations in the range of 0.04-10.2 µg/kg. Lower concentrations were observed in cooked green bell peppers. The change from green to red color was also accompanied by shifts in the proportions of stereoisomers in favor of the (S)-enantiomers of the mercaptoheptanones and of the (4S)-configured stereoisomers of 4-mercapto-2-heptanol.

Analysis and Sensory Evaluation of the Stereoisomers of a Homologous Series (C5-C10) of 4-Mercapto-2-alkanols.

A homologous series of 4-mercapto-2-alkanols (C5-C10) was used to investigate the impact of the stereochemistry on the sensory properties of a class of naturally occurring polyfunctional thiols having a 1,3-oxygen-sulfur functionality. Stereoisomers were obtained via syntheses of racemic mixtures and subsequent lipase-catalyzed kinetic resolutions. Analytical separations of the stereoisomers were achieved by capillary gas chromatography (GC) using chiral stationary phases. The absolute configurations were assigned via NMR analysis. Sensory evaluations by means of GC/olfactometry revealed odor threshold minima for the medium-chain homologues (C7-C9) of the 4-mercapto-2-alkanol stereoisomers. Except for the C5 homologue, the lowest odor thresholds were determined for the (2R,4R)-configured stereoisomers. The variability in odor qualities was mainly determined by the chain length. None of the 4-mercapto-2-alkanol stereoisomers showed consistent odor qualities for all homologues.

Reinvestigation of the Absolute Configurations of Chiral ß-Mercaptoalkanones Using Vibrational Circular Dichroism and 1H NMR Analysis.

The absolute configurations of chiral ß-mercaptoalkanones were previously assigned on the basis of the 1H NMR anisotropy method using (S)-2-methoxy-2-(1-naphthyl)propionic acid ((S)-MαNP) as the chiral auxiliary. This study presents a reinvestigation of the configurations of 4-mercapto-2-pentanone 1, 4-mercapto-2-heptanone 2, and 2-mercapto-4-heptanone 3. Enantiomers of 1, 2, and 3 were obtained by lipase-catalyzed hydrolyses of the respective acetylthioalkanones. Upon derivatization with (S)-MαNP, the configurations of the reaction products were deduced based on the order of the HPLC elution of the diastereoisomeric thioesters, assuming that the sector rule previously developed for secondary alcohols is also valid for thiols. In addition, the configurations were experimentally determined by vibrational circular dichroism (VCD) and 1H NMR analyses after esterification with (R)-hydratropic acid (HTA) and 2-methoxy-2-phenylacetic acid (MPA). The assignments of the configurations using VCD and NMR analyses of HTA- and MPA-thioesters were in agreement. However, they were opposite to those deduced for (S)-MαNP thioesters via the sector rule. Consequently, the formerly assigned configurations of ß-mercaptoalkanones deduced via investigation of (S)-MαNP-derivatives have to be revised.

Influence of the stereochemistry on the sensory properties of 4-mercapto-2-heptanol and its acetyl-derivatives.

4-Mercapto-2-heptanol, previously described in cooked bell pepper, was used to determine the impact of the stereochemistry on the sensory properties of a thiol with a 1,3-oxygen-sulfur functionality. In addition, the acetyl-derivatives 4-acetylthio-2-heptanol, 4-mercapto-2-heptyl acetate and 4-acetylthio-2-heptyl acetate were investigated. The synthesized stereoisomers were separated via capillary gas chromatography (GC) using chiral stationary phases. The GC orders of elution were determined by assigning the absolute configurations via NMR analysis in combination with lipase-catalyzed kinetic resolutions. Odor thresholds and odor properties were determined by means of GC/Olfactometry. The data revealed that the sensory properties of the investigated compounds are not only significantly influenced by the acetylation but also by the configurations of the two asymmetric centers.

Metabolite profiling of two novel low phytic acid (lpa) soybean mutants.

A GC-based approach was applied to compare the metabolite profiles of two low phytic acid (lpa) soybean mutants and their respective wild-types. The lpa mutants (Gm-lpa-TW75-1 and Gm-lpa-ZC-2) were grown together with the wild-types (Taiwan 75 and Zhechun no. 3) in three and four field trials, respectively. HPLC analysis revealed a phytic acid reduction of -53% for Gm-lpa-TW75-1 and of -46% for Gm-lpa-ZC-2. For Gm-lpa-TW75-1, no accumulation of lower inositol phosphates was observed, whereas Gm-lpa-ZC-2 exhibited significantly increased contents of the lower inositol phosphates InsP(3), InsP(4), and InsP(5) compared to the corresponding wild-type. The metabolite profiling revealed that compared to the wild-types, 40% (Gm-lpa-TW75-1) and 21% (Gm-lpa-ZC-2) of the detected peaks were statistically significantly different in the lpa mutants grown at one field trial. However, the majority of these differences were shown to be related to environmental impact and natural variability rather than to the mutation event. Identification of consistent metabolic changes in the lpa mutants revealed decreased contents of myo-inositol, galactinol, raffinose, stachyose, and the galactosyl cyclitols galactopinitol A, galactopinitol B, and fagopyritol B1 compared to the wild-type. These consistently pronounced changes in Gm-lpa-TW75-1 confirmed the suggested mutation target. Consideration of the metabolic changes observed for Gm-lpa-ZC-2 (accumulation of lower inositol phosphates and increased myo-inositol contents) indicated a mutation event affecting the latter biosynthetic steps leading to phytic acid. The study demonstrated the applicability of metabolite profiling for the detection of changes in the metabolite phenotype induced by mutation breeding and its power in assisting in the elucidation of mutation events.