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

Abstract

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.

Absolute Configurations and Sensory Properties of the Stereoisomers of a Homologous Series (C6–C10) of 2-Mercapto-4-alkanols

“…The (2 S ,4 S )- and (2 S ,4 R )-stereoisomers of this β-mercaptoalkanol are quantitatively dominating in cooked bell pepper. Biogenetically, this is in line with the reported high preponderance of the potential precursor ( S )-2-mercapto-4-heptanone 8 in cooked bell pepper . (2 S ,4 S )-2-Mercapto-4-heptanol 2a ′ and (2S,4 R )-2-mercapto-4-heptanol 2b showed the lowest and highest thresholds, respectively, for the C7 homologues (Table ).…”

“…2-Mercapto-4-heptanone 7 , 2-mercapto-4-heptanol 2 , and the positional isomers 4-mercapto-2-heptanone and 4-mercapto-2-heptanol were among the sulfur-containing volatiles identified in bell peppers . Distributions of the stereoisomers of these C7 homologues in cooked red bell pepper have previously been investigated . The data elaborated in the present study allow complementing the missing assignments of the configurations of the 2-mercapto-4-heptanol stereoisomers.…”

“…8 Distributions of the stereoisomers of these C7 homologues in cooked red bell pepper have previously been investigated. 30 The data elaborated in the present study allow complementing the missing assignments of the configurations of the 2-mercapto-4-heptanol stereoisomers. The (2S,4S)-and (2S,4R)-stereoisomers of this β-mercaptoalkanol are quantitatively dominating in cooked bell pepper.…”

“…Biogenetically, this is in line with the reported high preponderance of the potential precursor (S)-2-mercapto-4-heptanone 8 in cooked bell pepper. 30 (2S,4S)-2-Mercapto-4-heptanol 2a′ and (2S,4R)-2-mercapto-4-heptanol 2b showed the lowest and highest thresholds, respectively, for the C7 homologues (Table 5). For both stereoisomers, the odor qualities showed a wide range, from sweet, vegetable to burnt, pungent; none of the panelists described a specific bell pepper flavor (Table 6).…”

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

“…Determinations of odor thresholds in water and calculations of odor activity values would, for example, be required to assess the contributions of β-mercaptoalkanones and β-mercaptoalkanols to the aroma of bell pepper. A first study on the naturally present distributions of the stereoisomers of the C7-homologues demonstrated consistent ratios independent of origin and date of purchase; however, the data also revealed an impact of the state of maturity on the optical purities of these volatiles …”