StructureâOdor Correlations in Homologous Series of

Article pubs.acs.org/JAFC

Structure−Odor Correlations in Homologous Series of Mercaptoalkanols Johannes Polster and Peter Schieberle* Lehrstuhl für Lebensmittelchemie, Technische Universität München, Lise-Meitner-Straße 34, D-85354 Freising, Germany S Supporting Information *

ABSTRACT: To study the influence of molecular structure on the sensory properties of mercaptoalkanols, homologous series of 1-mercaptoalkan-3-ols, 3-mercaptoalkan-1-ols, 2-mercaptoalkan-1-ols, 4-mercaptoalkan-2-ols, 3-mercapto-3-methylalkan-1-ols, 1-mercapto-2-methylalkan-3-ols, 3-mercapto-2-methylalkan-1-ols, and 2-ethyl-3-mercaptoalkan-1-ols were synthesized. Odor thresholds in air and odor qualities were determined, and the results obtained were correlated to the chemical structures. Sensory properties were strongly influenced by steric effects: All homologous series revealed a minimum in odor thresholds between five and seven carbon atoms, and increasing the length of the carbon chain led to an exponential increase in odor thresholds. The olfactory power of the thiols was considerably improved by methyl or ethyl substitution in the α-position to the thiol group as well as by an additional methyl or ethyl group at the mercapto-containing carbon atom. By using comparative molecular similarity indices analysis, a 3D-quantitative structure−activity relationship model could be created, which was able to predict the odor thresholds of mercaptoalkanols in good agreement with the experimental results. Retention indices, NMR data, and mass spectra for 49 mercaptoalkanols, most of them synthetically prepared for the first time, are supplied. KEYWORDS: mercaptoalkanols, thiols, QSAR, CoMSIA, structure-odor relationships, odor threshold, odor quality

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INTRODUCTION To date, more than 100 volatile compounds bearing a mercapto group have been identified in foods. Of these, about 20−30% are mercaptoalkanols, suggesting that this is probably among the most important sulfur compound class with aroma activity. Although mercaptans usually occur only in trace amounts, due to their often extremely low odor thresholds, they can show a major impact on the overall aroma of many foods. For example, 3-mercaptohexan-1-ol is a key aroma compound of passion fruit,1 wine,2 and guava3 and 3-mercapto-2-methylpentan-1-ol is a major contributor to the odor of onion, chive, and leek.4 Although this is not fully explored yet, the mercapto group in aroma-active sulfur compounds is suggested to stem from a degradation of the amino acid cysteine by either an enzymatic or a thermal reaction with unsaturated intermediates. Therefore, a quite high number of mercaptans can be expected to be formed in foods, but some may remain to be identified. One approach to this question can be the determination of structure−activity correlations in synthesized mercaptans to clarify which structural elements define the structural space for components with high odor potency. Despite the great importance of thiols in aroma chemistry, only a few studies on such structure−odor correlations are available. Recently, it was shown5 that the odor perception of alkanethiols is strongly influenced by steric effects, such as the length of the carbon chain or an additional methyl group in the carbon chain, but in particular the free SH group was essential for low odor thresholds. Building on these results, the aim of the present study was to reveal the influence of an additional hydroxy group on the olfactory properties of mercaptans. For this purpose, homologous series of mercaptoalkanols (C3−C10) with different substitution patterns were synthesized and their odor © 2017 American Chemical Society

thresholds as well as their odor qualities were determined. Furthermore, using comparative molecular similarity indices analysis (CoMSIA), aroma-relevant molecular substructures (odotopes) should be identified and a 3D-quantitative structure−activity relationship (3D-QSAR) model should be created to simulate the experimental results. Additionally, as a basis for future investigations on sulfur-containing food aroma constituents, the synthesized compounds were fully characterized by retention indices and spectroscopic data (MS-CI, MS-EI, NMR).

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MATERIALS AND METHODS

Chemicals. Chemicals for syntheses were purchased from SigmaAldrich (Steinheim, Germany), Alpha-Aesar (Karlsruhe, Germany), and TCI Europe Laboratory Chemicals (Eschborn, Germany). Solvents were obtained from VWR (Darmstadt, Germany), and deuterated solvents for NMR spectroscopy were supplied by Eurisotop (Saarbrücken, Germany). Dichloromethane, diethyl ether, and pentane were distilled prior to use. Reference Odorants. 3-Mercaptopropan-1-ol was purchased from TCI Europe Laboratory Chemicals (Eschborn, Germany), and 3mercaptohexan-1-ol was supplied by Alpha-Aesar (Karlsruhe, Germany). Syntheses. A total of 24 mercapto compounds were synthesized and characterized for the first time. All reactions were carried out in oven-dried glassware under argon atmosphere unless otherwise mentioned. Experimental procedures and spectral data (NMR, MS) of the synthesized compounds and intermediates are given in the Supporting Information. Received: Revised: Accepted: Published: 4329

March 21, 2017 May 3, 2017 May 8, 2017 May 8, 2017 DOI: 10.1021/acs.jafc.7b01266 J. Agric. Food Chem. 2017, 65, 4329−4340

Article

Journal of Agricultural and Food Chemistry

Figure 1. Synthetic route used for the preparation of 3-mercapto-3-methylalkan-1-ols. As an example, the synthetic route used in the preparation of the 3methyl-3-mercaptoalkan-1-ols is detailed below. 3-Mercapto-3-methylalkan-1-ols. The 3-mercapto-3-methylalkan-1-ols were synthesized from the corresponding 3-methylalk-2enals by the addition of thioacetic acid followed by reduction with LiAlH4 (Figure 1). If the respective α,β-unsaturated aldehyde was not commercially available, it was prepared by a carboalumination of the respective terminal alkynes6,7 followed by reaction with paraformaldehyde and oxidation with Dess−Martin periodinane. 3-Methylalk-2-enals. To a suspension of zirconocene dichloride (11.7 g; 40 mmol) in 1,2-dichloroethane (100 mL, abs) was slowly added trimethylaluminum (40 mL; 2.0 mol/L in hexane; 80 mmol) at 0 °C. The mixture was stirred for 1 h at 0 °C followed by 1.5 h at room temperature. Then, the respective alkyne (40 mmol) in 1,2dichloroethane (20 mL, abs) was added, and the solution was stirred for 3 h at room temperature. The volatile compounds were removed by rotary evaporation at 45−50 °C, and the residue was extracted with hexane (4 × 30 mL, abs). To the combined organic layers was slowly added n-butyl-lithium (16 mL; 2.5 mol/L in hexane; 40 mmol) at 0 °C, and the mixture was stirred for 1.5 h at room temperature. Then, dry THF (70 mL) was added to dissolve the precipitate, and the resulting solution was slowly added to a suspension of paraformaldehyde (6 g; 200 mmol) in dry THF (100 mL) and stirred for 20 h at room temperature. After the mixture had cooled to 0 °C, ice and a saturated aqueous ammonium chloride solution (100 mL) were added, and the reaction was acidified with aqueous hydrochloric acid (2.5 mol/L) until the mixture became a clear yellow solution at a pH of 2− 3. The organic layer was separated, and the aqueous layer was extracted with diethyl ether (2 × 150 mL). The combined organic layers were washed with a saturated aqueous sodium bicarbonate solution (100 mL) and dried over anhydrous sodium sulfate, and the solvent was removed by rotary evaporation or was distilled off by means of a Vigreux column (50 × 1 cm), depending on the boiling point of the product. Thioacetylation with Thioacetic Acid. Thioacetic acid (2.28 g; 30 mmol) was slowly added to an ice-cooled solution of piperidine (40 mg; 0.5 mmol) and the respective 3-methylalk-2-enal (20 mmol). The mixture was stirred for 24 h at room temperature. After dilution with diethyl ether (80 mL), the solution was washed with aqueous hydrochloric acid (2 mol/L; 2 × 25 mL), followed by a saturated aqueous sodium bicarbonate solution (30 mL). After drying over anhydrous sodium sulfate, the solvent was removed by rotary evaporation. Reduction with LiAlH4. The acetylthio derivative (10 mmol) obtained was dissolved in anhydrous diethyl ether (15 mL) and added slowly to a suspension of lithium aluminum hydride (0.76 g; 20 mmol) in anhydrous diethyl ether (20 mL) at 0 °C. The solution was stirred for 2 h at room temperature, and a saturated aqueous ammonium chloride solution (20 mL) was added slowly at 0 °C. Then, aqueous hydrochloric acid (2 mol/L; 50 mL) was added to dissolve the precipitate formed, the ethereal layer was separated, and the aqueous layer was extracted with diethyl ether (3 × 70 mL). The combined organic layers were washed with a saturated aqueous sodium bicarbonate solution (50 mL), followed by water (50 mL). After drying over anhydrous sodium sulfate, the solvent was removed by rotary evaporation. Purification of the compound was performed by

column chromatography on silica gel using pentane/diethyl ether (90:10, by vol) as the eluent. Gas Chromatography−FID (GC-FID) and Gas Chromatography−Olfactometry (GC-O). GC-FID and GC-O analyses were performed by means of a Fisons gas chromatograph type 8160 (Fisons Instruments, Mainz, Germany) using helium as the carrier gas. Samples were applied by the cold-on-column injection technique. Capillaries used were DB-5 and DB-FFAP (each 30 m × 0.32 mm i.d., 0.25 μm film thickness, 70 kPa head pressure; J&W Scientific) (Chromatographie-Handel Mueller, Fridolfing, Germany). The temperature programs were adapted to the boiling point of each analyte. For GC-O applications, the end of the capillaries were connected to a deactivated Y-shaped glass splitter (Chromatographie Handel Mueller, Fridolfing, Germany) dividing the effluent into two equal parts, which were transferred via two deactivated fused silica capillaries (50 cm × 0.25 mm) to a sniffing port and an FID, respectively. The sniffing port consisted of a cylindrically shaped aluminum device (80 mm × 25 mm i.d.) with a beveled top and a central drill hole (2 mm) housing the capillary. It was mounted on a detector base of the GC and heated to 200 °C. The FID was operated at 250 °C with hydrogen (20 mL/min) and air (200 mL/min). Nitrogen (30 mL/min) was used as the makeup gas. The injection volume was 1.0 μL. During a GC-O run, the nose of the panelist was placed closely above the top of the sniffing port, and the odor of the effluent was evaluated. The evaluation was performed by three panelists, and the results were averaged. Panelists were trained in a “flavor language” in weekly training sessions for at least 6 months, in which pure reference odorants were used.14 Gas Chromatography−Mass Spectrometry (GC-MS). Mass spectra were recorded after chromatography on a gas Hewlett-Packard chromatograph 5890 series II (Hewlett-Packard, Waldbronn, Germany) connected to a Finnigan sector field mass spectrometer MAT 95 S (Finnigan, Bremen, Germany). Mass spectra in the electron ionization mode (MS-EI) were recorded at 70 eV ionization energy and mass spectra in the chemical ionization mode (MS-CI) at 115 eV with isobutane as the reactant gas. Determination of Odor Thresholds and Odor Qualities in Air. Thresholds were determined by aroma extract dilution analysis of a mixture containing known amounts of the target odorant and (E)-2decenal as the internal standard. Thresholds were calculated from the FD factors determined by using the method previously described8 and a threshold of 2.7 ng/L for (E)-2-decenal.9 Odor qualities were assigned during GC-O at the threshold level. NMR Spectroscopy. The 1H, 13C, and 2D NMR experiments were performed using a Bruker 400 MHz DMX spectrometer (Bruker, Rheinstetten, Germany). Samples were dissolved in CDCl3, and chemical shifts were determined using tetramethylsilane as the internal standard (0.03%) in the proton dimension and from the carbon signal of CDCl3 (77.0 ppm) in the carbon dimension. Molecular Modeling and Alignment. All molecular modeling studies, CoMSIA, and partial least-squares approaches were performed using the SYBYL-X 1.2 program package.10 Initial template structures were built using Sybyl’s Concord module. Energy minimizations were carried out using Tripos Force Field. Gasteiger−Hückel charges were calculated for all molecules. For molecular alignment, 2-methylpentane-2-thiol, showing the lowest odor thresholds of all investigated sulfur-containing compounds,5 was used as the reference molecule. 4330

DOI: 10.1021/acs.jafc.7b01266 J. Agric. Food Chem. 2017, 65, 4329−4340

Article


Figure 2. Selected mass spectra (MS-EI) of straight-chain mercaptoalkanols: (A) 1-mercaptohexan-3-ol; (B) 3-mercaptohexan-1-ol; (C) 4mercaptohexan-2-ol; (D) 2-mercaptohexan-1-ol.

Table 1. Retention Indices, MS-EI Data, and Sensory Properties of 1-Mercaptoalkan-3-ols RI compound

FFAP

DB-5

MS-EI: m/z (intensity in %)a

3-mercaptopropan-1-ol

1653

890

4-mercaptobutan-2-ol

1607

918

1-mercaptopentan-3-ol

1684

1012

1-mercaptohexan-3-ol

1775

1109

1-mercaptoheptan-3-ol

1882

1213

1-mercaptooctan-3-ol

1987

1316

1-mercaptononan-3-ol

2092

1422

1-mercaptodecan-3-ol

2197

1525

58 (100), 92 (82, M+), 57 (76), 41 (62), 74 (54), 61 (45), 47 (38), 46 (34), 45 (26), 59 (21) 45 (100), 72 (61), 55 (41), 88 (38), 47 (36), 43 (34), 60 (27), 73 (18), 57 (16), ..., 106 (5, M+) 57 (100), 59 (95), 87 (84), 102 (81), 47 (74), 73 (46), 41 (41), 45 (41), 86 (41), ..., 120 (4, M+) 87 (100), 55 (90), 47 (47), 57 (41), 73 (40), 41 (35), 43 (35), 67 (32), 82 (29), ..., 134 (2, M+) 87 (100), 69 (69), 101 (58), 55 (47), 41 (46), 47 (40), 57 (40), 81 (30), 60 (25), ..., 148 (1, M+) 87 (100), 55 (85), 101 (56), 41 (42), 115 (41), 83 (37), 57 (36), 47 (33), 81 (28), ..., 162 (1, M+) 87 (100), 55 (87), 69 (47), 101 (47), 41 (42), 115 (40), 43 (38), 57 (37), 129 (34), M+ absent 87 (100), 69 (67), 55 (59), 101 (42), 41 (42), 57 (38), 129 (34), 43 (29), 115 (26), M+ absent

odor thresholdc (ng/L in air)

reported previously as food constituentd

leek, onion

8.3

wine,18 beer,19 hops20

leek, onion

4.9

burned

0.59

burned

0.15

burned, mushroom mushroom, fatty mushroom, fatty fatty

0.18

odor qualityb

beer,19 hops20

0.33 4.7 40

a For further spectroscopic data (MS-CI, NMR) see the Supporting Information. bOdor quality as perceived at the sniffing port during GC-O at threshold level. cOdor thresholds in air were determined as previously reported.8 dFirst report.

Partial Least-Squares (PLS) Regression. PLS regression was conducted to establish and validate the 3D-QSAR model. Thereby, the optimal number of principal components was determined using leaveone-out cross-validation.

Superimposition was performed with the main focus on maximal alignment of the SH− group. Comparative Molecular Similarity Indices Analysis (CoMSIA). For CoMSIA, the aligned molecules were positioned inside a three-dimensional grid box with a grid spacing of 2 Å. Steric, electrostatic, hydrophobic, and hydrogen-bond donor and acceptor descriptors were evaluated using the SYBYL standard parameters (sp3 carbon probe atom with a radius of 1.0 Å, charge +1, hydrophobicity +1, hydrogen-bond donor and acceptor properties +1). The value of the attenuation factor was set to 0.30.

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RESULTS AND DISCUSSION 1-Mercaptoalkan-3-ols. The mass spectra (EI) of the 1mercaptoalkan-3-ols, illustrated by 1-mercaptohexan-3-ol (Figure 2A), revealed only weak signals for the molecular ion at m/ 4331


Article

Journal of Agricultural and Food Chemistry Table 2. Retention Indices, MS-EI Data, and Sensory Properties of 3-Mercaptoalkan-1-ols RI odor qualityb



compound

FFAP

DB-5

MS-EI: m/z (intensity in %)a


1652

924

onion, leek

1.8

wine,11 hops20


1755

1028

grapefruit

0.23

wine,11 hops21


1842

1119

72 (100), 57 (70), 55 (69), 61 (54), 41 (41), 59 (31), 43 (31), 39 (22), 60 (18), ..., 106 (14, M+) 41 (100), 57 (89), 86 (76), 69 (63), 61 (47), 73 (32), 47 (25), 55 (22), 39 (19), ..., 120 (13, M+) 55 (100), 41 (68), 57 (56), 61 (39), 100 (37), 67 (29), 82 (24), 47 (22), 83 (21), ..., 134 (8, M+)

grapefruit

0.053


1942

1222

grapefruit

0.058


2050

1321

hops20

2156

1429

grapefruit, burned fatty, soapy

0.26

3-mercaptononan-1-ol

13

3-mercaptodecan-1-ol

2264

1530

55 (100), 41 (53), 57 (41), 61 (34), 81 (30), 69 (26), 114 (25), 43 (21), 68 (20), ..., 148 (6, M+) 55 (100), 41 (66), 69 (66), 57 (49), 68 (39), 61 (39), 81 (37), 43 (29), 82 (29), ..., 162 (8, M+) 55 (100), 69 (81), 82 (57), 57 (52), 95 (50), 68 (50), 81 (45), 61 (41), 83 (39), ..., 176 (6, M+) 55 (100), 69 (65), 57 (54), 68 (44), 82 (40), 67 (39), 83 (36), 61 (35), 81 (33), ..., 190 (2, M+)

passion fruit,1 wine,2 grapefruit,22 beer,19 guava23 wine,11 hops20

fatty, soapy

140

a

For further spectroscopic data (MS-CI, NMR) see the Supporting Information. bOdor quality as perceived at the sniffing port during GC-O at threshold level. cOdor thresholds in air were determined as previously reported.8 dFirst report.

Table 3. Retention Indices, MS-EI Data, and Sensory Properties of 4-Mercaptoalkan-2-ols RI

compound 4-mercaptopentan-2-ol 4-mercaptohexan-2-ol

4-mercaptoheptan-2-ol 4-mercaptooctan-2-ol 4-mercaptononan-2-ol 4-mercaptodecan-2-ol

FFAP

DB-5

MS-EI: m/z (intensity in %)

a

b

odor quality

1579 1602 1668 1690

(a) (b) (a) (b)

943 957 1043 1055

(a) (b) (a) (b)

45 (100), 86 (48), 71 (41), 61 (40), 43 (34), 41 (31), 60 (26), 69 (25), 42 (17), ..., 120 (3, M+)

onion, meat

45 (100), 71 (58), 41 (43), 100 (43), 55 (40), 43 (28), 56 (26), 74 (24), 83 (22), ..., 134 (2, M+)

1753 1768 1853 1869 1953 1969 2056 2071

(a) (b) (a) (b) (a) (b) (a) (b)

1135 1144 1235 1244 1338 1347 1440 1449

(a) (b) (a) (b) (a) (b) (a) (b)

45 (100), 55 (96), 71 (68), 43 (50), 41 (34), 114 (34), 61 (21), 70 (21), 87 (20), ..., 148 (2, M+)

grapefruit (a) meat, grapefruit (b) grapefruit

45 (100), 71 (76), 69 (75), 43 (57), 41 (52), 55 (50), 128 (35), 74 (25), 61 (24), ..., 162 (2, M+)

grapefruit

45 (100), 71 (91), 55 (80), 69 (61), 43 (58), 41 (55), 142 (45), 83 (36), 56 (27), ..., 176 (2, M+)

grapefruit, fatty

71 (100), 45 (92), 55 (85), 43 (68), 69 (60), 41 (57), 156 (47), 83 (38), 102 (32), ..., 190 (1, M+)

burned, fatty

odor thresholdc (ng/L in air) 2.1 0.45 0.11 0.41

(a) (b) (a) (b)

0.072 0.27 0.083 0.44 0.11 0.78 6.0 12

(a) (b) (a) (b) (a) (b) (a) (b)


bell pepper12

bell pepper12

a


z 134. The secondary hydroxy group was clearly indicated by the loss of water (m/z 116), whereas the presence of the mercapto function resulted in characteristic fragments at m/z 101 [M − SH]+ and m/z 100 [M − H2S]+. All compounds of this homologous series showed a strong signal for m/z 87 attributed to the ion C4H7S+. The odor thresholds of the 1-mercaptalkan-3-ols showed a strong dependency on the length of the alkyl chain. After a continuous decline in odor thresholds from 3-mercaptopropan1-ol to 1-mercaptohexan-3-ol (Table 1), a minimum was found at a chain length of six to seven carbon atoms. A further elongation of the alkyl chain resulted in a clear increase in odor thresholds with the threshold of 1-mercaptodecan-3-ol being by a factor of 270 higher than that of 1-mercaptohexan-3-ol. As for the odor thresholds, also a change in odor qualities was observed. Whereas the short-chain homologues 3-mercaptopropan-1-ol and 4-mercaptobutan-2-ol exhibited leek- and onion-like odors, the C6/C7 compounds showed burned aroma notes. With an increasing number of carbon atoms the odor quality finally changed to mushroom-like and fatty. Except for 3-mercaptopropan-1-ol and 1-mercaptopentan-3-ol, to the

best of our knowledge, none of these compounds have been identified in foods so far. 3-Mercaptoalkan-1-ols. In contrast to the 1-mercaptoalkan-3-ols, the mass spectra of the 3-mercaptoalkan-1-ols, for example, 3-mercaptohexan-1-ol (Figure 2B), revealed stronger signals for the molecular ion at m/z 134, whereas the signals for [M − H2O]+ were considerably weaker, proving the presence of a primary hydroxy group. The mercapto function was indicated by [M − H2S]+ (m/z 100). Except for the signal at m/z 87 (C4H7S+), showing only weak intensities in the group of 3-mercaptoalkan-1-ols, their fragmentation patterns were quite similar to those of the corresponding 1-mercaptoalkan-3ols. The odor thresholds of the 3-mercaptoalkan-1-ols showed a similar dependency on the chain length as found for the 1mercaptoalkan-3-ols. With an increasing number of carbon atoms from C3 to C6, the odor thresholds decreased to a minimum at C6/C7 (Table 2). Although there was a slight increase in odor thresholds at C8, a further elongation of the alkyl chain from C8 to C10 resulted in an exponential increase. For example, the odor threshold of 3-mercaptodecan-1-ol was 4332


Article

Journal of Agricultural and Food Chemistry Table 4. Retention Indices, MS-EI Data, and Sensory Properties of 2-Mercaptoalkan-1-ols RI

a

compound

FFAP

DB-5


2-mercaptopropan-1-ol

1493

800


1586

900


1678

996


1779

1098


1892

1200


2000

1309

61 (100), 60 (24), 92 (22, M+), 74 (19), 59 (13), 41 (13), 45 (8), 39 (6), 59 (6), 62 (6) 75 (100), 41 (69), 74 (50), 47 (42), 106 (32, M+), 55 (27), 59 (26), 88 (23), 39 (15), 45 (14) 55 (100), 89 (61), 47 (47), 88 (35), 120 (27, M+), 41 (27), 59 (22), 69 (17), 102 (15), 60 (15) 69 (100), 41 (45), 103 (24), 60 (20), 55 (17), 59 (13), 47 (13), 134 (12, M+), 102 (12), 61 (11) 55 (100), 83 (63), 41 (55), 117 (45), 60 (29), 56 (22), 43 (20), 57 (19), 59 (16), ..., 148 (12, M+) 55 (100), 41 (40), 131 (38), 43 (31), 97 (29), 69 (27), 60 (22), 87 (15), 56 (15), ..., 162 (7, M+)

odor quality

b


cabbage, burned

0.34

onion, meat

0.17

grapefruit, burned

0.23

grapefruit, burned

0.14

burned

0.72

burned

1.5


a


Figure 3. Selected mass spectra (MS-EI) of methyl- and ethyl-branched mercaptoalkanols: (A) 1-mercapto-2-methylhexan-3-ol; (B) 3-mercapto-2methylhexan-1-ol; (C) 2-ethyl-3-mercaptohexan-1-ol; (D) 3-mercapto-3-methylhexan-1-ol.

most known homologue and has been reported as a key aroma compound of passion fruit,1 wine,2 and guava.3 4-Mercaptoalkan-2-ols. Besides the molecular ion at m/z 134, characteristic fragments for the 4-mercaptoalkan-2-ols, represented by 4-mercaptohexan-2-ol (Figure 2C), were m/z 116 [M − H2O]+ and m/z 100 [M − H2S]+, proving the presence of the OH− and SH− groups. The base peak at m/z 45 results from β-cleavage and indicates the presence of a hydroxy function in the 2-position. Due to a sufficient gas chromatographic separation, odor thresholds for the diastereomers could be determined individually; however, the absolute configurations were not

2600-fold higher than that of 3-mercaptohexan-1-ol. Also, the odor qualities changed with chain length: whereas 3mercaptobutan-1-ol revealed an onion-, leek-like odor, with C5−C8 interesting grapefruit-like odor qualities appeared, but at C9/C10 these were replaced by fatty, soapy odors. Among all mercaptoalkanols, the 3-mercaptoalkan-1-ols occur most often in foods, and except for 3-mercaptononan-1-ol and 3mercaptodecan-1-ol, all other mercaptans have previously been identified in foods. In addition, the 3-mercaptoalkan-1ols are often reported with a major impact on the overall food aroma, for example, for 3-mercaptobutan-1-ol and 3-mercaptopentan-1-ol in wine.11 In particular, 3-mercaptohexan-1-ol is the 4333


Article

Journal of Agricultural and Food Chemistry Table 5. Retention Indices, MS-EI Data, and Sensory Properties of 1-Mercapto-2-methylalkan-3-ols RI

compound 4-mercapto-3-methylbutan-2-ol 1-mercapto-2-methylpentan-3-ol 1-mercapto-2-methylhexan-3-ol 1-mercapto-2-methylheptan-3-ol

FFAP 1668 1675 1723 1731 1808 1811 1909 1911

(a) (b) (a) (b) (a) (b) (a) (b)

a

b

DB-5


odor quality

994

45 (100), 41 (37), 42 (33), 86 (26), 43 (23), 102 (23), 69 (20), 47 (17), 71 (17), ..., 120 (3, M+)

burned, grapefruit

1081

55 (100), 43 (45), 41 (45), 73 (42), 42 (27), 71 (25), 87 (18), 36 (17), 47 (17), ..., 134 (1, M+)

burned, grapefruit

1175

55 (100), 43 (45), 41 (45), 73 (42), 42 (27), 71 (25), 87 (18), 36 (17), 47 (17), ..., 148 (1, M+)

1276

69 (100), 41 (62), 87 (41), 55 (28), 42 (24), 57 (23), 101 (22), 43 (19), 36 (19), ..., 162 (1, M+)

rubber, burned (a) burned, meat (b) burned, rubber

odor thresholdc (ng/L in air) 0.49 0.98 0.059 0.052 0.080 0.088 19 42


(a) (b) (a) (b) (a) (b) (a) (b)

a

For further spectroscopic data (MS-CI, NMR) see Supporting Information. bOdor quality as perceived at the sniffing port during GC-O at threshold level. cOdor thresholds in air were determined as previously reported.8 dFirst report.

Table 6. Retention Indices, MS-EI Data, and Sensory Properties of 3-Mercapto-2-methylalkan-1-ols RI

compound

FFAP

DB-5

3-mercapto-2-methylpropan-1-ol 3-mercapto-2-methylbutan-1-ol 3-mercapto-2-methylpentan-1-ol

1690

948

1732

1013

1810 (a) 1818 (b)

1106

3-mercapto-2-methylhexan-1-ol 3-mercapto-2-methylheptan-1-ol

1892 1900 1989 1997

1194 1196 1292 1294

3-mercapto-2-methyloctan-1-ol

2089 (a) 2100 (b)

(a) (b) (a) (b)

a


b

odor quality

72 (100), 41 (91), 57 (87), 55 (80), 47 (77), 42 burned (43), 39 (40), 75 (36), 73 (28), 106 (28, M+) 60 (100), 61 (92), 41 (89), 86 (80), 55 (70), 71 burned, onion (63), 45 (49), 39 (34), 43 (31), ..., 120 (13, M+) 41 (100), 74 (73), 75 (37), 55 (36), 71 (33), 100 onion, meat (30), 47 (25), 39 (19), 43 (17), ..., 134 (5, M+) (a) (b) (a) (b)

1398 (a) 1402 (b)

55 (100), 88 (38), 41 (32), 114 (31), 71 (30), 47 onion, meat (20), 43 (19), 89 (17), 54 (13), ..., 148 (8, M+) 69 (100), 41 (72), 55 (68), 60 (52), 71 (39), 43 meat, onion (34), 128 (25), 87 (22), 102 (20), ..., 162 (6, M+) 55 (100), 69 (91), 83 (68), 41 (56), 71 (54), 60 burned, (50), 87 (44), 142 (37), 82 (24), ..., 176 (8, M+) grapefruit, fatty


wine,24 hops20

7.4

beer,13 wine,11 hops20

0.032 0.0014 (a) 0.73 (b) 0.0080 0.032 1.8 14


onion,25 leek, chives, scallions,4 wine,11 hops20

(a) (b) (a) (b)

8.6 (a) 39 (b)

a


determined. Therefore, the assignment of these compounds was done due to elution order found on the FFAP column (diastereomers I and II, Table 3). For 4-mercaptopentan-2-ol, diastereomer I showed a slightly higher threshold than diastereomer II, whereas for the other compounds the threshold of the first eluting diastereomer was always a bit lower. Up to a carbon number of nine, both diastereomers revealed relatively constant threshold values, followed by a significant increase in odor threshold for 4-mercaptodecan-2-ol. Apart from 4-mercaptohexan-2-ol, exhibiting a meatier aroma impression for diastereomer I and a more grapefruit-like flavor for diastereomer II, no significant differences in odor qualities among the diastereomers were observed. Wheras for most of the 4-mercaptopentan-2-ols, meaty, onion-like odors were observed, the C6−C9 compounds elicited a grapefruit-like aroma. However, with increasing odor thresholds at C10, again a fatty aroma occurred. 4-Mercaptoheptan-2-ol and 4-mercaptononan-2-ol have been identified in bell pepper,12 but the influence of these compounds on the overall bell pepper flavor was not yet evaluated. 2-Mercaptoalkan-1-ols. The mass spectra of the 2mercaptoalkan-1-ols revealed signals for M+ (m/z 134) and [M − H2O]+ (m/z 116), as exemplified for 2-mercaptohexan-1ol (Figure 2D). In contrast to the other mercaptoalkanols, these compounds showed only very weak signals for [M − H2S]+

(m/z 100). Further characteristic fragments for this homologous series were C5H11S+ (m/z 103), resulting from βelimination, and m/z 69, representing [M − CH3O − H2S]+. Up to a carbon number of six, the odor thresholds of the 2mercaptoalkan-1-ols remained relatively constant (Table 4), but with increasing chain length, the homologous 2-mercaptoheptan-1-ol and 2-mercaptooctan-1-ol showed higher thresholds. The short-chain compounds (C3, C4) exhibited cabbage- and onion-like flavors, switching to grapefruit-like and burned aroma notes at C5 and C6. This grapefruit-like aroma quality no longer appeared for the C7 and C8 compounds, which showed burned aroma qualities. Interestingly, none of these 2mercaptoalkan-1-ols have ever been reported as food constituents. 1-Mercapto-2-methylalkan-3-ols. The mass spectra of the 1-mercapto-2-methylalkan-3-ols, represented by 1-mercapto-2-methylhexan-3-ol (Figure 3A), showed a very similar fragmentation as the 1-mercaptoalkan-3-ols, but with certain differences in the intensities of the fragments. In addition to a weak signal for M+ (m/z 148), signals for [M − H2O]+ (m/z 130) and [M − H2S]+ (m/z 114) appeared. Further significant fragments at m/z 101 and 87 resulted from the fragments C5H9S+ and C4H7S+. The 1-mercapto-2-methylalkan-3-ols also revealed a minimum in odor thresholds at C5/C6 (Table 5). An extension of 4334


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Journal of Agricultural and Food Chemistry Table 7. Retention Indices, MS-EI Data, and Sensory Properties of 2-Ethyl-3-mercaptoalkan-1-ols RI

compound 2-(mercaptomethyl)butan-1-ol 2-ethyl-3-mercaptobutan-1-ol 2-ethyl-3-mercaptopentan-1-ol 2-ethyl-3-mercaptohexan-1-ol 2-ethyl-3-mercaptoheptan-1-ol

FFAP 1798 1780 1810 1872 1892 1945 1966 2037 2060

DB-5 1047

(a) (b) (a) (b) (a) (b) (a) (b)

1102 1105 1191 1196 1279 1285 1376 1384

(a) (b) (a) (b) (a) (b) (a) (b)


a

odor quality

b


86 (100), 57 (89), 55 (86), 41 (52), 47 (52), 69 (40), 56 (36), 68 (31), 60 (30), ..., 120 (19, M+) 61 (100), 60 (71), 100 (67), 55 (59), 71 (57), 41 (56), 69 (27), 83 (25), 56 (22), ..., 134 (13, M+)

onion, meat

0.61

onion, meat

74 (100), 55 (93), 75 (63), 41 (58), 85 (45), 114 (44), 83 (21), 47 (21), 101 (20), ..., 148 (11, M+)

onion, meat

55 (100), 88 (48), 69 (33), 128 (30), 89 (29), 41 (27), 85 (22), 57 (19), 56 (16), ..., 162 (7, M+)

onion, meat

69 (100), 55 (49), 60 (41), 41 (40), 142 (25), 85 (23), 87 (20), 57 (19), 67 (17), ..., 176 (7, M+)

burned, onion

0.0089 3.3 0.0058 0.047 0.027 0.040 1.2 1.8


(a) (b) (a) (b) (a) (b) (a) (b)

a For further spectroscopic data (MS-CI, NMR) see the Supporting Information. bOdor quality as perceived at the sniffing port during GC-O at threshold level. cOdor thresholds in air were determined as previously reported.8 dFirst report.

Table 8. Retention Indices, MS-EI Data, and Sensory Properties of 3-Mercapto-3-methylalkan-1-ols RI

a

b


reported previously as food constituentd coffee,15 wine,2 passion fruit,16 blackcurrant,26 beer,19 hops20

FFAP

DB-5


odor quality

3-mercapto-3-methylbutan-1-ol 3-mercapto-3-methylpentan-1-ol 3-mercapto-3-methylhexan-1-ol

1658

975

sweat, onion

0.42

1800

1093

sweat, onion

0.045

1879

1183

grapefruit

0.0047

3-mercapto-3-methylheptan-1-ol 3-mercapto-3-methyloctan-1-ol 3-mercapto-3-methylnonan-1-ol 3-mercapto-3-methyldecan-1-ol

1973

1281

grapefruit

0.0048

2079

1381

grapefruit

0.078

2185

1486

2293

1590

69 (100), 41 (76), 86 (54), 71 (49), 75 (34), 87 (25), 39 (18), 43 (15), 55 (15), ..., 120 (5, M+) 55 (100), 83 (58), 41 (47), 71 (40), 100 (33), 67 (20), 101 (18), 43 (17), 39 (16), ..., 134 (3, M+) 55 (100), 97 (53), 41 (48), 71 (48), 81 (44), 69 (42), 56 (26), 115 (25), 114 (24), ..., 148 (3, M+) 69 (100), 55 (81), 41 (53), 71 (47), 56 (45), 81 (26), 129 (25), 87 (22), 43 (22), ..., 162 (2, M+) 69 (100), 55 (73), 41 (55), 71 (46), 56 (35), 83 (29), 81 (21), 57 (21), 68 (16), ..., 176 (1, M+) 69 (100), 55 (91), 41 (72), 71 (68), 83 (64), 56 (52), 81 (36), 43 (34), 68 (28), ..., 190 (1, M+) 55 (100), 69 (83), 41 (80), 71 (67), 56 (67), 43 (44), 83 (43), 97 (41), 81 (39), ..., 204 (1, M+)

compound

grapefruit, fatty meat, fatty

11 62

a


Odor thresholds for the diastereomers were determined individually, except for the 3-mercapto-2-methylbutan-1ols, which did not show a sufficient GC separation. Thereby, odor thresholds of the first eluting diastereomer were consistently lower than for the second eluting compound (Table 6). The biggest differences in odor thresholds were found for the diastereomers of 3-mercapto-2-methylpentan-1-ol (factor 520). The other homologues showed only slight differences in the range from factor 4 to factor 8. The first eluting diastereomer revealed a distinct minimum in odor thresholds at C5/C6 with extremely low thresholds for 3mercapto-2-methylpentan-1-ol and 3-mercapto-2-methylhexan1-ol (0.0014 and 0.0080 ng/L). With a higher number of carbon atoms, the threshold values exponentially increased, for example, by a factor of 6100 between C5 and C8. In contrast to the differences in odor thresholds, no differences were detected for the odor qualities of the diastereomers. At C 3 /C 4 predominantly burned odors were detected; with increasing length of the alkyl chain (C5−C7) onion-like, meaty aroma notes occurred, which finally changed to burned, grapefruit-like at C8. Some of these 3-mercapto-2-methylalkan-1-ols have been shown to cause a major influence on food aromas; for example, 3-mercapto-2-methylpentan-1-ol is a key aroma compound of

the alkyl chain to 1-mercapto-2-methylheptan-3-ol then resulted in a 240-fold (diastereomer I) or even 480-fold (diastereomer I) increase in odor thresholds. In contrast to the other homologous series, no significant differences in odor thresholds between the respective diastereomers were observed. The 3-mercapto-2-methylalkan-3-ols consistently exhibited burned odor notes. In addition, 4-mercapto-3-methylbutan-2ol and 1-mercapto-2-methylpentan-3-ol also showed grapefruitlike aroma qualities, whereas for 1-mercapto-2-methylhexan-3ol and 1-mercapto-2-methylheptan-3-ol rubber-like odors were detected. As for the odor thresholds, also for the aroma qualities no significant differences between the diastereomers occurred. To the best of our knowledge, none of these compounds has been identified in foods so far. 3-Mercapto-2-methylalkan-1-ols. The mass spectra of the 3-mercapto-2-methylalkan-1-ols, as shown for 3-mercapto2-methylhexan-1-ol (Figure 3B), revealed fragmentation patterns very similar to those of the corresponding 3mercaptoalkan-1-ols. Characteristic signals occurred at m/z 148 (M+), m/z 130 ([M − H2O]+), and m/z 114 ([M − H2S]+). However, in contrast to the 3-mercaptoalkan-1-ols, compounds of this series showed considerably higher signals for [M − CH3O]+ (m/z 117). 4335


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Journal of Agricultural and Food Chemistry onion, leek, and chive,4 and 3-mercapto-2-methylbutan-1-ol was identified in wine11 and beer.13 2-Ethyl-3-mercaptoalkan-1-ols. Although the fragmentation patterns of the 2-ethyl-3-mercaptoalkan-1-ols, shown for 2ethyl-3-mercaptohexan-1-ol (Figure 3C), were very similar to those of the corresponding 3-mercapto-2-methylalkan-1-ols, a differentiation of the compounds was possible by signal intensities. As for the 3-mercapto-2-methylalkan-1-ols, odor thresholds of the diastereomeric 2-ethyl-3-mercaptoalkan-1-ols were determined individually. For the 2-ethyl-3-mercaptobutan-1ols, a difference in odor thresholds by a factor of 370 was observed (Table 7). In contrast, the diastereomers of 2-ethyl-3mercaptopentan-1-ol showed only a smaller difference in their thresholds. For 2-ethyl-3-mercaptohexan-1-ols and 2-ethyl-3mercaptoheptan-1-ols no significant differences among the diastereomers were observed. This homologous series showed a minimum in odor thresholds at 2-ethyl-3-mercaptopentan-1-ol. An extension of the alkyl chain again resulted in a significant increase in odor thresholds, with 2-ethyl-3-mercaptoheptan-1-ol showing by a factor of 310 a higher threshold than 2-ethyl-3mercaptopentan-1-ol. In contrast to the differences in odor thresholds, no differences in odor qualities among the diastereomers were detected. To the best of our knowledge, none of these compounds has been identified in foods so far. 3-Mercapto-3-methylalkan-1-ols. In contrast to the primary and secondary thiols, the mass spectra of the tertiary 3-mercapto-3-methylalkan-1-ols, exemplarily shown for 3mercapto-3-methylhexan-1-ol (Figure 3D), showed considerably higher signals for [M − SH]+ (m/z 115). The characteristic fragments at m/z 114 and 130 were formed by the loss of H2S and H2O. The remaining signals were very similar to those for the corresponding 3-mercaptoalkan-1-ols with differences in signal intensities. The odor thresholds of the 3-mercapto-3-methylalkan-1-ols again were strongly influenced by the length of the alkyl chain. A decrease by a factor of 90 from 3-mercapto-3-methylbutan-1ol to 3-mercapto-3-methylhexan-1-ol was determined with a minimum for 3-mercapto-3-methylhexan-1-ol and 3-mercapto3-methylheptan-1-ol (Table 8). With increasing number of carbon atoms the thresholds again clearly increased, with 3mercapto-3-methyldecan-1-ol showing a 13,000-fold higher odor threshold than 3-mercapto-3-methylhexan-1-ol. Whereas 3-mercapto-3-methylbutan-1-ol and 3-mercapto-3-methylpentan-1-ol showed sweaty, onion-like odor notes, for 3-mercapto3-methylhexan- and -heptan-1-ol an interesting grapefruit-like aroma was observed. With thresholds of 0.0047 and 0.0048 ng/ L, respectively, these compounds belong to the most potent odorants analyzed in this study. With increasing chain length (3-mercapto-3-methylnonan-1-ol and 3-mercapto-3-methyldecan-1-ol), however, again fatty aroma notes occurred. Although for 3-mercapto-3-methylhexan-1-ol an influence on the overall aroma of axillary sweat was discussed14 and 3-mercapto-3methylbutan-1-ol was identified in coffee,15 wine,2 and passion fruit,16 the other compounds of this homologous series have not been identified in foods so far. Influence of Structural Modification on Odor Properties of Mercaptans. Influence of an Additional Hydroxy Function. In comparison to the alkanethiols recently analyzed,5 the mercaptoalkanols of this study showed a striking similarity regarding the dependency of the odor thresholds on structural elements (Figure 4). The additional hydroxy function in the molecule had no significant influence on the shape of the

Figure 4. Comparison of odor thresholds of alkane-1-thiols (data according to Polster et al.5) and 3-mercaptoalkan-1-ols.

threshold curve or on the threshold values. However, in contrast to the odor thresholds, clear differences in the odor qualities were observed. The alkanethiols predominantly exhibited burned, roasty aroma notes at minimum level,5 whereas for the mercaptoalkanols mainly grapefruit-like or onion-like odors were perceived. In general, it was found that an additional hydroxy function in the molecule resulted in more pleasant, fruity aroma qualities at minimum level. Chain Length. The biggest effect on odor thresholds was observed for the length of the alkyl chain. In general, in all homologous series a minimum in odor thresholds for compounds with a chain length of five to seven carbon atoms was observable. By contrast, at C10 the odor thresholds were always several orders of magnitude higher than at minimum level. Similar results have recently been found for structure− odor correlations of alkanethiols.5 This result suggests that, besides the importance of the mercapto group on the odor activity of thiols, the odor perception of mercaptans is also strongly influenced by steric effects. Above a certain length of the alkyl chain, the sensitivity for the detection of thiols at receptor level seems to be rapidly decreasing. Apart from odor thresholds, also odor qualities were influenced by the length of the alkyl chain. With increasing number of carbon atoms (C9, C10) the odor often changed to a less sulfury, but more fatty, soapy aroma note. Methyl Substitution at the Mercapto-Containing Carbon Atom. Primary (1-mercaptoalkan-3-ols), secondary (3-mercaptoalkan-1-ols), and tertiary thiols (3-mercapto-3-methylalkan-1ols) revealed very similar dependencies on the chain length (Figure 5): After a significant decline in odor thresholds from C3/C4 to a minimum at C6/C7, an exponential increase in threshold values was observed up to C10. With odor thresholds of 0.0047 and 0.0048 ng/L, 3-mercapto-3-methylhexan-1-ol and 3-mercapto-3-methylheptan-1-ol belong to the most potent odorants identified so far. With increasing substitution at the mercapto-containing carbon atom, the thresholds at minimum level decreased. Therefore, the primary thiols (1-mercaptoalkane-3-ols) showed by a factor of 30 higher odor thresholds at minimum level than the tertiary thiols (3-mercapto-3methylalkan-1-ols). This is also in agreement with the results observed for the alkanethiols,5 but for the mercaptoalkanols this behavior was even more pronounced. 4336


Article


Figure 7. Comparison of odor thresholds of 2-mercaptoalkan-1-ols and 3-mercaptoalkan-2-ols.

Figure 5. Influence of chain length on odor thresholds of homologous primary, secondary, and tertiary mercaptoalkanols.

function from the β- to the α-position resulted in slightly higher thresholds at minimum level (C6). With increasing length of the alkyl chain (C7, C8), also for the 2-mercaptoalkan-1-ols an increase in threshold values was observed. However, the shift of the SH− function did not have a significant influence on the odor qualities. For both homologous series a similar tendency regarding the aroma qualities was observed. Alkyl Substitution. To study the influence of alkyl substitution on the sensory properties of mercaptoalkanols, in addition to the 3-mercapto-3-methylalkan-1-ols, the 3-mercapto-2-methylalkan-1-ols and the 1-mercapto-2-methylalkan-3-ols as well as the 2-ethyl-3-mercaptoalkan-1-ols were synthesized. In comparison to the 3-mercaptoalkan-1-ols, the additional methyl substitution clearly lowered the odor thresholds at minimum level (Figure 8). The biggest effect on odor thresholds was found for a methyl substitution in the αposition to the SH− group. The 3-mercapto-2-methylalkan-1ols showed a 40-fold lower threshold at minimum level than the 3-mercaptoalkan-1-ols and revealed the most distinct minimum

Position of the SH− and OH− Groups. A shift of the OH− and SH− function within the alkyl chain from the 3mercaptoalkan-1-ols to the 4-mercaptoalkan-2-ols did not much affect the threshold values at C5−C7 (Figure 6). The

Figure 6. Comparison of odor thresholds of 3-mercaptoalkan-1-ols and 4-mercaptoalkan-2-ols.

compounds with the same carbon atoms showed almost identical thresholds and similar odor qualities at the minimum level. Whereas for the 3-mercaptoalkan-1-ols a clear increase in odor thresholds was observed after C7, the 4-mercaptoalkan-2ols revealed a rather constant threshold value up to C9 and, thus, showed the broadest minimum of all homologous series examined. With increasing chain length (>C9), also for the 4mercaptalkan-2-ols the odor thresholds significantly increased. To show the influence of a shift of the SH− group from the β- to the α-position, odor thresholds of the 3-mercaptoalkan-1ols were compared to those of the corresponding 2mercaptoalkan-1-ols (Figure 7). Whereas the short-chain homologous (C3/C4) 2-mercaptoalkan-1-ols revealed lower thresholds than the 3-mercaptoalkan-1-ols, the shift of the SH−

Figure 8. Influence of methyl substitution on odor thresholds of mercaptoalkanols. 4337


Article

Journal of Agricultural and Food Chemistry of all homologous series analyzed. Their threshold values decreased by a factor of 5300 from C3 to C5. After the minimum, the thresholds increased as rapidly as they had decreased (factor 6100 from C5 to C8). With 0.0014 ng/L, 3mercapto-2-methylpentan-1-ol exhibited the lowest odor thresholds of all mercaptoalkanols considered in this study. The threshold of this compound was in the same range as found previously for the (2S3R)-enantiomer,17 but was higher than the threshold of the (2R3S)-enantiomer (0.00007−0.0002 ng/L).17 In accordance with these results, a big difference in odor thresholds of the diastereomers (factor 520) was also observed for 3-mercapto-2-methylpentan-1-ols (Table 6). All other homologous mercaptans showed only slight differences in the range of factor 4−8 between the respective diastereomers. The methyl substitution in the α-position to the mercapto function also resulted in a change in odor qualities. Whereas the 3-mercaptoalkan-1-ols and the 3-mercapto-3-methylalkan-1-ols exhibited grapefruit-like aroma notes, the 3-mercapto-2methylalkan-1-ols revealed onion-like, meaty aroma qualities at the minimum level. Because of the big influence of a 2-methyl substitution on odor thresholds of mercaptoalkanols, also the 2-ethyl derivatives were examined. This structural modification resulted in slightly higher thresholds at the minimum level (Figure 9).

Figure 10. Comparison of odor thresholds of 3-mercapto-2methylalkan-1-ols and 1-mercapto-2-methylalkan-3-ols.

The change in the positions of the mercapto and hydroxy functions had a significant influence on the odor thresholds. Although both homologous series showed a similar dependency on the chain length with minimum thresholds at C5, the odor threshold of 1-mercapto-2-methylpentan-3-ol was by a factor of 40 higher compared to 3-mercapto-2-methylpentan-1-ol. As previously shown for the alkan-1-thiols5 as well as for the 1mercaptoalkan-3-ols, primary thiols revealed lower odor thresholds than secondary thiols at minimum level. In contrast to the 3-mercapto-2-methylalkan-1-ols, for the 1mercapto-2-methylalkan-1-ols no significant differences in the odor thresholds of the diastereomers were observed. Besides the differences in odor thresholds, also different odor qualities were detected. Whereas the 3-mercapto-2-methylalkan-1-ols predominantly showed onion-like, meaty odor notes, the 1mercapto-2-methylalkan-3-ols exhibited burned, grapefruit-like odor qualities. 3D-QSAR Studies. To get a first insight into aroma-relevant molecular substructures defining the space for a low odor threshold in mercaptoalkanols, 3D-QSAR calculations were performed. Using CoMSIA, 3D-QSAR models were established to correlate the threshold data of the 49 thiols analyzed with their molecular structures. For the calculation of the final CoMSIA model, only steric and electrostatic descriptors were used. An inclusion of hydrophobic as well as hydrogen-bond donor and acceptor interactions did not improve the model. As already found for the alkanethiols,5 most of the differences in odor thresholds could be explained by steric effects. Electrostatic fields had only little influence on odor thresholds. The thresholds simulated by a first theoretical model were in good agreement with the experimentally determined data (Figure 11). Statistical validation showed a reliable model with a coefficient of correlation (r2) of 0.90 at a number of 11 principal components and a standard deviation (s) of 0.40. In conclusion, the results have shown how the sensory properties within the group of mercaptoalkanols are determined by their structures. The biggest influence on the odor thresholds was found for steric effects, that is, the length of the carbon chain. Interestingly, all homologous series analyzed revealed a minimum in odor thresholds between compounds with five to seven carbon atoms, whereas increasing the chain length led to an exponential increase in the odor

Figure 9. Comparison of odor thresholds of 3-mercapto-2methylalkan-1-ols and 2-ethyl-3-mercaptoalkan-1-ols.

Nevertheless, the 2-ethyl-3-mercaptoalkan-1-ols revealed a very similar dependency on the chain length as the 3-mercapto-2methylalkan-1-ols with very low odor thresholds between C4 and C6. As for the 3-mercapto-2-methylalkan-1-ols, for some of the homologues, big differences were observed in the odor thresholds of the diastereomers (Table 7), but with increasing length of the alkyl chain, the threshold differences of the diastereomers decreased. The change from methyl to ethyl substitution did, however, not affect the odor qualities of the compounds. Like the 3-mercapto-2-methylalkan-1-ols, the 2ethyl-3-mercaptoalkan-1-ols elicited onion-like, meaty aroma notes. To evaluate the influence of different positions of the SH− and the OH− groups on odor thresholds of methyl-substituted mercaptoalkanols, the 3-mercapto-2-methylalkan-1-ols were compared to the 1-mercapto-2-methylalkan-3-ols (Figure 10). 4338


Article

Journal of Agricultural and Food Chemistry Notes

The authors declare no competing financial interest.

■

ABBREVIATIONS USED CoMSIA, comparative molecular similarity indices analysis; GC-O, gas chromatography−olfactometry; PLS, partial leastsquares regression; QSAR, quantitative structure−activity relationship; r2, correlation coefficient; s, standard deviation; Mr, relative molecular mass

■

Figure 11. Correlation of determined and predicted odor thresholds of mercaptoalkanols.

threshold. The olfactory power of the thiols was considerably improved by methyl or ethyl substitution in the α-position to the mercapto group as well as by methyl substitution at the mercapto-containing carbon atom. Structure−odor correlations of the mercaptoalkanols revealed a great similarity to those for the alkanethiols previously analyzed.5 This indicates that chemoreception of these different thiols should occur by similar mechanisms or by activation of a limited number of similar olfactory receptors. As recently shown, obviously for some mercaptans, such as 3-mercapto-2-methylpentan-1-ol, specific odorant receptors do exist, allowing the detection of food odorants occurring in very low concentrations.27 In general, the results also allow the conclusion that the molecular space for yet unknown powerful mercaptans contributing to the aroma of foods is quite narrow and should be found in the range of five to seven carbon atoms. Furthermore, only certain odotopes are probably able to interact in an effective way with the odorant receptors as recently shown for structural modifications in the well-known grapefruit aroma compound 1-menthene-8-thiol.28 Thus, these results are a further indication that the human odorant receptors were developed in close connection with the odorants occurring in nature.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.7b01266. Experimental procedures and spectral data (NMR, MS) of the synthesized compounds and intermediates. Figures S1−S6 illustrate the synthetic pathways of the reactions. Figures S7−S112 show the NMR spectra of the synthesized sulfur compounds. (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*(P.S.) Phone: +49 8161 712932. Fax: +49 8161 712970. Email: [email protected]. ORCID

Peter Schieberle: 0000-0003-4153-2727 4339


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Journal of Agricultural and Food Chemistry (17) Luentzel, C. S.; Widder, S.; Voessing, T.; Pickenhagen, W. Enantioselective syntheses and sensory properties of the 3-mercapto-2methylpentanols. J. Agric. Food Chem. 2000, 48, 424−427. (18) Lavigne, V.; Henry, R.; Dubourdieu, D. Identification and determination of sulfur compounds responsible for “grilled” aroma in wines. Sci. Aliments 1998, 18, 175−191. (19) Vermeulen, C.; Lejeune, I.; Tran, T. T. H.; Collin, S. Occurrence of polyfunctional thiols in fresh lager beers. J. Agric. Food Chem. 2006, 54, 5061−5068. (20) Gros, J.; Nizet, S.; Collin, S. Occurrence of odorant polyfunctional thiols in the super alpha tomahawk hop cultivar. Comparison with the thiol-rich Nelson sauvin bitter variety. J. Agric. Food Chem. 2011, 59, 8853−8865. (21) Takoi, K.; Degueil, M.; Shinkaruk, S.; Thibon, C.; Maeda, K.; Ito, K.; Bennetau, B.; Dubourdieu, D.; Tominaga, T. Identification and characteristics of new volatile thiols derived from the hop (Humulus luplus L.) cultivar Nelson Sauvin. J. Agric. Food Chem. 2009, 57, 2493− 2502. (22) Lin, J.; Rouseff, R. L.; Barros, S.; Naim, M. Aroma composition changes in early season grapefruit juice produced from thermal concentration. J. Agric. Food Chem. 2002, 50, 813−819. (23) Steinhaus, M.; Sinuco, D.; Polster, J.; Osorio, C.; Schieberle, P. Characterization of the aroma-active compounds in pink guava (Psidium guajava, L.) by application of the aroma extract dilution analysis. J. Agric. Food Chem. 2008, 56, 4120−4127. (24) Bouchilloux, P.; Darriet, P.; Henry, R.; Lavigne-Cruege, V.; Dubourdieu, D. Identification of volatile and powerful odorous thiols in Bordeaux red wine varieties. J. Agric. Food Chem. 1998, 46, 3095− 3099. (25) Widder, S.; Luentzel, C. S.; Dittner, T.; Pickenhagen, W. 3Mercapto-2-methylpentan-1-ol, a new powerful aroma compound. J. Agric. Food Chem. 2000, 48, 418−423. (26) Varming, C.; Petersen, M. A.; Poll, L. Comparison of isolation methods for the determination of important aroma compounds in black currant (Ribes nigrum L.) juice, using nasal impact frequency profiling. J. Agric. Food Chem. 2004, 52, 1647−1652. (27) Noe, F.; Polster, J.; Geithe, C.; Kotthoff, M.; Schieberle, P.; Krautwurst, D. OR2M3: a highly specific and narrowly tuned human odorant receptor for the sensitive detection of the onion key food odorant 3-mercapto-2-methylpentan-1-ol. Chem. Senses 2017, 42, 195−210. (28) Schoenauer, S.; Schieberle, P. Structure-odor activity studies on monoterpenoid mercaptans synthesized by changing the structural motifs of the key food odorant 1-p-menthene-8-ol. J. Agric. Food Chem. 2016, 64, 3849−3861.

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