2-Deoxyglucosone – a new C6-α-dicarbonyl compound in the Maillard

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2‑Deoxyglucosone: A New C6‑α-Dicarbonyl Compound in the Maillard Reaction of D‑Fructose with γ‑Aminobutyric Acid Philipp Bruhns,*,† Martin Kaufmann,† Timo Koch,‡ and Lothar W. Kroh† †

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Fachgebiet Lebensmittelchemie und Analytik, Institut für Lebensmitteltechnologie und Lebensmittelchemie, Technische Universität Berlin, Gustav-Meyer-Allee 25, 13355 Berlin, Germany ‡ Pfeifer & Langen GmbH & Company KG, Aachener Straße 1042a, 50858 Köln, Germany ABSTRACT: In this study, α-dicarbonyl compounds consisting of a backbone with six carbon atoms resulting from the Maillard reaction of D-fructose with γ-aminobutyric acid were determined. The reaction was carried out under mild reaction conditions at 50 °C and water contents between 0 and 90%. A thus far unknown α-dicarbonyl compound was found as the main product in the first 24 h at water contents below 50%. After isolation of its stable quinoxaline derivative, it was possible to identify the compound as 2-deoxy-D-glycero-hexo-3,4-diulose (2-deoxyglucosone). For the first time, the four C6-α-dicarbonyl compounds, 1-deoxyglucosone, 2-deoxyglucosone, 3-deoxyglucosone, and 4-deoxyglucosone, could be identified in the Maillard reaction of a hexose at the same time. This indicates the formation of a 2,3-eneaminol from the Schiff base of D-fructose and the formation of 2-amino-2-deoxy-3-ketose as an alternative to the Heyns product. KEYWORDS: Maillard reaction, C6-α-dicarbonyl compounds, 2-deoxyglucosone, 2-amino-2-deoxy-3-ketose, 4-deoxyglucosone, Amadori product

1. INTRODUCTION The Maillard reaction of carbohydrates in the presence of amino compounds, such as amino acids or proteins, is one of the most important reactions for the color and aroma development during food processing and has been the object of a multitude of studies.1−6 The degradation of reducing carbohydrates leads to a wide range of products from acids, aldehydes, and heterocyclic compounds up to high-molecularmass melanoidins.7 Through the formation of manifold products, changes in flavor, taste, texture, and color can occur. The formation of these products and the respective reaction pathways are influenced by the reaction conditions, such as the temperature, type, and concentration of the solvent, pH value, and time.8 In addition, the presence of different carbohydrates and amino compounds leads to different products. Known precursors of most Maillard products are αdicarbonyl compounds, which are present in relatively small amounts; for example, Degen et al. determined the contents of α-dicarbonyl compounds in several foods and found amounts up to 1000 mg/kg.9 However, they are considered to be the most important intermediates during the Maillard reaction as a result of their high reactivity.10,11 Even some of the postulated melanoidine structures result from the polymerization of αdicarbonyl compounds.12−14 In the reaction of hexoses, such as, for example, D-glucose or D-fructose, D-arabino-hexo-2-ulose (D-glucosone), 1-deoxy-D-erythro-hexo-2,3-diulose (1-deoxyglucosone), and 3-deoxy-D-erythro-hexo-2-ulose (3-deoxyglucosone) are known α-dicarbonyl compounds with a backbone of six carbon atoms.5 Besides, 3-deoxy-D-threo-hexo-2-ulose (3deoxygalactosone) and 1,4-dideoxy-D-glycero-hexo-2,3-diulos (1,4-dideoxyglucosone) can be formed in small amounts.15,16 Another C6-α-dicarbonyl compound is 4-deoxy-D-glycero-hexo2,3-diulose (4-deoxyglucosone), which is known from the © XXXX American Chemical Society

degradation of 1,4-glycosidic-bound oligosaccharides but not from the reaction of monosaccharides.17−19 All mentioned αdicarbonyl compounds are formed via the Amadori product starting from D-glucose or via the Heyns product starting from D-fructose. The formation of the Heyns product occurs through the 1,2-enolization of the Schiff base, but also a 2,3enolization, leading to a ketone at C3, is conceivable, which is not described in the literature thus far. The aim of this study was to detect possible reaction products of this 2,3-enolization. In the reaction of D-fructose with γ-aminobutyric acid under mild reaction conditions at 50 °C in a low-moisture system, it was possible to detect a thus far unknown α-dicarbonyl compound with a backbone of six carbon atoms, which results from the 2,3-enolization.

2. MATERIALS AND METHODS The following chemicals were obtained commercially: D-(−)-fructose (Roth, Karlsruhe, Germany), γ-aminobutyric acid (Alfa Aesar, Karlsruhe, Germany), o-phenylenediamine (OPD, Fluka, Steinheim, Germany), dichloromethane (VWR, Darmstadt, Germany), sodium sulfate (Merck, Darmstadt, Germany), and methanol (VWR, Darmstadt, Germany) 2.1. Model Systems. D-Fructose was mixed in an equimolar ratio with γ-aminobutyric acid and homogenized in a mortar. A total of 500 mg of the mixture was transferred to a headspace vial (20 mL); the respective amount of water was added; and the vial was sealed, mixed, and heated in an aluminum block in a drying cabinet (Heraeus Function Line type T6) at 50 °C. After defined reaction times, the reaction was stopped by cooling in a freezer. The samples were filled up to a volume of 5 mL with purified water for further analysis. Received: Revised: Accepted: Published: A

August 13, 2018 October 17, 2018 October 18, 2018 October 18, 2018 DOI: 10.1021/acs.jafc.8b03629 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry 2.2. High-Performance Liquid Chromatography with Diode Array Detection (HPLC−DAD) Analysis of α-Dicarbonyl Compounds. The determination of α-dicarbonyl compounds was carried out with HPLC−DAD9,10,20 (pump, Agilent 1100 Series G1312A; degasser, Agilent 1100 Series G1379A; autosampler, Agilent 1100 Series G1313A; guard column, Macherrey-Nagel EC 4/3 Universal RP guard column; column, Marcherey-Nagel EC 250/5.6 NUCLEOSIL 120-5 C-18; detector, Hewlett-Packard Series 1100 G1315B; and software, ChemStation for LC 3D). For quantification, the α-dicarbonyl compounds were derivatized by trapping with OPD. This was performed by adding 750 μL of an OPD solution (50 mmol/L in water:methanol, 1:1, v/v) to 250 μL of the sample and subsequently storing the samples for 24 h. The analysis was carried out at 35 °C. The flow rate was 1 mL/min. Eluent A water and eluent B methanol were used. Eluent gradient: 0 min, 80% A; 15 min, 67% A; 19 min, 35% A; 28 min, 0% A; 32 min, 100% A; 40 min, 70% A; and 45 min, 80% A. The identification was made by comparison of the retention times and ultraviolet−visible (UV−vis) spectra to those of a standard mix, consisting of the quinoxalines of six α-dicarbonyl compounds, which had been synthesized in our workgroup10,21 (Dglucosone-q, tr = 10.5 min; 1-deoxyglucosone-q, tr = 12.9 min; 3deoxyglucosone-q, tr = 14.2 min; 3-deoxygalactosone-q, tr = 14.4 min; 1,4-dideoxglucosone-q, tr = 18.4 min; and 3,4-dideoxyglucosone-q, tr = 19.4 min). Isolated 2-deoxyglucosone-q (tr = 14.4 min) was used as a reference but quantified as 3-deoxyglucosone-q. The quantitation wavelength was 317 nm. 2.3. High-Performance Liquid Chromatography with Mass Spectrometry (HPLC−MS) Analysis of α-Dicarbonyl Compounds. For the identification of the unknown α-dicarbonyl compound, HPLC−MS was used (pump, Thermo Accela pump; degasser, Shimadzu DGU-20AS; autosampler, Thermo CTC PAL autosampler; column, Knauer ProntoSIL 60-5 Phenyl 250 × 4.6 with precolumn; detector, Thermo TSQ Vantage System ION Max Source (H-ESI II probe); and software, Thermo Xcalibur, version 2.1.0). The samples were trapped with OPD as described for the HPLC−DAD analysis. The analysis was carried out at 35 °C. The flow rate was 0.5 mL/min. Eluent A water and eluent B methanol were used. Eluent gradient: 0 min, 80% A; 5 min, 80% A; 35 min, 40% A; 40 min, 40% A; and 45 min, 80% A (D-glucosone-q, tr = 22.9 min; 1deoxyglucosone-q, tr = 26.4 min; 2-deoxyglucosone-q, tr = 29.1 min; and 3-deoxyglucosone-q, tr = 28.1 min). 2.4. Isolation of 2-Deoxy-D-glycero-hexo-3,4-diulose-quinoxaline (2-Deoxyglucosone-q). D-Fructose (20 g, 111 mmol, 1 equiv), γ-aminobutyric acid (11.45 g, 111 mmol, 1 equiv), OPD (2.4 g, 22.2 mmol, 0.2 equiv), and water (3.1 g, 172 mmol, 2.55 equiv) were homogenized in a mortar and transferred in a Duran glass bottle (250 mL). The mixture was heated in a drying cabinet at 50 °C for 24 h. The reaction was stopped by cooling in a freezer. For extraction, the mixture was dissolved in purified water (100 mL) and extracted 8 times with dichloromethane (each 100 mL). The organic phase was dried with anhydrous sodium sulfate, and the solvent was evaporated. The residual solid (902 mg) was purified by semipreparative HPLC (pump, Thermo Accela Pump; guard column, Phenomenex guard column; column, 2× Phenomenex, Luna 5 μm, C18(2), 100 Å, 250 × 10 mm; detector, Jasco UV-970; and software, Gynkosoft Chromatography Data System, version 5.60). The separation was carried out at 50 °C. The flow rate was 2.5 mL/min. Methanol/water (2:8, v/v) was used as the eluent. 2-Deoxy-D-glycero-hexo-3,4-diulosequinoxaline was obtained as a white solid. MS, m/z 235 [M + H]+; nuclear magnetic resonance (NMR), see Table 1. 2.5. 4-Deoxy-D-glycero-hexo-2,3-diulose-quinoxaline (4-Deoxyglucosone-q). Isolation was carried out as described for 2deoxyglucosone-q. 1 H NMR (DMSO-d6, δ in ppm): 3.11 (m, 1H, H-4), 3.18 (m, 1H, H-4′), 3.45 (m, 2H, H-6), 4.07 (m, 1H, H-5), 4.69 (m, 1H, OH-6), 4.78 (m, 1H, OH-5), 4.83 (m, 1H, H-1), 4.92 (m, 1H, H-1′), 5.39 (d, 1H, OH-1), 7.78 (m, 2H, Ar−H), 8.02 (m, 2H, Ar−H). 13 C NMR (DMSO-d6, δ in ppm): 38.3 (C4), 63.7 (C1), 65.8 (C6), 71.5 (C5). Quaternary carbons could not be assigned as a result of the low signal-to-noise ratio.

Table 1. Chemical Shifts and Coupling Constants of 2Deoxyglucosone-q Determined by Iterative Spectra Simulation Using the TopSpin Implement Daisy carbon

δ(13C) (ppm)

C1

60.3

proton H−C1

δ(1H) (ppm) 3.922

coupling constant (Hz) J1,1′ = −6.0 J1,OH1 = 5.4 3 J1,2 = 7.2 3 J1,2′ = 7.2 2 J1′1 = −6.0 3 J1′,OH1 = 5.2 3 J1′,2 = 6.6 3 J1′,2′ = 6.7 3 JOH1,1 = 5.4 3 JOH1,1′ = 5.2 2 J2,2′ = −14.6 3 J2,1 = 7.2 3 J2,1′ = 6.6 2 J2′,2 = −14.6 3 J2′,1 = 7.2 3 J2′,1′ = 6.7 2 3

C2

C3 C4 C5

37.5

154.9 156.0 71.5

H′−C1

3.920

OH−C1

4.76

H−C2

3.31

H′−C2

3.29

H−C5

5.09

3

J5,OH5 = 6.3 J5,6 = 5.7 3 J5,6′ = 6.3 3 JOH5,5 = 6.3 3 J6,5 = 5.7 2 J6,6′ = −10.9 3 J6,OH6 = 6.0 3 J6′,5 = 6.3 2 J6′,6 = −10.9 3 J6′,OH6 = 5.6 3 JOH6,6 = 6.0 3 JOH6,6′ = 5.6 3

C6

64.7

OH−C5 H−C6

5.47 3.83

H′−C6

3.90

OH−C6

4.72

2.6. NMR Spectroscopy. NMR data were recorded on a Bruker AVANCE III 500 NMR (Bruker Corporation) spectrometer operating at either 500.13 MHz (1H NMR) or 125.76 MHz (13C NMR) and applying standard one-dimensional (1D) and twodimensional (2D) NMR experiments [1H, 13C, distortionless enhancement by polarization transfer (DEPT), correlation spectroscopy (COSY), heteronuclear single quantum correlation (HSQC), and heteronuclear multiple bond correlation (HMBC)]. NMR chemical shifts were referenced to the residual signals of the solvent DMSO-d6.22 Chemical shifts as well as coupling constants were determined by iterative spectra simulations using the TopSpin implement Daisy (Bruker BioSpin GmbH).

3. RESULTS In most Maillard reaction studies of hexoses, the reaction of Dglucose via the Amadori product to the α-dicarbonyl compounds is investigated. In contrast to these studies, the reaction of D-fructose with γ-aminobutyric acid in an equimolar mixture at 50 °C at different water contents is presented here. The main focus was laid on the formation of the α-dicarbonyl compounds consisting of six carbon atoms. They were detected in the form of their stable quinoxaline derivatives after trapping with OPD. Derivatization was performed after the end of the reaction to avoid an impact of OPD on the αdicarbonyl compound formation. B

DOI: 10.1021/acs.jafc.8b03629 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry After 24 h at water contents below 50%, an unknown αdicarbonyl compound was detected in relatively high amounts in addition to 1-deoxyglucosone, 3-deoxyglucosone, and Dglucosone (Figure 1). The UV−vis spectrum of the unknown

Figure 2. D-Glucosone (GLUC), 1-deoxyglucosone (1-DG), 2deoxyglucosone (2-DG), and 3-deoxyglucosone (3-DG) contents in the reaction of D-fructose with γ-aminobutyric acid in an equimolar mixture at 50 °C and different water contents after 24 h.

The data shown in Figure 2 refer to a reaction time of 24 h at 50 °C. The amounts and ratio between the α-dicarbonyl compounds change with not only the water content but also the reaction time as a result of the different rates of formation and degradation. For a comprehensive overview on the impact on the reaction of a single α-dicarbonyl compound, a measurement of the kinetics over a longer reaction period is required. Figure 3 shows the amounts of D-glucosone, 1-

Figure 1. (A) EIC-MS chromatograms of the standard mix [Dglucosone-q (GLUC-q), 1-deoxyglucosone-q (1-DG-q), and 3deoxyglucosone-q (3-DG-q)]. (B) EIC-MS chromatograms of the reaction of D-fructose with γ-aminobutyric acid in an equimolar mixture at 50 °C and a water content of 10% (D-glucosone-q, 1deoxyglucosone-q, 3-deoxyglucosone-q, and 2-deoxyglucosone-q).

compound was typical for a quinoxaline, and the detected mass (m/z 235 [M + H]+) suggested that it consists of a backbone with six carbon atoms. For identification, the compound was isolated from an equimolar reaction mixture of D-fructose with γ-aminobutyric acid, 0.2 equiv of OPD, and a water content of 20%. OPD was added before reaction to enrich the αdicarbonyl compound by trapping as its stable quinoxaline derivative. The identification was performed applying 2D NMR spectroscopy. Chemical shifts as well as coupling constants were determined by iterative spectra simulations using the TopSpin implement Daisy (Table 1). It could be identified as 2-deoxy-D-glycero-hexo-3,4-diulose (2-deoxyglucosone), a thus far undescribed α-dicarbonyl compound in the Maillard reaction. Its constitution is similar to that of 1deoxyglucosone and 3-deoxyglucosone but with the αdicarbonyl group located at carbons C3 and C4. In the model reactions conducted at different water contents, the formation could be verified by comparing the elution times, the UV−vis spectra, and the molecular mass of the quinoxalines to the isolated reference. Dependent upon the water content and reaction time, 2deoxyglucosone is the main α-dicarbonyl compound in the reaction of D-fructose with γ-aminobutyric acid. Figure 2 shows the contents of the C6-α-dicarbonyl compounds after 24 h for water contents between 0 and 90%. 2-Deoxyglucosone is present in the highest amounts at water contents less than 50%. With an increasing water content, the amount of 2deoxyglucosone formed decreases. At water contents higher than 50%, the amount of D-glucosone and 3-deoxyglucosone formed exceeds the amount of 2-deoxyglucosone.

Figure 3. D-Glucosone (GLUC), 1-deoxyglucosone (1-DG), 2deoxyglucosone (2-DG), and 3-deoxyglucosone (3-DG) contents in the reaction of D-fructose with γ-aminobutyric acid in an equimolar mixture with 20% water at 50 °C.

deoxyglucosone, 2-deoxyglucosone, and 3-deoxyglucosone in the reaction of D-fructose with γ-aminobutyric acid in an equimolar mixture with 20% water at 50 °C dependent upon the time. After 24 h, 2-deoxyglucosone reaches the highest amounts with up to 0.06 mol %. With increasing the reaction time, the amounts decrease, and after 48 h, similar amounts of D-glucosone, 3-deoxyglucosone, and 2-deoxyglucosone are found. With an even longer reaction time, the amounts of Dglucosone and 3-deoxyglucosone increase, while the content of 2-deoxyglucosone further decreases and D-glucosone becomes the predominant α-dicarbonyl compound. 1-Deoxyglucosone is formed in minor amounts during the entire reaction, with up to 0.004 mol % after 24 h.

4. DISCUSSION In course of the reaction of an amino acid with D-fructose, 2amino-2-deoxy-aldose (Heyns product) is formed in analogy to the formation of 1-amino-1-deoxy-ketose (Amadori product) in the reaction of an amino acid with D-glucose.23,24 The first reaction step starting with D-fructose is the formation of a 2,2amino-alcohol followed by the formation of a Schiff base by elimination of a water molecule. Through a keto−enol tautomerism via 1,2-eneamiol, the Schiff base rearranges to the Heyns product (Figure 4). Likewise, the enolization to 2,3C

DOI: 10.1021/acs.jafc.8b03629 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Figure 4. Mechanism of the formation of 1-deoxyglucosone, 2-deoxyglucosone, 3-deoxyglucosone, and 4-deoxyglucosone via the Heyns product and the alternative Amadori product (2-amino-2-deoxy-3-ketose).

eneaminol is possible, which subsequently leads to the formation of 2-amino-2-deoxy-3-ketose. This compound is similar to an Amadori product with functional groups of a ketone and an amine. In the following, the product is called the alternative Amadori product. 4.1. Alternative Amadori Product. The Schiff base of Dfructose differs from the Schiff base of D-glucose, because it can form 2,3-eneaminol in addition to 1,2-eneamiol. The rearrangement of the Schiff base of D-glucose only leads to the Amadori product. Whether the Heyns product or alternative Amadori product is formed from the Schiff base of D-fructose depends upon the tendency of deprotonation at the carbon atoms C1 or C3. The acidity of the C−H bond is influenced by the electronegativity of the atoms involved in the respective bond. The two carbon atoms, C1 and C3, have hydroxy substituents, which have the same electron-withdrawing effect. The electron density should only differ slightly as a result of the electron-donating effect of the attached carbon atom of the carbohydrate residue at carbon C3. Through this effect, the acidity of the C−H bond at carbon C1 is slightly higher and the formation of the Heyns product via

1,2-eneaminol is preferred, but this does not exclude the deprotonation at C3 and, thus, the formation of 2,3-eneaminol in smaller quantities. In addition to being the intermediate in the formation of the alternative Amadori product, 2,3eneaminol is also a precursor for the formation of 1deoxyglucosone. A vinylogous β-elimination of the hydroxy group attached to carbon C1 followed by a hydrolysis of the amino acid leads to 1-deoxyglucosone. The formation of 1deoxyglucosone is a further indication for the existence of 2,3eneaminol and the pathway to the alternative Amadori product. Suarez et al. postulated a respective reaction pathway via the alternative Amadori product already in 1989.25 4.2. 2-Deoyglucosone. During the subsequent reactions, the alternative Amadori product can underlie the same rearrangements as the common Amadori product. The analogous mechanisms to the formation of 1-deoxyglucosone and 3-deoxyglucosone starting with the common Amadori product lead to the formation of 2-deoxyglucosone and 4deoxyglucosone when starting with the alternative Amadori product. In analogy to the formation of 1-deoxyglucosone, 2deoxyglucosone can be formed via a 3,4-enolization and a D

DOI: 10.1021/acs.jafc.8b03629 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry

pounds. This was also found by Fiedler et al. in model systems with γ-aminobutyric acid.14 As a result of the similar structure of 2-deoxyglucosone, 1deoxyglucosone, and 3-deoxyglucosone, it can be assumed that 2-deoxyglucosone can undergo comparable reactions and shows a similar reactivity in the Maillard reaction. Besides cleavage reactions to short-chain products, it can also cyclize to a five-membered heterocyclic compound or polymerize to melanoidins. Because 2-deoxyglucosone is formed especially at low water contents and only from the Heyns product, its formation can be expected in foods containing high amounts of D-fructose, which are processed at low water contents, for example, in the manufacture of dried fruits and juice concentrates or the processing of honey.

subsequent elimination of the amino substituent, leading to the formation of a ketone at carbon C4. In a further keto−enol tautomerism, 2-deoxyglucosone is formed (Figure 4). In this reaction pathway, the amino acid acts as the preferred leaving group compared to the hydroxyl group. 3-Deoxyglucosone is formed through the well-known mechanism from the Schiff base via a 1,2-enolization, a vinylogous β-elimination of the hydroxy group attached to carbon C3, and the hydrolysis of the amino acid. The analogous mechanism starting with the alternative Amadori product via a 2,3-enolization, a vinylogous β-elimination of the hydroxy group attached to carbon C4, and a subsequent hydrolysis of the amino acid leads to 4-deoxyglucosone. Via the postulated formation of an alternative Amadori product from the Schiff base of D-fructose, the formation of 2deoxyglucosone and 4-deoxyglucosone can be explained in addition to the well-known formation of 1-deoxyglucosone and 3-deoxyglucosone. Each of the described C6-α-dicarbonyl compounds, except for 4-deoxyglucosone, could be detected in the model systems of D-fructose with γ-aminobutyric acid by post-derivatization with OPD. 4-Deoxyglucosone is known from the Maillard reaction of 1,4-glycosidic-bound disaccharides, such as maltose, where the elimination of the carbohydrate residue at carbon C4 is favored in comparison to the elimination of the hydroxy group in monosaccharides.18,19,26,27 The formation of 2-deoxyglucosone and 4deoxyglucosone and the postulated reaction mechanism validate the formation of the alternative Amadori product as an intermediate in the Maillard reaction of D-fructose. Because 2-deoxyglucosone starts to decrease already after 24 h and, at the same time, the color development increases, it presumably is involved in the formation of colorants and its fast degradation indicates a high reactivity. During the isolation of 2-deoxyglucosone-quinoxaline from the reaction with pre-derivatization, a mixture of several quinoxaline derivatives was recovered after a preliminary HPLC separation. The NMR spectrum of this mixture consists of superimposed spectra of 2-deoxyglucosone-q, 3-deoxyglucosone-q, and 4-deoxyglucosone-q with low intensity. With application of 2D NMR spectroscopy and a comparison of the extracted spectral information to data given in the literature, the signals could be assigned to 4-deoxyglucosone-q.28 From the intensities of the signals in the NMR spectrum, a molar ratio between 2-deoxyglucosone, 3-deoxyglucosone, and 4deoxyglucosone of 100:50:2.5 could be determined. Thus, when model reactions were performed with pre-derivatization, the formation of 4-deoxyglucosone could be observed in addition to 1-deoxyglucosone, 2-deoxyglucosone, 3-deoxyglucosone, and D-glucosone. The highest amounts of these α-dicarbonyl compounds are found in the reaction mixture at a water content of 10%, because the reaction rate of the Maillard reaction is at its maximum under these conditions. The α-dicarbonyl compound content correlates with the reduction of the D-fructose content. With increasing the amount of water in the reaction mixture, the reactivity of D-fructose toward the formation of αdicarbonyl compounds decreases (Figure 2). In contrast, the content of D-glucosone is similar at high and low water contents. At high water contents (>70%) and low temperatures (50 °C), it is the main α-dicarbonyl compound formed in the reaction mixture. This is caused by the more favored oxidative formation of D-glucosone at low temperatures compared to the ionic mechanisms leading to the other α-dicarbonyl com-

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +49-0-30-314-72404. E-mail: [email protected]. ORCID

Philipp Bruhns: 0000-0001-8859-8243 Notes

The authors declare no competing financial interest.

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ABBREVIATIONS USED alternative Amadori product, 2-amino-2-deoxy-3-ketose; DAD, diode array detector; 1-DG, 1-deoxyglucosone, 1-deoxy-Derythro-hexo-2,3-diulose; 2-DG, 2-deoxyglucosone, 2-deoxy-Dglycero-hexo-3,4-diulose; 3-DG, 3-deoxy-D-erythro-hexo-2ulose; 3-deoxygalactosone, 3-deoxy-D-threo-hexo-2-ulose; 4deoxyglucosone, 4-deoxy-D-glycero-hexo-2,3-diulose; 1,4-dideoxyglucoson, 1,4-dideoxy-D-glycero-hexo-2,3-diulos; GLUC, D-glucosone, D-arabino-hexo-2-ulose; HPLC, high-performance liquid chromatography; MS, mass spectroscopy; NMR, magnetic nuclear resonance; q, quinoxaline

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REFERENCES

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DOI: 10.1021/acs.jafc.8b03629 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jafc.8b03629 J. Agric. Food Chem. XXXX, XXX, XXX−XXX