2-Deoxyglucosone: A New C6-α-Dicarbonyl Compound in the Maillard Reaction of d-Fructose with γ-Aminobutyric Acid

Mechanism of Dihydromyricetin-Induced Reduction of Furfural Derived from the Amadori Compound: Formation of Adducts between Dihydromyricetin and Furfural or Its Precursors

Dihydromyricetin (DMY) was employed to reduce the yield of furfural derived from the Amadori rearrangement product of l-threonine and d-xylose (Thr-ARP) by trapping Thr-ARP, 3-deoxyxyosone (3-DX), and furfural to form adducts. The effect of different concentrations of DMY at different pH values and temperatures on the reduction of furfural production was studied, and the results showed that DMY could significantly reduce furfural production at higher pH (pH 5-7) and lower temperature (110 °C). Through the surface electrostatic potential analysis by Gaussian, a significant enhancement of the C6 nucleophilic ability at higher pH (pH ≥ 5) was observed on DMY with hydrogen-dissociated phenol hydroxyl. The nucleophilic ability of DMY led to its trapping of Thr-ARP, 3-DX, and furfural with the generation of the adducts DMY-Thr-ARP, DMY-3-DX, and DMY-furfural. The formation of the DMY-Thr-ARP adduct slowed the degradation of Thr-ARP, caused the decrease of the 3-DX yield, and thereby inhibited the conversion of 3-DX to furfural. Therefore, DMY-Thr-ARP was purified, and the structure was identified by nuclear magnetic resonance (NMR). The results confirmed that C6 or C8 of DMY and carbonyl carbon in Thr-ARP underwent a nucleophilic addition reaction to form the DMY-Thr-ARP adduct. In combination with the analysis results of Gaussian, most of the DMY-Thr-ARP adducts were calculated to be C6-DMY-Thr-ARP. Furthermore, the formation of DMY-furfural caused furfural consumption. The formation of the adducts also shunted the pathway of both Thr-ARP and 3-DX conversion to furfural, resulting in a decrease in the level of furfural production.

Peracetylation transforms natural products beyond mere derivatization

Peracetylation of the methanolic extract of Benincasa hispida led to the isolation of a compound with a peracetylated hex-4-en-3-one backbone. Mechanistic insights revealed that the isolated compound is an outcome of the chemical transformation of a α-dicarbonyl compound.

Evaluation of Fructo-, Inulin-, and Galacto-Oligosaccharides on the Maillard Reaction Products in Model Systems with Whey Protein

The present study aimed to investigate the effects of fructo-, inulin-, and galacto-oligosaccharides (FOS, IOS, and GOS) on forming the Maillard reaction products such as browning, α-dicarbonyl compounds, and advanced glycation end products (AGEs). The model solutions at pH 6.8 containing each carbohydrate (mono-, di-, and oligosaccharides) and whey protein were incubated at 50 °C for 8 weeks. In the IOS model, sugars of DP3 or larger were significantly decreased at 4 weeks, whereas at 6 weeks in the FOS model. The residual amount of GOS after 8 weeks was higher than FOS and IOS; however, a large amount of 3-deoxyglucosone was formed compared to the other models. N^ε-Carboxymethyllysine (CML) concentrations in oligosaccharide models were about half of those in monosaccharide and lactose models. The highest concentrations of glyoxal- and methylglyoxal-derived hydroimidazolones 3 (G-H3 and MG-H3) were observed in the IOS model, indicating the involvement of fructose units linked by β-2 → 1 bonds. G-H3 and MG-H3 quantification could be a useful indicator to reflect the modification of an arginine residue by fructose if used acid-hydrolysis for AGE analysis. CML, G-H3, and MG-H3 were considerably formed even in the FOS model, which has no reducing terminal site, suggesting that degradation products of oligosaccharides probably participated in the formation of AGEs.

Temperature-Dependent Catalysis of Glycylglycine on Its Amadori Compound Degradation to Deoxyosone

The Amadori rearrangement product derived from xylose-glycylglycine (XGG-ARP) is reactive to be attacked by another glycylglycine to generate a xylose-glycylglycine cross-linking product (XGG-CP) as a secondary product of the ARP. In this research, the role of additional glycylglycine in the XGG-ARP degradation was studied, and the dependence of glycylglycine on temperature was further clarified. The yields of XGG-CP and its degradation products were significantly affected by the molar ratio of glycylglycine to XGG-ARP. At the similar total concentration of reactant XGG-ARP and glycylglycine, the yields of XGG-CP, 3-deoxyxylosone, and furfural were dramatically decreased as the molar ratio of glycylglycine to XGG-ARP was increased. However, when the reaction temperature was increased to 120 °C, the increased additional glycylglycine percentage showed an obvious catalytic effect on the XGG-ARP degradation to deoxyosone and thus improved the furfural yield as well. The results revealed that an increased glycylglycine dosage level enhanced both the conversion of XGG-ARP to XGG-CP and the conversion of XGG-CP to 3-deoxyosone. The high-temperature-induced unequal acceleration for XGG-CP formation and degradation at a high glycylglycine dosage further led to a catalytic effect on the ARP degradation to deoxyosone. The concentration of 3-deoxyosone was increased by 37.5% when the molar ratio of glycylglycine to XGG-ARP increased from 1:2 to 2:1 at a temperature of 120 °C.

Formation of melanoidins - Aldol reactions of heterocyclic and short-chain Maillard intermediates

In the course of the Maillard reaction, reducing sugars and amino compounds are converted to colorants, whose chemical structures are still mostly unknown. Active methylene compounds like norfuraneol that can initiate aldol condensation reactions are considered as key intermediates in this reaction. The aim of the present study was to characterize color formation of norfuraneol with different carbonyl compounds and to identify the underlying mechanisms of the reaction. Norfuraneol was incubated with methylglyoxal or diacetyl at elevated temperatures and the resulting reaction mixtures were analyzed by means of high-resolution mass spectrometry. It was demonstrated that aldol reactions lead to the formation of heterogeneous carbohydrate-based oligomers, which are likely to contribute to the elevated browning observed in the reaction mixtures. Furthermore, redox reactions were identified as another important part of the reaction, resulting in an increasing number of double bonds in the detected reaction products.

Isotope-Coding Derivatization for Quantitative Profiling of Reactive α-Dicarbonyl Species in Processed Botanicals by Liquid Chromatography-Tandem Mass Spectrometry

α-Dicarbonyls (α-DCs) are key reactive Maillard intermediates with structural diversity and are widely found in foods and in vivo, but little is known regarding the complete molecular profiles of these potentially harmful electrophiles. Herein, we reported a novel isotope-coding derivatization (ICD) strategy for the broad-spectrum, quantitative profiling of (non)target α-DC species in natural foodstuffs. It utilized differential isotope labeling (DIOL) with a reagent pair o-phenylenediamine (OPD)/OPD-d₄ (deuterated) to form stable quinoxalines for class-specific fragmentation-dependent acquisition using liquid chromatography-hybrid quadrupole linear ion trap mass spectrometry (LC-QqLIT). A combination of facile one-pot quantitative labeling and convenient cleanup protocol afforded satisfactory sensitivity, linearity, accuracy (81-116%), and process recovery (86-109% with RSDs < 10%) by matrix-matched ICD-internal standard calibration, without significant matrix interference (-9 to 5%), isotopic effect (<0.5%), and cocktail effect. A more generic DIOL-based LC-QqLIT algorithm integrated double precursor ion and neutral loss scan to trigger enhanced product ions with the unique isobaric doublet tags (4 Da shift), enabling simultaneous screening and relative quantitation of nontarget α-DC analogues in a single analysis. This study has widened the vision on complex α-DC profiles in traditional botanicals, which revealed a wide occurrence of α-DCs in such processed sugar-rich products, yet their abundance varied greatly among different samples.

Pathways of Non-enzymatic Lysine Acylation

Posttranslational protein modification by lysine acylation is an emerging mechanism of cellular regulation and fine-tunes metabolic processes to environmental changes. In this review we focus on recently discovered pathways of non-enzymatic lysine acylation by reactive acyl-CoA species, acyl phosphates, and α-dicarbonyls. We summarize the metabolic sources of these highly reactive intermediates, demonstrate their reaction mechanisms, give an overview of the resulting acyl lysine modifications, and evaluate the consequences for cellular regulatory processes. Finally, we discuss interferences between lysine acylation and lysine ubiquitylation as a potential molecular mechanism of dysregulated protein homeostasis in aging and related diseases.

Basic Structure of Melanoidins Formed in the Maillard Reaction of 3-Deoxyglucosone and γ-Aminobutyric Acid

Melanoidins are formed in foods during processing through the Maillard reaction between carbohydrates and amino compounds. The aim of this study was to draw conclusions about the formation mechanism and the structure of melanoidins formed at low water contents and low temperatures. In the Maillard reaction of d-glucose and γ-aminobutyric acid at low water contents 3-deoxyglucosone is the most important intermediate. Therefore, we used the reaction of 3-deoxyglucosone with γ-aminobutyric acid or β-alanine as a simplified model system. The degradation of 3-deoxyglucosone and the color formation of the formed melanoidins were determined. In addition, the reaction mixture was analyzed with high-resolution mass spectrometry and a Kendrick analysis was applied. Oligomers consisting of up to four molecules of 3-deoxyglucosone and three amino acids and their respective dehydration products with furanoidic structure were detected. The melanoidin structure of C-C linked monomeric units postulated by Kroh et al. could be confirmed.