Assessing the non-specific hydroxyl radical scavenging properties of melanoidins in a Fenton-type reaction system

Soluble high molecular weight fraction (>10 kDa, named melanoidins) was isolated from Maillard reaction model systems, coffee, beer and sweet wine by ultrafiltration. Deoxyribose method was adjusted for measuring the hydroxyl radical scavenging properties of melanoidins. The presence of competitive melanoidins with deoxyribose for OH decrease the rate of deoxyribose degradation. Possible interferences to the deoxyribose method have been evaluated. Most of isolated melanoidins exhibited a variable and measurable non-site-specific hydroxyl scavenging activity in a Fenton-type reaction system. The iron reducing properties of melanoidins at the reaction conditions were evaluated with ferrozine. It has established a kinetic approach to assess the second rate constants of hydroxyl radical scavenging reactions of melanoidins. This approach may be a valuable tool for addressing the structure-activity relationships of melanoidins in a future. There is no correlation between browning (absorbance at 420 nm) and efficiency for scavenging hydroxyl radicals in solution.     

美拉德反应的研究进展

美拉德反应主要指还原糖与氨基酸、蛋白质之间的复杂反应,它与食品加工、疾病生理过程等有重要关系.除该反应对食品品质影响的研究仍在进行外,近来美拉德反应的研究更多地集中在蛋白质交联、类黑素、动力学以及丙烯酰胺等与人类健康关系更密切的方面,本文从这些方面综述了美拉德反应的进展.     

热重-固相微萃取/气相色谱-质谱联用研究葡萄糖/天冬酰胺模拟体系非水相Maillard反应

利用热重-固相微萃取/气相色谱-质谱(TG-SPME/GC-Ms)联用对葡萄糖/天冬酰胺模拟体系非水相Maillard反应热学性质进行了研究,并分析了模拟体系46种热解逸出挥发性产物相对含量的动态变化情况.探讨了有效减少Maillard反应产物丙烯酰胺的方法.结果表明:葡萄糖/天冬酰胺模拟体系非水相Mail-lard反...     

半纤维素模型化合物热解机理的理论研究

针对半纤维素模型化合物4-O-甲基葡萄糖醛酸的热解,提出了六种可能的反应路径,对各种反应路径中的反应物、产物、中间体和过渡态的结构进行了几何结构全优化,计算了各步反应的标准动力学参数。结果表明,4-O-甲基葡萄糖醛酸热解时,首先通过分子内的氢原子转移发生开环反应而形成链状中间体,然后中间体进一步分解,主要产物是甲醇、乙醇醛、2-羟基-3-甲氧基丁醛酸、乙二醛和2-羟基丁醛酸等;主要的热解竞争产物是甲酸、CO_2、CO、4-羟基-3-丁烯酮和甲基乙烯醚等。在半纤维素的热解中,CO_2是通过不饱和反应物或中间体脱羧基反应而形成,乙酸则是通过脱O-乙酰基反应而形成。     

The Maillard Reaction as Source of Meat Flavor Compounds in Dry Cured Meat Model Systems under Mild Temperature Conditions

Flavor is amongst the major personal satisfaction indicators for meat products. The aroma of dry cured meat products is generated under specific conditions such as long ripening periods and mild temperatures. In these conditions, the contribution of Maillard reactions to the generation of the dry cured flavor is unknown. The main purpose of this study was to examine mild curing conditions such as temperature, pH and a_w for the generation of volatile compounds responsible for the cured meat aroma in model systems simulating dry fermented sausages. The different conditions were tested in model systems resembling dry fermented sausages at different stages of production. Three conditions of model system, labeled initial (I), 1st drying (1D) and 2nd drying (2D) and containing different concentrations of amino acid and curing additives, as well as different pH and a_w values, were incubated at different temperatures. Changes in the profile of the volatile compounds were investigated by solid phase microextraction and gas chromatography mass spectrometry (SPME-GS-MS) as well as the amino acid content. Seventeen volatile compounds were identified and quantified in the model systems. A significant production of branched chain volatile compounds, sulfur, furans, pyrazines and heterocyclic volatile compounds were detected in the model systems. At the drying stages, temperature was the main factor affecting volatile production, followed by amino acid concentration and a_w. This research demonstrates that at the mild curing conditions used to produce dry cured meat product volatile compounds are generated via the Maillard reaction from free amino acids. Moreover, in these conditions a_w plays an important role promoting formation of flavor compounds.     

Iterated reaction graphs: simulating complex Maillard reaction pathways

This study investigates a new method of simulating a complex chemical system including feedback loops and parallel reactions. The practical purpose of this approach is to model the actual reactions that take place in the Maillard process, a set of food browning reactions, in sufficient detail to be able to predict the volatile composition of the Maillard products. The developed framework, called iterated reaction graphs, consists of two main elements: a soup of molecules and a reaction base of Maillard reactions. An iterative process loops through the reaction base, taking reactants from and feeding products back to the soup. This produces a reaction graph, with molecules as nodes and reactions as arcs. The iterated reaction graph is updated and validated by comparing output with the main products found by classical gas-chromatographic/mass spectrometric analysis. To ensure a realistic output and convergence to desired volatiles only, the approach contains a number of novel elements: rate kinetics are treated as reaction probabilities; only a subset of the true chemistry is modeled; and the reactions are blocked into groups.     

The role of peptides and proteins in melanoidin formation

High‐molecular‐weight (HMW) coloured compounds called melanoidins are widely distributed, particularly in foods. It has been proposed that they originate through the Maillard reaction, a non‐enzymatic browning reaction, due to the interaction between protein or peptide amino groups and carbohydrates. The melanoidin structure is not definitively known, and they have been generally defined as HMW nitrogen‐containing brown polymers. In order to gain information on the nature of melanoidins, a simple in vitro model was chosen to investigate the products of the reactions between sugars and peptide/proteins. This approach would elucidate whether melanoidin formation is due to the binding of different sugar units to a peptide/protein or vice versa. With this aim, the reactivity of two different peptides, EPK177 and physalaemin, and a low‐molecular‐weight (LMW) protein, lysozyme, was tested towards different saccharides (glucose, maltotriose (MT), maltopentaose and dextran 1000) in aqueous solutions at different temperatures. The incubation mixtures were analysed at different reaction times by MALDI/MS. Furthermore, in order to verify the possible role of sugar pyrolysis products in melanoidin formation, the products arising from the thermal treatment at 200 °C of MT were incubated with lysozyme, and the reaction products were analysed by the same MS approach. The obtained results allowed the establishment of some general views: melanoidins cannot simply originate by reactions of sugar moieties with proteins. In fact, the reaction easily occurs, but it does not lead to any coloured product, as melanoidins have been described to be; melanoidins cannot originate from the thermal degradation products of glycated proteins. In fact, the thermal treatment of glycated lysozyme leads to a severe degradation of the protein with the formation of LMW species, far from the view of melanoidins as HMW compounds; experimental evidence has been gained on the melanoidin formation through reaction of intact protein with the pyrolysis products of MT. This hypothesis has been supported either from MALDI measurements or from spectroscopic data that show an absorption band in the range 300–600 nm, typical of melanoidins. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterization of natural and synthetic humic substances (melanoidins) by stable carbon and nitrogen isotope measurements and elemental compositions

Stable carbon isotope ratios of the starting sugars (SG) and amino acids (AA) are mostly preserved in laboratory synthetic melanoidins. Stable nitrogen isotope ratios may not be “imprinted” in melanoidins by SG and AA precursors in the same way, indicating that nitrogen fractionation could occur during the rate-determining step in the Maillard reaction. Carbon isotope ratios support the stoichiometric ratios for combination of sugars with amino acids, which are based on the elemental composition data of melanoidins. These findings may provide clues to asses the role of the Maillard reaction in the formation of natural humic substances.     

Simulated Sunlight Selectively Modifies Maillard Reaction Products in a Wide Array of Chemical Reactions

The photochemical transformation of Maillard reaction products (MRPs) under simulated sunlight into mostly unexplored photoproducts is reported herein. Non-enzymatic glycation of amino acids leads to a heterogeneous class of intermediates with extreme chemical diversity, which is of particular relevance in processed and stored food products as well as in diabetic and age-related protein damage. Here, three amino acids (lysine, arginine, and histidine) were reacted with ribose at 100 °C in water for ten hours. Exposing these model systems to simulated sunlight led to a fast decay of MRPs. The photodegradation of MRPs and the formation of new compounds have been studied by fluorescence spectroscopy and nontargeted (ultra)high-resolution mass spectrometry. Photoreactions showed strong selectivity towards the degradation of electron-rich aromatic heterocycles, such as pyrroles and pyrimidines. The data show that oxidative cleavage mechanisms dominate the formation of photoproducts. The photochemical transformations differed fundamentally from “traditional” thermal Maillard reactions and indicated a high amino acid specificity.     

Containerless reaction monitoring in ionic liquids by means of Raman microspectroscopy

Reaction monitoring by Raman microspectroscopy in levitated room temperature ionic liquid (RTIL) droplets is reported. Due to their non-volatility, RTIL droplets are well-suited to act as wall-less microreactors. The droplets were produced by a piezoelectric flow-through microdispenser connected to an automated flow injection system and were levitated by an acoustic trap. Taking advantage of the flow system versatility, the sequence of reagents was easily changed to study a model organic reaction: the Knoevenagel condensation. The reaction was followed by Raman microspectrometry and the obtained spectra were analysed using multivariate curve resolution to retrieve the concentration profiles and pure spectra of reactants, intermediates and products involved in the reaction. In addition, information about solvation interactions was obtained by monitoring the desolvation process taking place when a volatile co-solvent evaporated from the droplet.     

Observation and Kinetic Characterization of Transient Schiff Base Intermediates by CEST NMR Spectroscopy

In aqueous solution, many biochemical reaction pathways involve reaction of an aldehyde with an amine, which progresses through generally unstable, hydrated and dehydrated, Schiff base intermediates that often are unobservable by conventional NMR. There are 4 states in the relevant equilibrium: 1) gem‐diol, 2) aldehyde, 3) hemiaminal, and 4) Schiff base. For the reaction between protein amino groups and DOPAL, a highly toxic metabolite of dopamine, the ¹H resonances of both the hemiaminal and the dehydrated Schiff base can be observed by CEST NMR, even when their populations fall below 0.1 %. CEST NMR reveals the quantitative exchange kinetics between reactants and Schiff base intermediates, explaining why the Schiff base NMR signals are rarely observed. The reactivity of DOPAL with N^α‐amino groups is greater than with lysine N^?‐amines and, in the presence of O₂, both types of Schiff base DOPAL–peptide intermediates rapidly react with free DOPAL to irreversibly form dicatechol pyrrole adducts.     

Microwave-assisted rapid photocatalytic degradation of malachite green in TiO2 suspensions: mechanism and pathways

Microwave-assisted photocatalytic (MPC) degradation of malachite green (MG) in aqueous TiO2 suspensions was investigated. A 20 mg/L sample of MG was rapidly and completely decomposed in 3 min with the corresponding TOC removal efficiency of about 85%. To gain insight into the degradation mechanism, both GC-MS and LC-ESI-MS/MS techniques were employed to identify the major intermediates of MG degradation, including N-demethylation intermediates [(p-dimethylaminophenyl)(p-methylaminophenyl)phenylmethylium (DM-PM), (p-methylaminophenyl)(p-methylaminophenyl)phenylmethylium (MM-PM), (p-methylaminophenyl)(p-aminophenyl)phenylmethylium (M-PM)]; a decomposition compound of the conjugated structure (4-dimethylaminobenzophenone (DLBP)); products resulting from the adduct reaction of hydroxyl radical; products of benzene removal; and other open-ring intermediates such as phenol, terephthalic acid, adipic acid, benzoic acid, etc. The possible degradation mechanism of MG included five processes: the N-demethylation process, adduct products of the hydroxyl radical, the breakdown of chromophores such as destruction of the conjugated structure intermediate, removal of benzene, and an open-ring reaction. To the best of our knowledge, it is the first time the whole MG photodegradation processes have been reported.     

A detailed chemical kinetic mechanism for methanol combustion in laminar flames

On the basis of existing detailed kinetic schemes a general and consistent mechanism of the oxidation of methanol was compiled for computational studies covering a wide range of lean to rich flames. The proposed model, featuring 21 species and 115 reactions, has been validated using three data sets and the computed reactants, products and intermediates mole fractions. This scheme was compared to those by Held-Dryer, Egolfopolous and Pauwels under the same conditions. The developed mechanism predicts well the concentrations of the major reactants, intermediates, and products at all the studied equivalence ratios and it gives the best calculated values, as compared to the other used models, as well. The production rates analysis of selected species allowed the identification of the major formation and depletion pathways. A reaction path analysis snowed that the main channels in methanol consumption involved H, OH and O attack and the resulting radicals CH₂OH and CH₃O produced formaldehyde.     

Molecular Shape Analysis of a Maillard Reaction Intermediate

Abstract

The Maillard (browning) reaction involving the polycondensation of sugars and amino acids is believed to be an important abiotic pathway for humic substance formation in nature. However, a major drawback is that the Maillard reaction is extremely slow at temperatures encountered under normal environmental conditions. In order to elucidate some details of this process molecular shape analysis was applied to investigate the initial reaction between D-glucose and glycine to form the Amadori compound fructosylglycine which is an intermediate product in the Maillard reaction. The structure of the Amadori compound was optimized at a quantum mechanical level and its ground state electron energy calculated. Molecular Iso-Density Contours (MIDCO's), electron density contour surfaces of constant electron density, were constructed for D-glucose, glycine and fructosylglycine in order to study the steric conditions for the reaction. The calculations indicate that the Amadori compound and water on one hand and the separate entities D-glucose and glycine on the other hand are very similiar to each other in terms of their ground state energy. This agrees with the experimental observation that the reaction between D-glucose and glycine to form the Amadori compound is slow.     

In vitro antioxidant activities of maillard reaction products produced in the steaming process of Polygonum multiflorum root

The root of Polygonum multiflorum Thunb. (Heshouwu in Chinese) is one of the most popular herbs used in traditional Chinese medicine (TCM). However, after steam processing (Zhi-heshouwu in Chinese), the root is known to have different properties and medicinal values compared with Heshouwu. Eleven volatile Maillard reaction products were identified in the extract of Zhi-heshouwu, but not in that of Heshouwu. The new products were four furanones, two furans, two nitrogen compounds, one pyran, one alcohol and one sulfur compound. The antioxidant activities were compared between the extracts from Zhi-heshouwu and Heshouwu. The results showed that the extract from Zhi-heshouwu presented a higher 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity than the extract from Heshouwu, with IC50 values of 0.43 mg/mL and 2.9 mg/mL, respectively (p < 0.05). The hydroxyl radical scavenging activities of the two were similar (IC50 0.98 mg/mL and 1.45 mg/mL, respectively; p > 0.05). 5-Hydroxymethyl-furfural, a main compound in the extract of Zhi-heshouwu, showed IC50 values for scavenging DPPH radicals and hydroxyl radicals of 1.6 mg/mL and 0.24 mg/mL, respectively. The antioxidant activities of the extract from Zhi-heshouwu could partly explain the different therapeutic effects of Heshouwu and Zhi-heshouwu in TCM.     

Model studies of 6,7-indolequinone cofactors of quinoprotein amine dehydrogenases

The electronic effects of the C-4 substituent on the physicochemical properties and reactivity of the 6,7-inodolequinone cofactors (CTQ and TTQ) have extensively been investigated with use of a series of C-4 substituted 6,7-inodolequinone derivatives (1-4). The one-electron reduction potentials of the 6,7-inodolequinone derivatives decrease with increasing the electron donating ability of the C-4 substituent (with the following order of E degrees': 4>1>2>3). The reaction of indolequinones 1-3 with benzylamine proceeds stepwise through the iminoquinone and the product-imine intermediates to give aminophenol as the final product as the case of TTQ model compound 4. The rate constants of each step have been determined by the detailed kinetic analysis, and the kinetic deuterium isotope effects have also been examined to confirm the rate-determining step. The reactivity of CTQ model compound 1 toward the amines is by one order of magnitude lower than that of TTQ model compound 4. The reactivity of indolequinones 2 and 3 is further decreased due to their stronger electron-donating substituents at C-4. A more important difference between CTQ model compound 1 and TTQ model compound 4 is the reactivity of the iminoquinone intermediate: the reaction of the CTQ model compound with amines stops at the iminoquinone formation stage at room temperature, whereas the reaction of the TTQ model compound with amines proceeds up to the aminophenol formation. Thus, the energy barrier for the rearrangement of the iminoquinone to the product-imine is higher in the CTQ model system than in the TTQ model system.     

1,4-Addition of benzene to a dihydrocyclopent[a]indene diradical: synthesis and DFT study

Photochemical cyclization of compound 1, a homoenediyne (-CCC=CCH2CC-) bearing two ethynylanthracene chromophores, yields two isomeric dihydrocyclopent[a]indene ring systems, spiro-fused to the 9-position of a 9,10-dihydroanthracene moiety. Evidence of a photochemically initiated diradical cyclization pathway is proposed on the basis of (i) hydrogen abstraction from reaction with 1,4-cyclohexadiene (1,4-CHD) and (ii) the observation of 1,4-addition of benzene (solvent). The reaction was further analyzed by a complete density functional theory (DFT) study, using an unrestricted approach (UBLYP) with a 6-31G* basis set for the open-shell triplet states of the reactants, products, and diradical intermediates to model the photochemical nature of observed transformation. A mechanism detailing the observed cyclization/addition reaction is proposed.     

Specific Features of a Two-Phase Bimolecular Chemical Reaction in the Case of the Association of Both Reactants

The effect of the association of both reactants on the kinetics of their bimolecular reaction in the liquid phase is studied. The mathematical modeling of chemical reactions that are described by nonlinear differential equations is performed. The steady states, the conditions for the emergences of intermediates, and the nature of their concentration oscillations in the reaction system are described. It is found that the concentration of the intermediates has two types of oscillations (harmonic and relaxation oscillations) characterized by significantly different times. The relationship between the observed rate constant of the process, the rate constants for the elementary stages, and the reactant concentrations is found.     

有机锗化合物对氨基酸非酶糖化反应的抑制作用

在模拟体内条件下，研究了有机锗化合物Ｇｅ－１３２对４种氨基酸（Ｌ－组氨酸、Ｌ－精氨酸、Ｌ－缬氨酸、甘氨酸）和葡萄糖发生非酶糖化反应（Ｍａｉｌａｒｄ反应）的抑制作用，结果表明：Ｇｅ－１３２在１～１０ｍｍｏｌ／Ｌ范围内，对这４种氨基酸非酶糖化反应都有较强的抑制作用．     

Maillard Reaction between Leucine and Glucose in O/W Microemulsion Media in Comparison to Aqueous Solution

The Maillard reaction is controlled by temperature, pH, reactant nature (sugars and amino acids), and water activity. We carried out reactions between glucose and leucine in U‐type nonionic microemulsions to obtain regioselectivity and control reaction rates. Reactants were oriented at the interface and water activity was adjusted using blends of surfactant and propylene glycol (PG). U‐type microemulsions, previously studied by us, served as microreactors for the Maillard reaction. The reactions in the microemulsion media were slower than those carried out in aqueous solution and formed unique aroma compounds. Reaction rates increased when using systems richer in water, as the water activity was enhanced. The surfactant plays a key role in determining water activity and reagent reactivity in all the microemulsions. The presence of PG slows the reaction, mainly when it resides at the interface, facilitating the formation of a bicontinuous structure. Phase transitions within the U‐type microemulsions were determined by viscosity and SD‐NMR and were correlated to the interfacial presence of the reactants and their reactivity.