Model Reaction Systems Research Articles

Study on the Hydroxyl Radical Scavenging Activity Changes of Ginseng and Ginsenoside-Rb2 by Heat Processing

The free radical scavenging activities of Panax ginseng C.A. MEYER are known to increase by heat processing. Phenolic acids and Maillard reaction products (MRPs) have been suggested as active free radical scavenging components from our previous research, but heat processing-induced chemical and activity changes of ginsenosides considering the Maillard reaction have not yet been fully elucidated. In this study, we investigated the hydroxyl radical (.OH) scavenging activity changes of ginsengs and ginsenoside-Rb2 (Rb2)) by heat processing using an electron spin resonance spectrometer. Especially, Rb2 was heat processed with the same amount of glycine, a frequently used amino acid in the Maillard reaction model system. As a result, the .OH scavenging activities and brown compound levels of ginseng and glycine-Rb2 mixture were increased by heat processing. However, the increase in .OH scavenging activities were not in accordance with the extents of browning. On the other hand, less-polar ginsenosides such as Rg3, Rg5, and Rk1 were generated from the glycine-Rb2 mixture by heat processing. The sugar moieties at carbon-20 of Rb2 were separated by the steaming process, less-polar ginsenosides were produced, and then the separated sugar moieties were thought to form MRPs with glycine. From the .OH scavenging activity tests of Rb2, glycine, less-polar ginsenosides, and maltol, the increase in .OH scavenging activity was thought to be more closely related to the generation of .OH scavenging ginsenosides such as 20(S)-Rg3 and Rg5 by heat processing than MRPs.

Proton Transfer in Ionic Water Clusters

How waterlike are ionic water clusters? Clusters with 10 to 100 water molecules are frequently used as model systems for reactions in aqueous solution. Concerted proton transfer, a key feature of liquid water, can be probed by observing the formation of HDO when D2O is used as a reactant (see scheme). Mass spectrometry shows that proton transfer occurs in H(H2O)n+, whereas the individual water molecules stay intact in (H2O)n− and O2(H2O)n−.

Model reaction system에서 가열조건이 acrylamide 및 기타 화합물들의 생성에 미치는 영향

아크릴아마이드의 형성과정에서 가열조건이 아크릴아마이드의 생성량과 아크릴아마이드 형성에 관여 또는 동반 생성하는 화합물을 조사하기 위하여, 아스파라긴과 포도당의 등량 혼합물을 125, 150, 175 및 <TEX>$200^{\circ}C$</TEX>에서 각각 10, 20 및 30 분간 가열하는 모델반응을 이용하였다. 가열한 반응물을 ethyl acetate 및 methanol 2종의 용매로 달리 추출하고 FFAP capillary column과 HP-5MS 5% phenyl methyl siloxane column으로 GC/MS에서 분석한 결과, 아크릴아마이드는 methanol추출물을 FFAP capillary column으로 분석한 결과에서 retention time 23.53분에서 검출되었고 최저검출한계는 4 ng 이었다. 아크릴아마이드 생성량은 <TEX>$175^{\circ}C$</TEX>보다. 낮은 125 및 <TEX>$150^{\circ}C$</TEX>에서는 온도와 가열시간을 증가시킬수록 점차 증가하여, <TEX>$175^{\circ}C$</TEX>에서 10분간 처리하였을 때 최대량인 <TEX>$116{\mu}g/g$</TEX>을 생성하였지만 175 및 <TEX>$200^{\circ}C$</TEX>에서는 온도 및 가열시간을 증가시킬수록 감소하는 결과를 나타내었다. 아크릴아마이드이외 반응생성물들로서, 1,3-dihydroxypropanone, 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyrane-4-one 및 5-hydroxymethylfurfural이 다량으로 검출되었으며, 또 다른 화합물인 5-methylfurfural, 2-acetylpyrrole 및 N,N-dimethylcyclohexamine 등은 소량 생성되었음을 확인하였다. The Formation of acrylamide was studied in Maillard model reaction systems based on asparagine-glucose. The mixture of asparagine and glucose in equal molar ratio, and then heated at 125, 150, 175 and <TEX>$200^{\circ}C$</TEX> for 10, 20 and 30 minute, respectively. The reaction products were extracted with ethyl acetate and methanol, and then isolated and detected on FFAP capillary column and HP-5MS 5% phenyl methyl siloxane column by using GC/MS. Acrylamide was detected only from methanol extracts and on FFAP capillary column, at retention time 23.53 min., and the detection limit was 4.6 ng. Acrylamide content mainly increased with increasing temperature and processing time till <TEX>$175^{\circ}C$</TEX>, therefore, maximal acrylamide formation occurred at <TEX>$175^{\circ}C$</TEX> for 10 minute (<TEX>$116{\mu}g/g$</TEX>), while, above <TEX>$175^{\circ}C$</TEX>, higher temperatures or prolonged processing times caused a decrease of acrylamide levels, finally disappeared at <TEX>$200^{\circ}C$</TEX> for 30 minute. Three major compounds were identified as 1,3-dihydroxypropanone, 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyrane-4-one and 5-hydroxymethylfurfural, and three minor compounds also as 5-methylfurfural, 2-acetylpyrrole and N,N-dimethylcyclohexamine, from ethyl acetate or methanol extracts on FFAP or HP-5MS capillary column.

Determination of Acrylamide Formed in Asparagine/d-Glucose Maillard Model Systems by Using Gas Chromatography with Headspace Solid-Phase Microextraction

A gas chromatographic method, along with a headspace solid-phase microextraction (HS-SPME), was developed for the determination of acrylamide formed in Maillard reaction model systems. The developed method was validated by liquid chromatography/mass spectrometry. A headspace sample was collected from an aqueous acrylamide solution (100 microg/mL) by SPME and directly injected into a gas chromatograph equipped with a nitrogen-phosphorus detector. The recovery of acrylamide from an aqueous solution was satisfactory, i.e, >93% under the conditions used. Acrylamide formed in an asparagine/D-glucose (molar ratio, 1/2) Maillard reaction model system heated at 150 and 170 degrees C for 20 min was collected and analyzed by the newly developed method using gas chromatography with nitrogen-phosphorus detection and HS-SPME. The amounts of acrylamide were 318 +/- 33 microg/g asparagine from a sample heated at 150 degrees C and 3329 +/- 176 microg/g asparagine from a sample heated at 170 degrees C. Addition of cysteamine or glutathione to the above model system reduced acrylamide formation. Acrylamide formation was not observed when cysteamine or glutathione was added to asparagine in the above model systems to obtain equimolar concentrations of both compounds. This newly developed method is simple and sensitive, and requires no solvent extraction.

Antioxidant and Pro-oxidant Activities of Aqueous Extracts and Crude Polyphenolic Fractions of Rooibos (Aspalathus linearis)

Unfermented rooibos tea is known to contain higher levels of total polyphenols and flavonoids than its fermented counterpart, making it the obvious choice for the preparation of flavonoid-enriched fractions. Evaluation of aqueous extracts and crude polyphenolic fractions of unfermented and fermented rooibos showed anti- and/or pro-oxidant activities, using a linoleic acid-Tween-buffer emulsion for lipid peroxidation and the deoxyribose degradation assay, based on a Fenton reaction model system containing FeCl3-EDTA and H2O2 for the generation of hydroxyl radicals. Except for the ethyl acetate fraction, with the highest total polyphenol (TP) content and offering the least protection presumably due to pro-oxidant activity, the inhibition of lipid peroxidation by the samples correlated moderately with their TP content in a linear relationship (r = 0.896, P < 0.01). Using the deoxyribose degradation assay, the pro-oxidant activity of the aqueous extracts and their crude polymeric fractions (0.1 mg/mL in the reaction mixture) was linear with respect to their dihydrochalcone (aspalathin and nothofagin) (r = 0.977, P = 0.023) and flavonoid (r = 0.971, P = 0.029) content. Pro-oxidant activity was demonstrated for pure aspalathin. Using the same assay, but with ascorbate added to regenerate Fe3+ to Fe2+, the aqueous extract and crude polymeric fraction of fermented rooibos displayed hydroxyl radical scavenging activity. Fermentation (i.e., oxidation) of rooibos decreased the pro-oxidant activity of aqueous extracts, which was contributed to a decrease in their dihydrochalcone content. The in vitro pro-oxidant activity displayed by flavonoid-enriched fractions of rooibos demonstrates that one must be aware of the potential adverse biological properties of potent antioxidant extracts utilized as dietary supplements.

A proposed mechanism for the inhibition of soybean lipoxygenase by β‐carotene

AbstractThe inhibition mechanism of soybean lipoxygenase (LOX) by β‐carotene was studied. Addition of β‐carotene into the reaction mixture decreased the rate of conjugated diene formation. Increasing the concentration of β‐carotene in the reaction mixture resulted in a decrease in the rate of conjugated diene formation. Although the rate of conjugated diene formation was lower in the presence of β‐carotene, the same amounts of linoleic acid hydroperoxides were formed by the enzyme at the end of the reaction, both with and without β‐carotene in the reaction medium. The rates of conjugated diene formation for 40, 20, 10 and 4 U mL−1 LOX enzyme were almost equal to zero when the concentrations of β‐carotene were 20, 17.5, 15 and 10 µmol L−1 in model reaction systems, respectively. β‐Carotene directly influences the amount of enzyme in the reaction medium available for the catalytic conversion of linoleic acid into corresponding hydroperoxides. The results obtained here suggest that β‐carotene reacts with linoleyl radical (L•) at the beginning of the chain reaction, preventing the accumulation of conjugated diene forms (LOO•, LOO− and LOOH). Since L• transforms back to its original form of LH, the enzyme cannot complete the chain reaction and thus remains at inactive Fe(II) form. Copyright © 2005 Society of Chemical Industry

Reactivity of Epicatechin in Aqueous Glycine and Glucose Maillard Reaction Models: Quenching of C2, C3, and C4 Sugar Fragments

Mechanisms of how epicatechin alters the pathways of the Maillard reaction were investigated. Carbon-13 and nitrogen-15 labeling studies were utilized to define the reactivity of epicatechin with glucose, glycine, and/or reaction products in an aqueous model (pH 7, 125 degrees C for 30 min) via GC, GC/MS and HPLC/MS analysis. Quantification of the volatile reaction product isotopomers by GC/MS from a 1:1 labeled to unlabeled glucose (carbohydrate module labeling technique) plus glycine model system indicated the formation of 2,3-butanedione and acetol were primarily formed via intact C4 and C3 sugar fragments, whereas pyrazine, methylpyrazine, 2,5-dimethylpyrazine, 2,3,5-trimethylpyrazine, and cyclotene were primarily formed via intact C2/C2, C2/C3, C3/C3, C3/C3, and C3/C3 sugar fragment pairs, respectively. The formation of these seven compounds was also reported by GC analysis to be dramatically inhibited when epicatechin was added to the glucose/glycine model system (observed 9-113-fold reduction). HPLC/MS analysis of both the glucose-labeled and glycine-labeled model systems with and without epicatechin indicated that epicatechin reacted directly with C2, C3, and C4 sugar fragments, while epicatechin did not report any direct reactivity with glycine. In conclusion, the quenching of sugar fragmentation products via epicatechin was correlated with the observed inhibition on volatile compound formation when epicatechin was added to a glucose/glycine aqueous reaction model system.

Does allo‐immune reactivity play a role in the prenatal programming of childhood allergy?

Does allo‐immune reactivity play a role in the prenatal programming of childhood allergy?

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.

Time-resolved study of the symmetric SN2-reaction I−+CH3I

Time-resolved photoelectron spectroscopy of negative ions has been applied to study the title reaction as a model system for gas phase SN2 reactions. Starting from the precursor cluster I2−⋅CH3I, the interaction of the reactants I− and CH3I is initiated by a pump pulse and the subsequent dynamics are observed with a delayed probe pulse used to detach the excess electron and measure their photoelectron spectra. Using two different pump photon energies, which lead to different amounts of internal energy available to the reaction complex, a number of dynamical features have been observed. For small internal excitation, the reactants only form stable, albeit vibrationally excited, I−⋅CH3I complexes. However, with increased internal excitation, complexes are formed that exhibit biexponential decay back to I− and CH3I reactants with time scales of 0.8 and 10 ps. Similar dynamics are expected for entrance channel complex formed in the first step of a gas phase SN2 reaction.

The Synthesis and Catalytic Application of a New Class of Imprinted Silica

AbstractThe effect of chemical environment surrounding a synthetic heterogeneous catalyst active site is investigated using the hydrophilic imprinting of silica. Two model reaction systems have been used for this study: (i) Knoevenagel condensation of 3-nitrobenzaldehyde and malononitrile and (ii) Suzuki coupling of bromobenzene and phenylboronic acid. Using a catalyst in which isolated imprinted amines are surrounded by an acidic silanol-rich environment led to rate accelerations of over 120-fold relative to catalysts in which the amines are surrounded by a hydrophobic environment consisting of trimethylsilyl functional groups for system (i). This result parallels our previous study on the effect of the outer sphere composition on rate acceleration of Knoevenagel reactions using isophthalaldehyde as the aldehyde reactant. We also extended our method for the hydrophilic imprinting of bulk silica to organometallic systems, by successfully synthesizing a tethered palladium complex within the imprinted pocket. This material was used as an active catalyst for (ii). Our results show that a hydrophobic framework environment results in higher initial turnover frequencies than an acidic silanol-rich framework for the Suzuki coupling reaction of bromobenzene and phenylboronic acid, albeit with a lower overall effect than observed in the Knoevenagel system (i). Altogether, these results demonstrate the control of chemical reactivity via the rational design of the outer sphere using an imprinting approach.

Automatic Reaction Modeling in Chemical Vapor Depositions Using Multiple Process Simulators

ABSTRACTThe identification of appropriate reaction models is very helpful for developing chemical vapor deposition (CVD) processes. In this paper we propose a novel system to analyze experimental data of various CVD reactors and identify reaction models automatically using Genetic Algorithms (GA) with multiple process simulators and modeled functions. We demonstrate that this system is able to adequately model reaction systems, and that complex analysis of various experimental data increased the reliability of the reaction modeling results.

Racemization of amino acids during classical and microwave oven hydrolysis — application to aspartame and a Maillard reaction system

Abstract The conventional acid hydrolysis routinely used in protein analysis induces partial racemization of the amino acids. An alternative method, using microwave heating, was evaluated with the aim of assessing its efficiency in preserving the amino acids in their original configuration. The method was employed on the dipeptide aspartame. The results showed that some advantages, implying less racemization, could be gained by using the microwave technique, but the method was applicable only at temperatures under 150°C. Mild, conventional hot oven-hydrolysis, involving a shorter hydrolysis time, could have the same mild effect. The different techniques were applied to aspartame treated at different pH, a soft drink and a Maillard reaction model system. The results showed that aspartame was insusceptible to racemization in acid and neutral environments, and not until very high pH did racemization start. The soft drink contained a small amount of d -amino acids, 0.27 mg d -asp and 0.14 mg d -phe/100 ml. The model system exhibited no racemization.

Molecular oxygen on the Si(111)-7×7 surface

The adsorption of neutral molecular oxygen at room temperature on the Si(111)-7\ifmmode\times\else\texttimes\fi{}7 surface has for more than 20 years been a central issue in physics and chemistry as it offers a unique model system for surface reactions with a maximum number of observable parameters. A comprehensive set of new details of this system come from synchrotron radiation induced photoelectron spectroscopy, in terms of changes of the population of the surface states and of the number of chemically shifted atoms at the surface. The reaction steps between oxygen molecules and different surface bonds and atoms in the unit cell are now followed in full detail including the existence of a stable molecular configuration up to exposures of 6 L.

An ab Initio Study of Hydrogen Adsorption on ZnO(101̄0)

The interaction of hydrogen with the stoichiometric ZnO(101̄0) surface has been studied using ab initio, all-electron total energy calculations. A highly accurate hybrid density functional (B3LYP) implemented within a local Gaussian basis set has been employed. Details of the geometric and electronic structure of the surface have been determined. The results are discussed in terms of the role of this surface as a model system for both catalytic reactions and gas sensing applications. The Zn−O dimers on the (101̄0) surface provide a model system for the type I adsorption sites known to be important in surface mediated methanol synthesis reactions. The interaction of hydrogen with the surface does not result in surface metallisation, and hence, the (101̄0) surface is unlikely to be an active gas sensing material.

Understanding performance improvement of forced periodic operation of stirred tank reactors

It is well-known that the performance of chemical reaction processes can be improved by periodic forcing. The π-criterion has been introduced and applied extensively to quantitatively assess the potential of periodic forcing in a particular case, though it is difficult to understand the physical reasons for the performance improvement. A theoretical analysis in the frequency domain and a study of typical model reaction systems in stirred tanks including parallel, consecutive and reversible reactions is presented here. It is shown that the performance improvement can clearly be related to resonance phenomena displayed by the linearized dynamics in conjunction with a nonlinear variation of the process gain in the region of forcing. The theoretical findings are illustrated by a number of examples.

Porphyrin-o-quinones as Model Systems for Electron Transfer and Catecholase Reactions

Various porphyrin-o-quinones 1−8 were prepared as model compounds for electron transfer reactions. The synthesis of these compounds could only be accomplished by using o-quinone components stabilized by sterically demanding substituents in the o,o′-positions and by using protected catechol aldehydes for the initial porphyrin condensation step. The stability of the target compounds strongly depends on the steric influence of substituents on the o-quinone, which are necessary to stabilize the o-quinones. To test this steric influence we synthesized the porphyrin-o-quinones both without and with methyl and tert-butyl groups and investigated the influence of the bridging position on the reactivity. The 1H- and 13C-NMR spectroscopic characteristics of all compounds are given. The electron transfer properties were determined using optical spectroscopy and cyclic voltammetry. The free base porphyrin-o-quinone 1 with two tert-butyl groups in the o,o′-positions of the quinone readily underwent degradation reactions to muconic acid derivatives similar to those observed in catecholase reactions. Nevertheless, this is the first report of such reactions that do not involve the addition of metal-containing catalysts. The respective zinc(II) complex 6 and the two separated components tetraphenylporphyrin 26 and 3,6-di-tert-butyl-o-quinone (27) did not undergo this reaction, and we surmise a photochemical mechanism that depends on the steric demand of the o-quinone.

Micromixing and Macromixing Effects in Unsteady Chemical Reaction System

This study presents a novel model that combines the tank-in-series model and the random replacement IEM (interaction by exchange with a mean environment) model for investigating the effects of both imperfect micromixing and macromixing. Two unstable chemical reaction model systems are simulated, the Oregonator model and the pH oscillator model. Dynamical behaviors include steady state, relaxation oscillation, small oscillation, and chemical chaos. Simulation results revealed significant effects of mixing; however, the ways the system dynamics had been altered depended on the chemical reaction systems.

Process and product engineering – achievements, present and future challenges

In this paper, present and future challenges faced by chemical engineers are discussed. To place the development of this discipline in its industrial and social context, a short history of chemical engineering in one of the major global chemical companies (BASF) is given. The ‘scientific part’ of chemical engineering consists in breaking down real complex systems into subsystems, which are then described using our understanding of fundamental chemical and physical processes. The ‘engineering part’ of chemical engineering consists in using this new found knowledge in the design and construction of a working plant which is capable of producing the desired product, even if our understanding of the single subsystems is today incomplete. From the components that make up our discipline, process engineering has in the last several decades attained a high degree of scientific maturity. Further developments are needed and expected in the improved description of fundamental chemical and physical processes necessary particularly to model reaction systems. The other field, product engineering, is a younger, less mature area where the scientific elucidation of the structure/property relationship at molecular and microscopic levels first needs to be tackled. This knowledge is required to model disperse systems so as to design products and develop appropriate production facilities. Improving the design and evaluation of complex systems for the production of real products will require further research into methodologies, tools and strategies. Firstly, these improvements will enable us to combine various unit operations so as to obtain an optimal overall process within an optimally designed production plant. And secondly, this individual production plant should in turn be optimally integrated into the entire production site. Such procedures must take into account both the requirements of customers as well as environmental concerns. The challenges faced by the chemical engineering community can only be met if two preconditions are fulfilled: both organisations carrying out basic research and R&D departments focussed on applied research within companies must be at the cutting edge of technology. And furthermore, they must work even more closely together if we are to meet the challenges described in this paper. It is also essential that young engineers and scientists as part of their education should be integrated into this research effort and become fully committed to it.

Three-pulse photon echoes for model reactive systems

A theoretical description of the three-pulse photon echo peak shift for model reaction systems is presented. An electronic two-state system with a finite upper-state lifetime and a three-state system in which electronic transitions can occur are considered. A probabilistic argument is employed to incorporate the incoherent transitions. New pathways describing the transition of electronic population are introduced and the nuclear propagator in the electronic population state is written by a convolution integral between those of the nonreactive two-state system weighted by some factors for the electronic transition. The response functions are given by multitime correlation functions and are analyzed by the cumulant expansion method. Some numerical calculations are presented and the influence of incoherent reactions on the peak shift is discussed. Comparison with experimental data confirms the existence of the effects predicted here.