Formation in model systems

Ascorbate, cysteine, hydroquinone, and NADH are capable of acting as re-ductants for NOMb formation in model systems containing sodium nitrite and Mb (Fox and Ackerman, 1968). Ascorbate, cysteine, and hydroquinone all form nitroso-reductant intermediates which released NO, forming a NO-MMb complex which was then reduced to NOMb. Release of NO from the reductant-NO complex was rate limiting in production of NOMb. For NADH as reductant, reduction of NOMMb to NOMb was the rate limiting step. In summary, two reduction steps were required, the reduction of nitrite (as nitrous acid or its anhydride, N2O3) to NO, and reduction of NOMMb to NOMb. 

G. P. Rizzi, A mechanistic study of alkylpyrazine formation in model systems, J. Agric. Food Chem., 1972, 20, 1081-1085. 

Chocarom Pyrazine isomers were isolated from the skin and flesh of potato Solanum tuberosum L.) cultivars after baking 4). 3,5-Dimethyl-2-isobutylpyrazine [2,5-dimethyl-3-(2-methylpropyl)-pyrazine] was isolated by Oruna-Concha, Craig, Duckham and Ames from the following potato cultivars - Cara, Nadine, Flanna and Marfona. 3,6-Dimethyl-2-isobutyl-pyrazine [3,5-dimethyl-2-(2-methylpropyl)pyrazine], was found by the same team in Cara and Marfona potato cultivars. 2,5-Dimethyl-3-isobutylpyrazine was also detected by Welty, Marshall and Grun in chocolate ice cream prepared from cocoa flavor (5). Both pyrazines were also found as key odorant compounds in dark chocolate by Counet, Callemien, Ouwerx and Collin (6). The role of amino acids in alkyl-substituted pyrazines formation in model systems containing pyruvaldehyde was examined by Mea (7). 2,5-Dimethyl-3-isobutylpyrazine was formed in the model system with valine. Both isomers were prepared synthetically by Chen (S) by reacting acetol, isobutyraldehyde and ammonium acetate, with low yield of 22.3%. Subsequent proprietary work by the author has improved the yield to 65%. 

Stadler R.H., Varga N.,HauJ.,ArceV.F.,WdtiD.H. Alkylpyridiniums. 1. Formation in model systems via thermal degradation of trigonelline. Journal of Agricultural and Food Chemistry, 50 1192-1199 (2002). 

This compilation of data proves the importance of configuration and population of conformations in the intramolecular excimer formation in model systems of polyvinylaromatics. Besides the above mentioned parameters another important aspect In the photophysical behaviour of the compounds is the role of the substitution pattern on the chromophore. 

To prevent insolubility resulting from uncontrolled aggregation of extended strands, two adjacent parallel or antiparallel yS-peptide strands can be connected with an appropriate turn segment to form a hairpin. The / -hairpin motif is a functionally important secondary structural element in proteins which has also been used extensively to form stable and soluble a-peptide y9-sheet arrangements in model systems (for reviews, see [1, 4, 5] and references therein). The need for stable turns that can bring the peptide strands into a defined orientation is thus a prerequisite for hairpin formation. For example, type F or II" turns formed by D-Pro-Gly and Asn-Gly dipeptide sequences have been found to promote tight a-pep-tide hairpin folding in aqueous solution. Similarly, various connectors have been... 

Mechanism of NPYR Formation The consistent occurrence of NPYR in fried bacon and cooked-out fat has led to an intensive search for both the precursors and mechanism that could account for its formation. Although model system studies have implicated a number of compounds including proline, collagen, putreseine, spermidine, pyrrolidine and glycyl-L-glycine as possible... 

Smoking. The effects of smoking on the formation of N-nitros-amines in bacon has been investigated recently by Bharucha et al. ( ). They reported that unsmoked bacon samples generally tended to contain more N-nitrosamines, presumably because of their higher nitrite content at the time of frying. Sink and Hsu (55) showed a lowering of residual nitrite in a liquid smoke dip process for frankfurters when the pH also was lowered. The effects of smoke seem to be a combination of pH decrease and direct C-nitrosation of phenolic compounds to lower the residual nitrite in the product (56). This is an area which requires further study since certain C-nitrosophenols have been shown to catalytically transnitrosate amines in model systems (57). 

Insights into the mechanisms of carotenoid degradation can be followed in model systems that are more easily controlled than foods and the formation of initial, intermediate, and final products can also be more easily monitored. However, extrapolation to foods must be done with caution because simple model systems may not reflect the nature and complexity of a multicomponent food matrix and the interactions that can occur. In addition, even in model systems, one must keep in mind that carotenoid analysis and identification are not easy tasks. 

Rodriguez, E. B. and D. B. Rodriguez-Amaya (2007). Formation of apocarotenals and epoxycarotenoids from beta-carotene by chemical reactions and by autoxidation in model systems and processed foods. Food Chem. 101(2) 563-572. 

RP-HPLC methods have been frequently applied for the investigation of various chemical, biochemical and biophysical processes in in vitro model systems. Thus, the separation of new compounds achieved by enzymatic oxidation of phloridzin was carried out by semi-preparative RP-HPLC. Phloridzin was incubated with a polyphenol oxidase prepared from apple pulp for 6h at 30°C under air agitation. After incubation the suspension was filtered, stabilized by NaF and injected into the RP-HPLC column using diluted acetic acid-ACN gradient. The new compounds were isolated and identified by NMR and MA techniques. The proposed mechanism of the formation of new phloridzin derivatives 3 and 4 is shown in Fig. 2.159. The results illustrate that RP-HPLC can be successfully used for the study of enzymatic processes in model systems [331],... 

The yield of HAs in food systems is affected by the concentration of substrates, enhancers and inhibitors, duration and temperature of heating, water activity, and pH. Some HAs are formed in mixtures of substrates heated for several weeks at relatively low temperature, about 37 to 60°C at 150 to 200°C the rate of reaction is much higher. However, in model systems prolonged heating may also bring about a decrease of the concentration of some HAs. Low water activity in the surface layers of the heated products favors the formation of HAs. In presence of lipids, Fe, and Fe, the rate of reaction increases, probably due to oxidation and generation of radicals (Jagerstad et ah, 2000). 

Jiigerstad, M., Skog, K., Arvidsson, P. and Solyakov, A. (1998). Chemistry, formation and occurrence of genotoxic heterocyclic amines identified in model systems and cooked foods, Z. Lebensm. Unters. Forsch A, 207, 419—427. 

There are a number of findings suggesting that agents that facilitate elimination of protein carbonyls (by either proteolytic elimination or by enzymically mediated chemical reduction) may suppress neurodegenerative conditions in model systems (Botella et al., 2004). Consequently, as carnosine may also react with protein carbonyls, it is theoretically possible that it could suppress formation and /the reactivity of protein carbonyls in the brain. Whether carnosine participates in carbonyl reductase activity has not been investigated but it is also a reasonable speculation. 

M. lijima, H. Tohda, Y. Moriwaki, Growth and structure of lamellar mixed crystals of octacalcium phosphate and apatite in model system of enamel formation, J. Cryst. Growth 116 (1992) 319-326. 

The goal of this contribution is to review the formation of at least some of these compounds in model systems (consisting of aldehydes, acetoin, and ammonium sulfide), their identification, and analytical characterization (mass spectra, Kov ts indices) accomplished by using the GC-MS-SPECMA data bank. 

The characteristic aroma of wheat bread crust has been attributed to 2-acetyl-1-pyrroline, and its formation depends on the presence of bakers yeast [31]. In model systems it was demonstrated that the acetylpyrroline is formed from the reaction of proline with pyruvaldehyde or dihydroxyacetone. Other compounds with bread-like aromas formed in the reaction of proline with pyruvaldehyde include l-acetonyl-2-pyrroline and 2-acetyltetrahydropyridine (Scheme 12.5). These compounds are unstable, which explains why the characteristic aroma of freshly baked bread disappears quickly during storage. 

The formation of brown pigments via the Maillard reaction, especially in model systems (e.g. glucose-glycine), usually follows zero-order kinetics, but the loss of reactants has been found to follow first- or second-order kinetics in foods and model systems. Activation energies of 109, 116 and 139 kJ mol-1 have been reported for the degradation of lysine, the formation of brown pigments and the production of hydroxymethylfurfural (HMF), respectively. 

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