Acrylamide formation mechanisms

Zyzak, D.V. et al. Acrylamide formation mechanism in heated foods, J. Agric. Food Chem., 51, 4782, 2003. 

Wenzl, T., de la CaUe, B., and Anklam, E. 2003. Analytical methods for the determination of acrylamide in food products A review, FoodAddit. Contam., 20 885-902. Gertz, C., and Klostermann, S. 2002. Analysis of acrylamide and mechanisms of its formation in deep-fried products, Eur. J. Lipid Sci. Technol., 104 762-771. 

New findings about the presence of this substance in a number of food items, inevitably provoked intense international research, aimed not only to clarify the mechanism of acrylamide formation and evaluation of the health risks from dietary exposure for consumers, but also focusing on finding appropriate strategies to minimise food contamination. 

Flqure 12.3 Mechanism of acrylamide formation In the presence of a-hydroxycarbonyl compounds. 

Acrylamide is an unstable compound and may undergo, for example, the addition of nucleophilic amino or sulJhydryl groups of amino acids and other nucleophiles (Michael addition), which can cause its ehmination. Therefore, the reported amounts of acrylamide are the result of simultaneous formation and ehmination reactions. Outputs of extensive studies aimed at elucidating the mechanisms of acrylamide formation in thermally processed products identified the foUowing critical factors that influence the level of contamination of the final products ... 

We begin with Barron, et al, who reported mobilities of restriction fragments in solutions of hydroxyethylcellulose, hydroxypropylcellulose, and linear poly-acrylamide(5,6). Barron, et al. found that dilute polymer solutions are effective separatory media for DNA fragments. Dilute polymer solutions had previously not been expected to be effective separatory media, because the theoretical models being invoked in the electrophoresis literature referred only to gels and nondi-lute solutions. In these models, interpenetrating polymer coils were claimed to form evanescent separatory pores. In dilute solution, polymer coils do not interpenetrate, so the hypothesized pores should not be present, and therefore there was expected to be no separation. Barron, et al. concluded that they had evidence for a new mechanism for DNA separation at low matrix concentration, a mechanism distinct from the pore formation mechanisms presumed active at large c(6). 

Acrylamide polymerization by radiation proceeds via free radical addition mechanism [37,38,40,45,50]. This involves three major processes, namely, initiation, propagation, and termination. Apart from the many subprocesses involved in each step at the stationary state the rates of formation and destruction of radicals are equal. The overall rate of polymerization (/ p) is so expressed by Chapiro [51] as ... 

The temperature-sensitive poly(A-isopropyl acrylamide) and pH-sensitive poly(methacrylic acid) were used as the two component networks in the IPN system. Since both A-isopropyl acrylamide (NIPAAm) (Fisher Scientific, Pittsburgh, PA) and methacrylic acid (MAA) (Aldrich, Milwaukee, Wl) react by the same polymerization mechanism, a sequential method was used to avoid the formation of a PNIPAAm/PMAA copolymer. A UV-initiated solution-polymerization technique offered a quick and convenient way to achieve the interpenetration of the networks. Polymer network I was prepared and purified before polymer network II was synthesized in the presence of network I. Figure I shows the typical IPN structure. 

The kinetics of the oxidation of isopropylamine by diperiodatocuprate(III) complex ion have been studied and the results are consistent with a mechanism in which dissociation of one of the periodate ligands is followed by an adduct formation between [Cu(HIOg)] and isopropylamine. Polymerization of acrylamide indicated the participation of free radicals The kinetics of the oxidation of several diols by diperiodatocuprate(III) (DPC) in aqueous alkaline media have been studied. ... 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

Evidence supporting this mechanism is presented for the case of acrylamide polymerization sensitized by riboflavin, but not for the case of fluorescein and its halogenated derivatives. Irradiation with a millisecond flash in the presence of air leads to polymer formation after an induction period of one hour. In contrast, when the irradiation is carried out with degassed solutions, polymerization starts only after the sample is exposed to atmospheric oxygen. 

Rhodium(I) or polymer supported rhodium(I) compounds catalyzed the formation ofFefCO - CNR L (x = 1 - 3 R = Bu, xylyl L = MA, citra-conic anhydride, acrylamide) (29, 30), and the dimer [CpFe(CO)2]2 catalyzed the stepwise substitution of carbonyl groups in CpFeI(CO)2 to give CpFeKCO - CNR) (x = 1,2 R = Bu, xylyl) in 60-80% yields. A nonchain free-radical mechanism was proposed for the latter reaction (28). The compounds CpFeX(CO)2 x(CNR)x (x = 1,2 X = halide, SiMe3) are known for a range of alkyl and aryl isocyanides (169-171). 

It has been suggested that the mechanism of bead formation occurring in the PEO macromonomer system is quite different from the mechanism proposed by Dimonie et al. [109] for ISP of acrylamide. According to these authors, phase inversion occurs after the start of the reaction. At high conversions, the gel breaks under stirring into small particles which remain as such until the end of polymerization. With PEO macromonomers, beads are present from the beginning up... 

The reaction was found to be first order with respect to amines and acrylamides and no base catalysis was observed. The reaction is believed to occur in a single step in which the addition of amine to Cp of acrylamide and proton transfer from amine to Ca of acrylamide take place concurrently with a four-membered cyclic transition-state structure. The Hammett (px) and Brpnsted (/3X) coefficients are rather small, suggesting an early transition state (TS). The sign and magnitude of the cross-interaction constant, pxy(= —0.26), is comparable to those found in the bond formation processes in the. S n2 and addition reactions. The normal kinetic isotope effect ( h/ d > 1.0) and relatively low A and large negative Avalues are also consistent with the mechanism proposed.192... 

More detailed studies were devoted to the mechanism of electrochemical polymerization applied to Fe(II), Ru(II) and Os(II) complexes containing 2,2 -dipyridyl and monofunctional monomer (L) such as 4-VP, bis(4-pyridyl)ethylene, trans-4-stilbazole or N-(4-pyridyl)acrylamide [86], The first stage of electrochemical polymerization is shown to be the formation of a radical-anion, e. g. by the following scheme... 

One of the most reported pathways for C=C and C=C bonds coupling involves the oxidative coupling and the ruthenacyde intermediate formation. The first ruthenium-catalyzed Unear codimerization of disubstituted alkynes and alkenes involved acrylates or acrylamides and selectively produced 1,3-dienes [33] (Eq. 23). The proposed mechanism involves a ruthenacyclopentene via oxidative coupUng on the Ru(0) catalyst Ru(COD)(COT). The formation of 1,3-di-ene results from intracyclic /1-hydride eUmination, this process taking place only when a favored exocyclic /1-elimination is not possible. 

Dainton and Tordoff (9) showed that in the case of acrylamide Fe+ OH is a significant terminator. I am not, however, convinced of the validity of their suggested extension of this mechanism to other vinyl monomers, and similar generalizations, particularly in view of the fact that under otherwise identical conditions in experiments concerning the oxidation of benzoic acid (4) a doubled maximum yield of ferrous ion formation was determined by straightforward analytical methods, indicating that in the latter case, but not in the former, Fe+ OH must be considered as a terminator. This is quite independent of the quantitative estimate of the Fe+30H primary yield, which has become controversial. It is, however, feasible that Fe+ Br did not act as photoinitiator of the polymerization of methyl methacrylate or acrylonitrile because of eflficient termination with Fe+ Bir, which would not be unexpected. 

Besides water, another small molecule in the pyrolysate is CO2. This small molecule may be eliminated by various mechanisms including a hydrolysis of the amide groups with the formation of acrylic acid, followed by decarboxylation. Some small peaks In the pyrolysis of polyacrylamide are identical with those from poly(acrylic acid). Even traces of propanoic acid and propenoic acid are present in the acrylamide pyrolysate. A comparison between a time window 25.0 min. to 70 min. from the pyrogram of poly(acrylic acid) and from the pyrogram of polyacrylamide is shown in Figure 6.7.17. 

The silicates and the acrylamides both require basic conditions for gel formation, but the gelation mechanisms are less related than those of the pheno- and aminoplasts. Silicates and acrylamides can be used together, but the latitude of workable proportions is small. Siprogel (Rhone Poulenc, France) is a commercial product using both materials, available as separate solutions containing... 

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