Beta elimination reactions

Beta-elimination reactions have been observed in a number of proteins. This reaction occurs primarily at alkaline pH conditions. Abstraction of the hydrogen atom from the alpha-carbon of a cysteine, serine, threonine, phenylalanine, or lysine residue leads to racemization or loss of part of the side chain and the formation of dehydroalanine (26). 

From this, it can be seen that the amount of KOH within the hydroxide mixture would probably be critical in removing organo-sulfur from coal. While the particular role of KOH has not been determined, evidence from the literature has shown that the size of the cation may be important in stabilizing intermediate carbanions. Wallace et al. (J ) conducted a series of base- catalyzed, beta-elimination reactions with isopropyl sulfide and measured the amount of olefin production. The proposed mechanism involved initial abstraction of a proton by the t-butoxide base, and formation of a carbanion, with subsequent elimination of the sulfur moiety (which can be considered a good leaving group) to form the olefin (Equation 5). 

In the bimolecular concerted beta elimination reaction, E2, heterolytic cleavage of the C—X and C—H bonds takes place within the same reaction step, without formation of an intermediate (see Vol. 9). It appears that the energy barrier for the concerted process is lower than each of the barriers for the separate steps involving either a carbanion or a carbonium ion intermediate. 

Sulfoxides and seienoxides are known to easily undergo beta-elimination reactions. Sugar sulfoxides obtained by oxidation of respective thioglycosides were efficiently transformed to glycals upon heating in toluene (O Scheme 20) [160]. 

Muller R, Andersson LI, Mosbach K (1993) Molecularly imprinted polymers facilitating a beta-elimination reaction. Makromol. Chem Rapid Commun 14(10) 637-641... 

At pH values of 5-6 pectin solutions are stable only at room temperature. As the temperature 1s raised, pectin chains cleave by a beta-elimination reaction (25-39) (Figure 1), a reaction which is stimulated by organic anions (40). Deesterification of pectin proceeds simultaneously with the beta-elimination depolymerization reaction, which occurs only at monosaccharide units that are esterlfied. At pH values above 6, deesterlfIcatlon eind depolymerization are rapid reactions even at room temperature, the rate of each reaction increasing with Increasing pH. 

All catalyze depolymerization by a beta-elimination reaction like that which occurs during base-catalyzed depolyraerization. Pectin lyases, all of which are endo-enzymes, catalyze beta-eliminations at esterified D-galacturonlc acid units (See Figure 1) (68). Pectate lyases catalyze beta-eliminations at nonesterifled D-galacturonic acid units. Both exo- and endo-pectate lyases are T[Pg.10]

Mazumder, A., Gerlt, J. A., Absalon, M. J., Stubbe, J., Cunningham, R. P., Withka, J., and Bolton, P. H. (1991). Stereochemical studies of the beta-elimination reactions at aldehydic abasic sites in DNA Endonuclease III from Escherichia coli, sodium hydroxide, and Lys-Trp-Lys. Biochemistry 30, 1119-1126. 

Beta-elimination reactions are used to release saccharides from protein... 

In the context of this discussion, the product of an ehmination reaction possesses a C=C double bond and is called an alkene. The chapter will focus on the structure of alkenes and their preparation via beta elimination reactions. 

FIGURE 64-3 OP-serine loses the OP (soman in this case) plus a molecule of water when the peptide is fragmented in the mass spectrometer. This beta-elimination reaction converts OP-serine to dehydroalanine. 

Kisfaludy, L., A. Patthy, and M. L6w A Beta-Elimination Reaction between Cysteine Derivatives Containing Free SH Group and Dicyclohexylcarbodiimide. Acta Chim. Acad. Sci. Hung. 59, 159 (1969). 

Athene formation requires that X and Y be substituents on adjacent carbon atoms By mak mg X the reference atom and identifying the carbon attached to it as the a carbon we see that atom Y is a substituent on the p carbon Carbons succeedmgly more remote from the reference atom are designated 7 8 and so on Only p elimination reactions will be dis cussed m this chapter [Beta (p) elimination reactions are also known as i 2 eliminations ] You are already familiar with one type of p elimination having seen m Section 5 1 that ethylene and propene are prepared on an industrial scale by the high temperature dehydrogenation of ethane and propane Both reactions involve (3 elimination of H2... 

In a substitution reaction, the leaving group is replaced with a nucleophile. In an elimination reaction, a beta ((3) proton is removed together with the leaving group, forming a double bond. In the previous chapter, we saw two mechanisms for substitution reactions (SnI and Sn2). In a similar way, we will now explore two mechanisms for elimination reactions, called El and E2. Let s begin with the E2 mechanism. 

Alkyl halides that are 2° or 3°, or are 1° with branching at the beta carbon, undergo elimination reaction predominantly. 

The mechanism of the Meerwein-Pondorf-Verley reaction is by coordination of a Lewis acid to isopropanol and the substrate ketone, followed by intermolecular hydride transfer, by beta elimination [41]. Initially, the mechanism of catalytic asymmetric transfer hydrogenation was thought to follow a similar course. Indeed, Backvall et al. have proposed this with the Shvo catalyst [42], though Casey et al. found evidence for a non-metal-activation of the carbonyl (i.e., concerted proton and hydride transfer [43]). This follows a similar mechanism to that proposed by Noyori [44] and Andersson [45], for the ruthenium arene-based catalysts. By the use of deuterium-labeling studies, Backvall has shown that different catalysts seem to be involved in different reaction mechanisms [46]. 

Beta-H elimination reactions, 20 157 P-HCG (beta subunit human chorionic gonadotropin), 9 64 P-hydroxy acids, 14 130, 131 P-hydroxyethyl esters, 10 487 Betaine, 2 65... 

Proteins, peptides, and other polymeric macromolecules display varying degrees of chemical and physical stability. The degree of stability of these macromolecules influence the way they are manufactured, distributed, and administered. Chemical stability refers to how readily the molecule can undergo chemical reactions that modify specific amino-acid residues, the building blocks of the proteins and peptides. Chemical instability mechanisms of proteins and peptides include hydrolysis, deamidation, racemization, beta-elimination, disulfide exchange, and oxidation. Physical stability refers to how readily the molecule loses its tertiary and/or sec-... 

Proteins are subject to a variety of chemical modification/degradation reactions, viz. deamidation, isomerization, hydrolysis, disulfide scrambling, beta-elimination, and oxidation. The principal hydrolytic mechanisms of degradation include peptide bond... 

Elimination reactions 526, 530, 677—690 beta, of cystine residues 85 conjugative 689 decarboxylative 689 facilitation by carbonyl group 681 of y substituent 746 of PLP-dependent enzymes 742 reversibility 690 Ellman s reagent 125,125s Elongation factor EF-Tu 558 Elongin complex 564... 

To ascertain the upper limit of protein thermostability and to evaluate the effect of additional disulfide bridges on the enhancement of protein thermostability, additional cysteine residues were introduced into several unrelated proteins by site-directed mutagenesis and deactivation behavior tested at 100°C (Volkin, 1987). All the proteins investigated underwent heat-induced beta-elimination of cystine residues in the pH 4—8 range with first-order kinetics and similar deactivation constants kj that just depended on pH 0.8 0.3 h-1 at pH 8.0 and 0.06 0.02 h 1 at pH 6.0. These results indicate that beta-elimination is independent of both primary amino acid sequence and the presence of secondary structure elements. Elimination of disulfides produces free thiols that cause yet another deleterious reaction in proteins, heat-induced disulfide interchange, which can be much faster than beta-elimination. 

Our own laboratory has studied these reactions and, in particular, beta eliminations involving disulfides (Figure 16) and... 

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