Mercaptoethanol, addition reactions

Tryptophan synthase (EC 4.1.2.20) normally catalyzes the synthesis of tryptophan from serine by the oc,p elimination-addition reaction outlined in Scheme 5 where X = OH and Z = indole. The B protein of the oligomeric enzyme will catalyze the dehydration of serine, and in the presence of PLP and mercaptoethanol, the intermediate 15 will form adduct 25. This will then react as in Scheme 9 to yield the ketoacid 26 and pyridoxamine-phosphate 6. The net transamination has been shown to involve protonation at the 4 -Si face in yielding PMP (30). When the apoenzyme of tryptophan synthase is reconstituted with the unnatural substrates (4 / )- or (4 S)-[4- H,]pyridox-amine-phosphate and indole-3-pyruvic acid, an unnatural transamination... 

The a- and p-L-fucosyl esters of UMP and dTMP have been prepared as anomeric mixtures by treating the nucleotides with orthoester (15) in DMF. In the absence of methylenetetrahydrofolate cofactor, thymidylate synthetase has been observed to catalyse nucleophilic addition reactions at the ethynyl moiety of 5-ethynyl-2 -deoxyuridylate. On the basis of isotope effects and other kinetic studies, it has been suggested that addition of a thiol group of the enzyme at C-6 of the nucleotides results in formation of the allenic intermediate (16), followed by addition of 2-mercaptoethanol, morpholine, or other nucleophiles at the jp-hybridized carbon of the allene, and subsequent elimination of the enzyme. 

One further aspect besides H-atom abstraction must, however, be considered in the reactions of thiyl radicals with PUFAs, namely, the possibility of concurrent RS additions to the double bonds. By analyzing a particular system in which the thiyl radical from mercaptoethanol had reacted with linolenic acid, Schoneich at came to the conclusion that the abstraction of bisallylic hydrogen and thiyl addition occur with comparable rates. The C-centered adduct radical formed in the addition reaction is, of course, prone for repair in the presence of thiols, i.e., will subsequently regenerate thiyl radicals via the back reaction of equilibrium 14. These, in turn, will re-enter into the abstraction / addition competition cycle and eventually all of the thiyl radicals will appear to have reacted via the abstraction route. Experimentally, overall efficiencies of up to 85 % have been measured and the difference to the limiting 100% efficiency may be accounted for by side and termination processes. But looking into the details, the situation is, in fact, even more complex. As shown by Schwinn at al., there is still another process which readily occurs within the RS -adducts to the double bonds, namely, cis-trans isomerization. Quantification of this is of imminent interest for the biological community. 

The reagent is prepared by the addition of a solution containing 70 mg of o-nitrophenyl-P-D-galactopyranoside (ONPG) per 100 mL of 0.1 AT potassium phosphate buffer, pH 7.0, containing 1 mM magnesium chloride and 0.01 AT2-mercaptoethanol. The reaction may be stopped by the addition of 0.25 vol of 2 AT sodium carbonate. 

The by-products of these reactions are sulfides. The sulfide formed in the synthesis of 2-mercaptoethanol, 3-thia-l,5-pentanediol (thiodiglycol), has a variety of uses ranging from lubricant additive intermediates to textile finishing. 

Wash particles (e.g., 100 mg of 1 pm carboxylated latex beads) into coupling buffer (i.e., 50 mM MES, pH 6.0 or 50 mM sodium phosphate, pH 7.2 buffers with pH values from pH 4.5 -7.5 may be used with success however, as the pH increases the reaction rate will decrease). Suspend the particles in 5 ml coupling buffer. The addition of a dilute detergent solution may be done to increase particle stability (e.g., final concentration of 0.01 percent sodium dodecyl sulfate (SDS)). Avoid the addition of any components containing carboxylates or amines (such as acetate, glycine, Tris, imidazole, etc.). Also, avoid the presence of thiols (e.g., dithiothreitol (DTT), 2-mercaptoethanol, etc.), as these will react with EDC and effectively inactivate it. 

Immediately after irradiation, stop the reaction by the addition of 7 pi of 4 X SDS electrophoresis loading buffer or the equivalent (with a high concentration of reducing agent present) 0.2M Tris, 8 percent SDS, 2.88M P-mercaptoethanol, 40 percent glycerol, 0.4 percent xylene cyanol, 0.4 percent bromophenol blue. Heat the sample at 95°C for 5 minutes and analyze the complexes formed by electrophoresis. 

The regioselectivity of Michael additions of thiolates to 2,4-dienones can be altered drastically by variation of the reaction conditions and addition of Lewis acids to the reaction mixture. Lawton and coworkers examined the reaction of 2-mercaptoethanol with l-(3-nitrophenyl)-2,4-pentadien-l-one and observed a high regioselectivity in favor of the 1,6-addition product at 45 °C (equation 42)123,124. Lowering of the reaction temperature caused an increase in the amount of 1,4-adduct, and at —40°C, a product ratio of 40 60 was found. These events suggest that kinetic control favors the 1,4-addition product whereas the 1,6-adduct is thermodynamically more stable. If, however, the reaction was carried out with a complex of the dienone and titanium tetrachloride, only the 1,4-adduct was isolated after hydrolytic workup123. Obviously, this product is trapped as a metal chelate which prevents formation of the 1,6-adduct by retro-Michael/Michael addition. In the absence of the chelating Lewis acid, the 1,4-addition product can indeed be converted... 

Figure 2. Illustration of the importance of the choice of reaction conditions on the determination of initial velocity. Shown are four conditions applied to examine the rate behavior of Escherichia coli NAD+-dependent coenzyme A-linked aldehyde dehydrogenase (Reaction NAD+ + CoA-SH + Acetaldehyde = NADH + Acetyl-S-CoA + H+). All assay mixtures contained enzyme, 0.5 mM NAD+, 8 /jlW CoA-SFI, 16 mM acetaldehyde, and 22.5 mM Tris buffer at pFI 8.1. (a) Time-course observed when enzyme was added to the standard assay (b) time-course observed when enzyme was added to standard assay augmented with 10 mM 2-mercaptoethanol (c) time-course observed when enzyme was first preincubated for 15 min with 8 /jlW CoA-SH, 16 mM acetaldehyde, 10 mM 2-mercaptoethanol, and 22.5 mM Tris buffer at pH 8.1, and the reaction was initiated by addition of NAD+ (d) time-course observed when enzyme was preincubated with lOmM 2-mercaptoethanol for 15 min andthen addedtostandard assay augmented with 10 mM 2-mercaptoethanol. The data are most compatible with the idea that the enzyme has an active-site thiol group that must be reduced to express full catalytic activity during assay.

To further exploit the potential usefiilness of this new family of clusters, monoadduct 54 was saponified into 55 (0.05 M NaOH, quant) and condensed to L-lysine methyl ester using 2-ethoxy-l-ethoxycarbonyl-l,2-dihydroquinoline (EEDQ) to give extended dimer 56 in 50 % yield together with monoadduct in 15 % yield [75]. Additionally, tert-butyl thioethers 52 could be transformed into thiols by a two step process involving 2-nitrobenzenesulfenyl chloride (2-N02-PhSCl, HOAc, r.t, 3h, 84%) followed by disulfide reduction with 2-mercaptoethanol (60%). Curiously, attempts to directly obtain these thiolated telomers by reaction with thioacetic acid f ed. These telomers were slightly better ligands then lactose in inhibition of binding of peanut lectin to a polymeric lactoside [76]. 

Other authors have described the lipase-catalyzed chemoselective acylation of alcohols in the presence of phenolic moities [14], the protease-catalyzed acylation of the 17-amino moiety of an estradiol derivative [15], the chemoselectivity in the aminolysis reaction of methyl acrylate (amide formation vs the favored Michael addition) catalyzed by Candida antarctica lipase (Novozym 435) [16], and the lipase preference for the O-esterification in the presence of thiol moieties, as, for instance, in 2-mercaptoethanol and dithiotreitol [17]. This last finding was recently exploited for the synthesis of thiol end-functionalized polyesters by enzymatic polymerization of e-caprolactone initiated by 2-mercaptoethanol (Figure 6.2)... 

Equation 7 applies for thiolate anion addition. Ethylene oxide is nearly three times more sensitive than 1 to the nucleophilicity of the thiol. The second-order rate constants for the reactions of 2-mercaptoethanol with 1,47, and 48 are 3.32, 1.58, and 2.04M"1 sec-1, respectively. This shows that 1 is only slightly more reactive and that the K-region does not necessarily make the... 

The elusive nature of the cyclic sulfenamide is exemplified by its instability in aqueous weakly acidic or neutral solution. However, under conditions which resemble those of the parietal cell (e.g., 0.1 M HC1), the cyclic sulfenamide was fairly stable if, in addition, the concentrations were in the micromolar range. With isolated cyclic sulfenamides in hand, the study of the reaction of this species with thiols (e.g., mercaptoethanol) in different solvents and in different acidic solutions could easily be performed, and compared with similar reactions starting with the... 

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