Reactions deesterification

The activity of PE was measured [25] from the increase of carboxylic groups during the pectin deesterification (5 g/L) in phosphate-citrate buffer 50 mM, pH 4.7. Continuous titration of the reaction mixture with NaOH 0.01 M was employed for product determination. 

Qualitatively, the results observed with ferric chloride and lead acetate were highly variable. It was first noted with ferric chloride that gels were forming within the reaction vessel We observed the formation of translucent gels in the reaction vessel with calcium chloride, spermidine and ferric chloride. In the case of lead acetate, a feathery type precipitate formed in the reaction vessel. Macdonald et al. (14) observed formation of a clear gel and flocculated precipitate in high ester pectin treated with lemon endocarp and peel PE isozymes, respectively. They hypothesized that the different gel structures were due to unique mechanism of deesterification by the PE isozymes. Our results with different cations and formation of different gel structure or precipitate seems to be similar to that reported by Macdonald et al. (14). If there is a different mechanism of de-esterification for plant PEs,... 

R.M.McCready et al.[l] demonstrate that the adding of O.IM NaCl to 0.33 M ammonia hydroxide enhances the deesterification more than twice. The adding of other chlorides, such as KCl, MgCb and CaCb also enhances the reaction. The effectiveness of the ions in relation to the deesterification and rate constant is arranged in the following order Ca>Mg>Na, K. The equal values of pH and ion strength prove that only the specific cation effect is responsible for the enhanced velocity. 

It may be noted from the data for Rd (Table 1) that at the conditions chosen by us for performing of deesterification with ammonia a degree of transformation above 80 % is reached. Both competitive reactions of hydrolysis and ammonolysis are commensurable in regard of their rate. This fact is very favourable for investigating cations influence on the course of the deesterification and the rate of the investigated competitive reactions(3), (4) and (5). 

Influence of the Monovalent Ions. The addition of ammonium salts retard the course of the deesterification by changing the reaction rate of hydrolysis and ammonolysis. By increasing the concentration of the ions the conversion of the ester groups is reduced from 83.3 % for 0,01 M to 62.8 % for 0.2 M (without added salt this value is 84.05 %), and the ratio hydrolysis ammonolysis is changed correspondingly from 53.8 76.2 to 37.3 62.7 (without added salt this ratio is 57.6 42.4). 

The behaviour of the added alkali cations is different. Na and K enhance the deesterification and the differences in the interaction of both cations at the separate reactions are inessential. The use of these ions in the deesterification of highly esterified pectin in either enzyme or chemical method shows that they only enhance the hydrolysis. Since in the investigated case they cannot influence the equilibrium (6), the enhancement of the hydrolysis leads to faster exhausting of the reactable -COOCH3 groups and thus is reduced the rate of the competitive ammonolysis. 

The two most striking features of these time-courses are that, though the initial speed of the reaction increases markedly with the pH, the final level does not, as the reaction stops much earlier. This was not due to a drop of pH below the values suitable for P-elimination, as the pH change were slight (-0.2 at pH 11, no measurable change at pH 9). In order to check wether this plateau was due to disparition of suitable reaction sites, i.e. methoxylated galacturonic acid residues, we have investigated in detail the deesterification reaction. 

The deesterification reaction, or more precisely the demethylation reaction, was followed by measuring the amount of methanol liberated (Fig. 2). 

The analogy between the RRDE and the deesterification reaction above goes further. The time t requiredfor analyte to travel from the disc (where it is formed) to the ring (where it is re-reduced ) is a function of both the distance between the disc and the ring, and the rate at which the RRDE is spun. Rapid rotation means that t is short, while slow rotation means that t is longer. We see that the time-scale of the observation is readily altered by variation of the rotation speed co. 

In nonaqueous medium (sodium methoxide in methanol), deesterification is replaced by transesterification that permits the C-4 elimination to proceed. Yields of 50% of methyl (methyl 4-deoxy-/3-L-threo-hex-4-enopyranosid)uronate have been obtained from methyl (methyl a-D-galactopyranosid)uronate.180,181 The same reaction has been studied by using other glycuronates.182-184 Evidence for C-4 elimination where esterification is internal is found in the reactions... 

Most experiments were performed with cotton or cotton linters as highly crystalline celluloses. Table I shows conditions leading to complete dissolution. A minimum amount of an acid which forms a cellulose ester (sulfuric or trifluoromethylsulfuric acid) (Entries 5 7-14) is necessary for the reaction. The dissolution is accelerated by a temperature increase (Entries 10-12 13, 14) and leads to water-soluble cellulose acetate hydrogensulfate. Whereas this primary hydrolysis can be achieved within 1-5 min, the deesterification and complete hydrolysis of the soluble cellulose derivative proved to be much more difficult. This is in contrast to the generally accepted view that the main resistance to the hydrolysis of cellulose lies in the crystalline nature or low accessibility determining the heterogeneous first step of the reaction. 

A third type of membrane reactor combines the functions of contactor and separator. An example of this combination membrane reactor is shown in Figure 13.16(c), in which the membrane is a multilayer composite. The layer facing the organic feed stream is an immobilized organic liquid membrane the layer facing the aqueous product solution contains an enzyme catalyst for the deesterification reaction... 

The length of the reaction, which may reach 10-12 days, could limit the reproducibility and applicability of this reaction. Additionally, the method used to recover the product at the end of the reaction can lead to side-reactions. In some procedures (e.g., Halpin and Richardson, 1985 Bertrand-Harb etui, 1991), the reaction medium at the end of the reaction time was combined with an equal volume of water, then dialyzed against water for several days, which can lead to deesterification (Bertrand-Harb et al., 1991). Another way to recover the reaction product was by drying under vacuum (Briand etal., 1995). 

Two additional carbonyl condensation reactions which made their appearence in the 1950 s provide ring closure capability of synthetic value. The first has been described by Paul and Wendel and utilizes 2-chlorocyclopentanone as starting material.39 Subsequent to alkylation with an acetoacetic ester and deesterification, the... 

The 5-nitro substituted dipeptide analogue was formed by selective reduction (BH,) of methyl 3-amidocarbonyll-5-nitrobenzoate, followed by deesterification (LiOH in THF) and reaction sequentially with Fmoc-Asp-(tBu)-OH and H-Gly-Asp(tBu -Gly-OH (dicyclohexyl carbodiimide (DCC), N-hydroxysuccinimide) to yield Fmoc-Asp(tBu)-5N023 amb-Gly-Asp (tBu)-GlyOH. CycUzation with 4-dimethylaminopyridine (DMAP) and 2-lH-benzotriazole-1-yl-l, 1,3,3,-tetramethyluronium tetrafluoroborate (TBTU) was achieved in 70% yield, followed by reduction (Hj, Pd/C) to give the amino-substituted peptide loop 2. (p. 2681)... 

Dissolved pectins undergo deesteriflcation and depolymerization in aqueous systems. The pH of greatest stability is about 4. At pH values both above and below 4, deesterlfIcatlon and depolymerization occur concurrently, with the rate of deesterification being greater than the rate of depolymerization. The presence of solutes, which lowers water activity, reduces the rates of both reactions. 

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. 

Other pectins with degree of esterification of about 10, 30 and 40% (Table I) were obtained by enzymic deesterification of the initial preparation. An orange pectinesterase was used and the pH of the reaction was 6 in order to avoid concurrent base-catalyzed saponification. The action of plant pectinesterase is known (16-18) to result in a blockwise arrangement of free carboxyl groups in the pectic molecules. The enzymic preparation does not contain depolymerase activities as shown by the constancy of the intrinsic... 

The six mono- and di-O-butyryl derivatives of 1,6-anhydro- -D-glucopyranose and the six corresponding palmitic acid esters have been prepared from the tributyrate and tripalmitate respectively, by partial methanolysis and hydrazinolysis. The experimental findings regarding the regioselectivity of the deesterification reactions were related to polar and steric effects identified by quantum-chemical calculations (PCILO). ... 

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