Reversed hydrolysis

Hexamethylenetetramine. Hexa, a complex molecule with an adamantane-type stmcture, is prepared from formaldehyde and ammonia, and can be considered a latent source of formaldehyde. When used either as a catalyst or a curative, hexa contributes formaldehyde-residue-type units as well as benzylamines. Hexa [100-97-0] is an infusible powder that decomposes and sublimes above 275°C. It is highly soluble in water, up to ca 45 wt % with a small negative temperature solubiUty coefficient. The aqueous solutions are mildly alkaline at pH 8—8.5 and reasonably stable to reverse hydrolysis. 

The mechanism of chemical adhesion is probably best studied and demonstrated by the use of silanes as adhesion promoters. However, it must be emphasized that the formation of chemical bonds may not be the sole mechanism leading to adhesion. Details of the chemical bonding theory along with other more complex theories that particularly apply to silanes have been reviewed [48,63]. These are the Deformable Layer Hypothesis where the interfacial region allows stress relaxation to occur, the Restrained Layer Hypothesis in which an interphase of intermediate modulus is required for stress transfer, the Reversible Hydrolytic Bonding mechanism which combines the chemical bonding concept with stress relaxation through reversible hydrolysis and condensation reactions. 

This last reaction finds use in volumetric analysis. The use of sulfamic acid to stabilize chlorinated water depends on the equilibrium formation of A-chlorosulfamic acid, which reduces loss of chlorine by evaporation, and slowly re-releases hypochlorous acid by the reverse hydrolysis ... 

If polycondensation is carried out at low temperature, removal of the liberated water is impossible. In this case, reverse hydrolysis must be taken into account unless equilibrium is shifted towards esterification by an excess of one of the reactants. 

Phosphate condensation reactions play an essential role in metabolism. Recall from Section 14.6 that the conversion of adenosine diphosphate (ADP) to adenosine triphosphate (ATP) requires an input of free energy ADP -I-H3 PO4 ATP +H2O AG° — +30.6kJ As also described in that section, ATP serves as a major biochemical energy source, releasing energy in the reverse, hydrolysis, reaction. The ease of interchanging O—H and O—P bonds probably accounts for the fact that nature chose a phosphate condensation/hydrolysis reaction for energy storage and transport. 

The results presented in Tables 3 and 4 deserve some comments. First, a variety of enzymes, including whole-cell preparations, proved suitable for the resolution of different hydroxyalkanephosphorus compounds, giving both unreacted substrates and the products of the enzymatic transformation in good yields and, in some cases, even with full stereoselectivity. Application of both methodologies, acylation of hydroxy substrates rac-41 and rac-43 or the reverse (hydrolysis of the acylated substrates rac-42 and rac-44), enables one to obtain each desired enantiomer of the product. This turned out to be particularly important in those cases when a chemical transformation OH OAc or reverse was difficult to perform. As an example, our work is shown in Scheme 3. In this case, chemical hydrolysis of the acetyl derivative 46 proved difficult due to some side reactions and therefore an enzymatic hydrolysis, using the same enzyme as that in the acylation reaction, was applied. Not only did this provide access to the desired hydroxy derivative 45 but it also allowed to improve its enantiomeric excess. In this way. 

This reaction is the reverse hydrolysis of iodine [22], In a further reaction, the product iodine reacts with iodide to give triodide. Both iodine and triodide absorb in the visible region, hence the reaction can be monitored optically. 

S. Singh, M. Scigelova, and D. H. G. Crout, Glycosidase-catalysed synthesis of mannobioses by the reverse hydrolysis activity of alpha-mannosidase Partial purification of alpha-mannosidases from almond meal, limpets and Aspergillus niger, Tetrahedron Asymmetry, 11 (2000) 223-229. 

Degradation and evaporation seem to be the major pathways for acrolein loss in water smaller amounts are lost through absorption and uptake by aquatic organisms and sediments (USEPA 1980 Reinert and Rodgers 1987). The half-time persistence of acrolein in freshwater is 38 h at pH 8.6 and 50 h at pH 6.6 degradation is more rapid when initial acrolein concentrations are less than 3000 pg/L (Bowmer and Higgins 1976). Nordone et al. (1998) show a half-time persistence of 2.9 to 11.3 h at initial nominal concentrations of 20 pg/L, and 27.1 to 27.8 h at 101 pg/L. At pH 5, acrolein reacts by reversible hydrolysis to produce an equilibrium mixture with 92% beta-hydroxy-... 

Two configurations of stirred-tank reactors are to be considered for carrying out the reversible hydrolysis of methyl acetate (A) to produce methanol (B) and acetic acid (C) at a particular temperature. Determine which of the following configurations results in the greater steady-state rate of production of methanol ... 

Reverse hydrolysis a thermodynamically controlled equilibrium process, in which a free monosaccharide reacts with a nucleophile under exclusion of a water molecule and hence chemically, can be considered a condensation reaction. 

Wu et al. [183] studied the reversible hydrolysis of penicillin G into 6-aminopeni-cillanic acid (6-APA) and phenylacetic acid (PAA) in a chromatographic reactor. E. coli cells containing penicillin acylase (the catalyst) were immobilized by entrapment into gelatine and further cross-linking with glutaraldehyde. The ad-... 

The reversibility of lactone hydrolysis in HMG-CoA reductase inhibitors has been demonstrated at gastric pH and temperature (pH 2.0, 37°) [186], For the prodrug lovastatin (8.148), reversible lactone hydrolysis to its active hydroxy acid (8.149) occurred with a f1/2 value of ca. 1 h and an equilibrium constant close to one. Similar results were obtained for some closely related compounds. In contrast, this reversible hydrolysis was much slower under the nearly neutral pH conditions of the intestine. 

Kita, K., Okino, N., and Ito, M., 2000, Reverse hydrolysis reaction of a recombinant alkaline ceramidase of Pseudomonas aeruginosa. B/oc/t/m. Biophys. Acta. 1485 111-120. 

From Eq. (12) we see that f is independent of both the concentrations of the enzyme and the substrate as well as the incubation time, as observed in the experiments. Since k and k/ are rate constants, the fraction f is a characteristic property of the system. Similar to the equilibrium constant for reversible hydrolysis [48], we define a quasi-steady state constant for irreversible hydrolysis as ... 

Fig. 3 Mechanisms for enzymatic supramolecular polymerisation (a) Formation of supramolecular assembly via bond cleavage, (b) Formation of supramolecular assemblies via bond formation. Examples are shown of biocatalytic supramolecular polymerisation of aromatic peptide amphiphiles via (i) phosphate ester hydrolysis, (ri) alkyl ester hydrolysis, and (iii) amide condensation or reversed hydrolysis using protease...

In addition to chemical composition, as discussed in Sect. 3, the route of self-assembly also significantly affects the resulting structure because different kineti-cally folded structures may be formed. For example, subtilisin-triggered formation of Fmoc-Ls via ester hydrolysis gives rise to hollow nanotubular structures [22], whereas Fmoc-Ls gel formed by the thermolysin-catalysed reversed hydrolysis of the Fmoc-L/L2 system gives rise to nanofibrillar morphology [21]. 

Das AK, Collins R, Ulijn RV (2008) Exploiting enzymatic (reversed) hydrolysis in directed self-assembly of peptide nanostructures. Small 4 279-287... 

Toledano S, Williams RJ, Jayawarna V, Ulijn RV (2006) Enzyme-triggered self-assembly of peptide hydrogels via reversed hydrolysis. J Am Chem Soc 128 1070-1071... 

Formation of an amide bond (peptide bond) will take place if an amine and not an alcohol attacks the acyl enzyme. If an amino acid (acid protected) is used, reactions can be continued to form oligo peptides. If an ester is used the process will be a kinetically controlled aminolysis. If an amino acid (amino protected) is used it will be reversed hydrolysis and if it is a protected amide or peptide it will be transpeptidation. Both of the latter methods are thermodynamically controlled. However, synthesis of peptides using biocatalytic methods (esterase, lipase or protease) is only of limited importance for two reasons. Synthesis by either of the above mentioned biocatalytic methods will take place in low water media and low solubility of peptides with more than 2-3 amino acids limits their value. Secondly, there are well developed non-biocatalytic methods for peptide synthesis. For small quantities the automated Merrifield method works well. 

If the yield for hydrolysis of 0.2 mol/1 penicillin G should be >90%, the yield of its synthesis (which is the reverse hydrolysis) should be <10%. From Figure 10.2 one can read that for initial concentrations of 0.2 mol/1 this yield will be obtained at pH >7.5. 

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