Substitution enzymatic

Except for perhaps the manufacture of cocoa butter substitutes, enzymatic interesterification processes have been confined to laboratories and pilot plants. The reasons are partially due to the relatively low value of products from these processes and more importantly to the many difriculties in solving the engineering problems of bioreactor designs for efficient, continuous processes. Application of biotechnology and computer aided techniques in bioreactor designs are consequently used to find solutions to these problems. The ultimate aim is to produce cost effident process designs for manufacturing products of even subtle value at competitive production costs to chemical catalysis. 

The need is acute for a simple, comprehensive microbiological assay for folio compounds. One way would be to assay PABA after liberating it with acid or alkali. Several potential assay bacteria have been mentioned, e.g. Acetdbactor suboxydans and L. plantarum 17-5. In using Tetrahymena for assay purposes one is substituting enzymatic liberation of FA compounds for chemical hydrolysis of FA to PABA the two methods may be complementary. The response of a thermophilic Tetrahymena (Holz et ai., 1959) to known bypas g compounds is under investigation. The results encourage the hope that a practical procedure is not too remote (Holz, 1958). 

The wM-diacetate 363 can be transformed into either enantiomer of the 4-substituted 2-cyclohexen-l-ol 364 via the enzymatic hydrolysis. By changing the relative reactivity of the allylic leaving groups (acetate and the more reactive carbonate), either enantiomer of 4-substituted cyclohexenyl acetate is accessible by choice. Then the enantioselective synthesis of (7 )- and (S)-5-substituted 1,3-cyclohexadienes 365 and 367 can be achieved. The Pd(II)-cat-alyzed acetoxylactonization of the diene acids affords the lactones 366 and 368 of different stereochemistry[310]. The tropane alkaloid skeletons 370 and 371 have been constructed based on this chemoselective Pd-catalyzed reactions of 6-benzyloxy-l,3-cycloheptadiene (369)[311]. 

The bactericidal and enzymatic action of dyes, particularly of vinyl derivatives of 3,4,5-substituted thiazolium, for example, 45 (Scheme 70) (139), have been systematically studied to know more about the basic mechanisms involved (140). 

Mutagenicity. The AJ-nitrosamines, in general, induce mutations in standard bacterial-tester strains (117). As with carcinogenicity, enzymatic activation, typically with Hver microsomal preparations, is required. Certain substituted A/-nitrosamine derivatives (12) induce mutations without microsomal activation (31,33,34). Because the a-acetoxy derivatives can hydroly2e to the corresponding a-hydroxy compounds, this is consistent with the hypothesis that enzymatic oxidation leads to the formation of such unstable a-hydroxy intermediates (13) (118). However, for simple /V-nitrosamines, no systematic relationship has been found between carcinogenicity and mutagenicity (117,119—123). 

Other examples illustrating the effect of substituent distribution on properties include (/) enzymatic stabiUty of hydroxyethjlceUulose (16,17) (2) salt compatibihty of carboxymethylceUulose (18,19) and (J) thermal gelation properties of methylceUulose (20). The enzymatic stabUity of hydroxyethylceUulose is an example where the actual position of the substituents within the anhydroglucose units is considered important. Increasing substitution at the C2 position promotes better resistance toward enzymatic cleavage of the polymer chain. Positional distribution is also a factor in the other two examples. 

Solutions of methylceUuloses are pseudoplastic below the gel point and approach Newtonian flow behavior at low shear rates. Above the gel point, solutions are very thixotropic because of the formation of three-dimensional gel stmcture. Solutions are stable between pH 3 and 11 pH extremes wiU cause irreversible degradation. The high substitution levels of most methylceUuloses result in relatively good resistance to enzymatic degradation (16). 

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

The first chemical synthesis of these substances, using a procedure which yields 1-ribofuranosyl derivatives by pyrimidine bases, was described by Hall. By using the mercuric salt of 6-azathymine and tribenzoate of D-ribofuranosyl chloride, he obtained a mixture of two monoribosyl derivatives and a diribosyl derivative. He determined the structure of the 3-substituted derivative by the similarity of spectra and other properties to those of 3-methyl-6-razauracil. The structure of the 1-ribosyl derivative was then determined from the similarity of the spectra with 6-azathymine deoxyriboside obtained enzymatically. 

Partial hydrolysis of a peptide can be carried out either chemically with aqueous acid or enzymatically. Acidic hydrolysis is unselective and leads to a more or less random mixture of small fragments, but enzymatic hydrolysis is quite specific. The enzyme trypsin, for instance, catalyzes hydrolysis of peptides only at the carboxyl side of the basic amino acids arginine and lysine chymotrypsin cleaves only at the carboxyl side of the aryl-substituted amino acids phenylalanine, tyrosine, and tryptophan. 

Substituting (5.7.1.24) into (5.7.1.22), the rate of enzymatic reaction with dual substrates is obtained ... 

Azetidine ring is an important structure because it is present in many compounds of pharmaceutical interest however, its manipulation must be done very carefully owing to the reactivity of these heterocycles of small size. An interesting application of the use ofbiocatalytic processes is the resolution of azetidine esters (Scheme 7.11). The procedure to choose for the resolution of these compounds is the enzymatic ammonolysis of the corresponding N-substituted azetidines [26]. 

The classical kinetic resolution of racemic substrate precursors allows only access to a theoretical 50% yield of the chiral ladone product, while the antipodal starting material remains unchanged in enantiomerically pure form. The regioseledivity for the enzymatic oxidation correlates to the chemical readion with preferred and exclusive migration of the more nucleophilic center (usually the higher substituted a-carbon). The majority of cydoketone converting BVMOs (in particular CHMOAdneto)... 

When you crack open a can of Coca Cola or Pepsi, you are tasting some of the fruits of bioohemioal engineering Most nondiet soft drinks sold in the United States are sweetened with high-fruotose oorn syrup (MFCS), a substitute for the natural sugar that oomes from cane and beets. MFCS, produced by an enzymatic reaction, is an example of the suooessful application of chemical engineering principles to bioohemioal synthesis. So successful, in fact, that more than 1.5 billion of MFCS was sold in the United States last year. 

Opitz F, Schenke-Layland K, Richter W, Martin DP, Degenkolbe I, Wahlers T, and Stock UA. Tissue engineering of ovine aortic blood vessel substitutes using applied shear stress and enzymatically derived vascular smooth muscle cells. Ann Biomed Eng, 2004, 32, 212-222. 

FIGURE 23 Rate of enzymatic surface erosion of a 1 1 copolymer of e-caprolactone and 6-valerolactone, crosslinked with a dilactone to form an elastomer. The effect of substitution of the e-caprolactone nucleus is also shown. (From Ref. 98). 

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