Acyl- substrate selectivity

Acyl substrate selectivity analysis. Acyl substrate competition assays employing various mixtures [1-l4C]acyl-ACPs contained 5 uM total ACPs, 0.25 M MOPS, pH 7.4, 2 mM sn-G3P, and no BSA. BSA has been omitted from the reaction mixes because the partially purified enzymes all behaved well in its absence and because BSA is not physiological in this system. These assays were carried out for 5 minutes at 25°C. After extraction into chloroform-methanol, the reaction products were dried under N2, redissolved in chloroform-methanol (2 1) and separated by TLC as described by Frentzen etal. (1983). Radioactivity comigrating with an LPA standard was detected using a Berthold radioactivity scanner, scraped from the plate, saponified, methylated and analyzed as described in the following section. 

A logical extension which emerges from a consideration of Murata s hypothesis is that Alls from chilling sensitive chloroplasts should be less selective in their preference for 18 1-ACP, but not to the extent that they actually display a selectivity for 16 0-ACP. This conclusion is reached from the realization that while di-saturated species of PG are present in higher proportions in chilling sensitive plants than in chilling resistant plants, no plant species much exceeds the value of 75%. This maximum in di-satu rated molecular species describes an ATI which is neutral in its acyl substrate selectivity. These expectations are fulfilled. 

Since amines react more readily than alcohols in noncatalyzed reactions with anhydrides, the reaction is more difficult and initially required stoichiometric catalyst loadings [107], but could be performed in a catalytic sense with an O-acylated azlactone as acylating agent, which does not react with a benzylic amine at —50°C, but is capable of acylating the catalyst [108, 109]. Depending on the buUdness of the substrate, selectivities ranged from S = 11 to 27 (s = [ enantiomer l]/[ enantiomer 2])-... 

Aresta and Quaranta studied the reactivity of alkylammonium N-alkylcarbamates (RNH3)02CNHR towards a different acylating substrate, such as dimethyl carbonate (DMC) [62a, b]. Carbamate salts (RNH3)02CNHR (R = benzyl, allyl, cyclohexyl), prepared in situ from aliphatic primary amines and C02, reacted with DMC to afford N-alkyl methylcarbamates (Equation 6.6). The reaction requires mild conditions (343-363 K 0.1 MPa C02 pressure) and can be carried out in DMC used as solvent and reagent. At 363 K, carbamate esters were obtained in satisfactory yield (45-92%) with high selectivity, as side products such as ureas, N,N-dialkylcarbamate esters, and alkylated amines were formed in very small amounts. 

A global electrophilicity index of common benzylating and acylating agents has been established from MO calculations and it shows a quantitative linear correlation with the experimental substrate selectivity index from a series of benzylation and acylation reactions.21 The values of relative rate coefficients predicted from the index may be accurate to within 10%. The reaction of /-butyl chloride with anisole catalysed by /Moluenesulfonic acid in supercritical difluoromethane has been subject to kinetic analysis.22 The proportions of substitution at the ortho -position and disubstitution increase at lower pressures, attributed to the decrease in the hydrogen-bonding ability of the solvent. 

The course consists of the acyl process (—OR process) and deacyl process, and (II) is called Michaelis complex. The formation of the Michaelis complex which is the predominant process of substrate selection serves the catalysis with high efficiency. Although it depends on pH a drastic decrease in activation entropy in this process shows that the free energy barrier of the Michaelis complex formation is entropic (2). The magnitude of the decrease is so large that it can not be explained by a decrease of the freedom due to a steric fit of the enzyme and the substrate, therefore changes in conformation of the enzyme and in structure of water molecules at the binding should be taken into account (4). 

Conversion of the carboxylic acid to its CoA ester derivative is the rate-limiting step. The enzyme that catalyzes the final reaction, acyl-CoA amino acid N-acyltransferase, is localized in the mitochondria of the kidney and liver. The amino acid substrate selectivity, which varies from species to species, resides in the specific N-acyltransferase that... 

It has been a widespread assumption that enzymes are fragile molecules that only work in aqueous environments. However, over the last 20 years numerous reports have appeared that feature a wide range of enzymes used for organic synthesis in non-aqueous environments. The discovery that many enzymes retain their catalytic activity in non-aqueous media is often attributed to Klibanov, despite a few much earlier reports [1]. A non-aqueous system may be required for a given transformation due to solubility properties or to drive a reaction equilibrium (such as lipases working in the synthesis/acylation direction) and can even lead to advantages such as enhanced thermostability or altered substrate selectivity. 

Kinetic studies of acylation reactions are somewhat limited by the insolubility of the acyl halide-Lewis acid complexes in many of the solvent systems that are used. However, useful results have been obtained and, as far as we are concerned, relative rates of reactions are of greater importance than absolute values. In any case it is not possible to distinguish between the two mechanistic extremes on the basis of the observed kinetics." Friedel-Crafts acylations are generally characterized by high substrate selectivity and frequently by high positional selectivity. Relative rate data show, as expected, that toluene is more reactive than benzene and that /n-xylene is the most reactive of the dimethylbenzenes. Values, relative to benzene, for benzoylation catalyzed by aluminum chloride were r-butylbenzene (72), toluene (1.1 X 10 ), p-xylene (1.4 x 10 ), o-xylene (1.12 x 10 ), and m-xylene (3.94 x 10- ). Competition data for the trifluoroacetylation of a number of heterocycles using trifluoroacetic anhydride at 75 "C gave the relative rates thiophene (1.0), furan (1.4 x lO ), 2-methylfuran (1.2 x 10 ) and pyrrole (5.3 x 10 ). ... 

The results can be rationalized by assuming that anti selectivity occurs via chelation control (as in 1), and syn selectivity via Felkin-Anh" control with the amino substituent acting as the largest" group (as in 2). In the case of A -acylated substrates the precise nature of the chelated intermediate is not entirely clear, but it is understandable that the control should be less efficient as chelate formation would involve either coordination by a nonbasic nitrogen or the formation of a seven-membered ring. 

Electroactive polymers may be coated on, or covalently attached to, the electrode surface. In the latter case, a facile pathrvay involving the acylation of amino groups on the el ode surface with partially derivatized activated polymer intermediates is shown in Fig. 22 [16]. Similar reactions based on the use of partially functionalized polymer intermediates provide suxess to a variety of modified surfaces with biocompatibility, substrate selectivity, fluorescence or ferromagnetic properties (proprietary work, unpublished). 

This selectivity factor has not been used very widely, however, perhaps because its experimental determination is difficult. The yield of -substitution products is often so low that accurate measurement of is difficult. Table 9.5 gives some data on the selectivity of some representative aromatic substitution reactions. The most informative datum in terms of substrate selectivity is fp, since the partial rate factors for ortho-substitution contain a variable steric component. Using/p as the criterion, halogenation and Friedel-Crafts acylation exhibit high selectivity, protonation and nitration are intermediate, and Friedel-Crafts alkylation shows low selectivity. 

Substrate selection is thus subject to controls at (a) the recognition level, (b) formation of the adenylate, (c) stability of the adenylate, (d) transfer of the acyl residue to the cofactor (aminoacylation or acylation), (e) stability of this acyl intermediate, (f) proper functioning of the acyl intermediate in peptide (or ester) bond formation, and (g)... 

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