Amino acids N-acetyl

D, L-N-acetyl amino acid L-amino acid D-N-acatyl amino acid... 

LOV MOLECULAR WEIGHT MODEL COMPOUNDS. The mechanisms of radiation effects on polymers are frequently investigated by studies of low molecular weight model compounds. Analysis of the chemical reactions is much easier than with high molecular weight polymers. Thus, N-acetyl amino acids can be studied as model compounds for poly(amino acid)s and hence for proteins. 

However, the chemical changes observed in low molecular weight compounds can be quite misleading as models for polymers. Difficulties include the high concentration of end groups, e.g. COOH in N-acetyl amino acids, which can dominate the radiation chemistry of the models. Low molecular weight compounds are usually crystalline in the solid state and reactions such as crosslinking may be inhibited or severely retarded. 

Gamma radiolysis of simple carboxylic acids and N-acetyl amino acids results in loss of the carboxyl group with formation of carbon monoxide and carbon dioxide. In the carboxylic acids, the ratio of C0/C02 produced is approximately 0.1, while in the N-acetyl amino acids the ratio is much smaller. In the poly carboxylic acids and poly amino acids, radiolysis also results in the loss of the carboxyl group, but here the ratio of C0/C02 is greater than 0.1. Incorporation of aromatic groups in the poly amino acids provides some protection for the carboxyl group. The degradation of the poly acids is believed to involve radical and excited state pathways. 

These observations are common to the series of N-acetylamino acids which have been investigated. Whereas the carbon monoxide to carbon dioxide yield for the aliphatic carboxylic acids are in the approximate ratio of 1 10, the corresponding ratio for the N-acetyl amino acids is much smaller, because of the much smaller yields for carbon monoxide. This suggests that there must be another site on which the ejected electron can be trapped with greater efficiency than that for trapping on the carboxyl group. 

A variety of radical products is observed following gamma radiolysis of the N-acetyl amino acids at 77 K (6), depending on the nature of the side chain of the parent amino acid. In the case of N-acetyl alanine, for example, the intermediates are (i) the anion radical IV (ii) the decarboxylation radical V (iii) the deamination radical VI and (iv) the alpha carbon radical VII. 

A similar behaviour has been found to occur with the other N-acetyl amino acids. In each case, the most stable radical observed at 303 K was the alpha carbon radical, as was also observed for the aliphatic carboxylic acids. In Table VI the radical yields observed following gamma radiolysis of a series of N-acetyl amino acids at 303 K are reported, together with the stable radical intermediates observed at this temperature (5). 

Again the close correspondence between the measured radical and carbon dioxide yields for 7-radiolysis of the N-acetyl amino acids in the solid state suggests that the mechanisms for radical production and carbon dioxide formation are closely related, as they were for the aliphatic carboxylic acids. The following mechanism has been proposed (5.) in order to account for the major degradation products and observed radical intermediates. 

POLYAMINO ACIDS Aliphatic polyamino acids irradiated in the solid state have been reported to undergo N-Ctf, main-chain, bond scission on gamma radiolysis (9) and the stable radical intermediates formed following radiolysis at 303 K are alpha carbon radicals, as observed in the N-acetyl amino acids. 

Owing to their importance in toxicology and as carcinogens, nitroamides must be considered seriously. The study of several N-nitroso-N-acetyl amino acids has been reported... 

Cacciatore, I., Cocco, A., Costa, M., Fontana, M., Lucente, G., Pecci, L., and Pinnen, F. (2005). Biochemical properties of new synthetic camosine analogues containing the residue of 2,3-diaminoproprionic acid The effect of N-acetylation. Amino Acids 28, 77-83. 

The starting material for the acylase process is a racemic mixture of N-acetyl amino acids which are chemically synthesized, most conveniently by acetylation of... 

Acylase (acylase I aminoacylase N-acetyl amino acid amidohydrolase E.C. 3.5.1.14), is one of the best-known enzymes as far as substrate specificity (Chenault, 1989) or use in immobilized (Takahashi, 1989) or membrane reactors (Wandrey, 1977, 1979 Leuchtenberger, 1984 Bommarius, 1992a) is concerned however, its exact mechanism or 3D structure is still not known (Gentzen, 1979 1980). Acylase is available in large, process-scale quantities from two sources, porcine kidney and the mold Aspergillus oryzae. 

A. S. Bommarius, K. Drauz, H. KLENKand C. Wandrey, Operational stability of enzymes - acylase-catalyzed resolution of N-acetyl amino acids to enantiomerically pure L-amino adds, Ann. N. Y. Acad. Sci. (Enzyme Eng. XI) 1992b, 672, 126-136. 

Enzymatic Kinetic Resolution of N-Acyl Amino Acids Coupled with Racemization by N-Acyl Amino Acid Racemase Acylases are enzymes hydrolysing the N-acetyl derivatives of amino acids. They require the free carboxylate for activity and have long been used for the kinetic resolution of amino acids. The unreacted enantiomer is usually racemized in a separate step by treatment with acetic anhydride. While acylases from hog kidney have an L-specificity, bacterial acylases with L- and D-specificity of various origins have been isolated and used for the kinetic resolution of N-acetyl amino acids. An industrial process for the production of L-Met and other proteinogenic and non-proteinogenic L-amino acids such as L-Val, L-Phe, L-Norval, or L-aminobutyric acid has been established. Currently, several hundred tons per year of L-methionine are produced by this enzymatic conversion using an enzyme membrane reactor [46]. 

The starting material for the acylase process is a racemic mixture of N-acetyl-amino acids 20 which are chemically synthesized by acetylation of D, L-amino acids with acetyl chloride or acetic anhydride in alkaU via the Schotten-Baumann reaction. The kinetic resolution of N-acetyl-D, L-amino acids is achieved by a specific L-acylase from Aspergillus oryzae, which only hydrolyzes the L-enantiomer and produces a mixture of the corresponding L-amino acid, acetate, and N-acetyl-D-amino acid. After separation of the L-amino acid by a crystallization step, the remaining N-acetyl-D-amino acid is recycled by thermal racemization under drastic conditions (Scheme 13.18) [47]. In a similar process racemic amino acid amides are resolved with an L-spedfic amidase and the remaining enantiomer is racemized separately. Although the final yields of the L-form are beyond 50% of the starting material in these multistep processes, the effidency of the whole transformation is much lower than a DKR process with in situ racemization. On the other hand, the structural requirements for the free carboxylate do not allow the identification of derivatives racemizable in situ therefore, the racemization requires... 

N -Acetylamino acid hydrolase N -Acetyl amino acids Amino acids 36, 37... 

Fauchere, J.L. and Pliska, V. (1983). Hydrophobic Parameters of Amino Acids Side Chain from the Partitioning of N-Acetyl-Amino-Acid Amides. Eur.J.Med.Chem., 4, 369-375. 

The method has been used to prepare esters of N-acetylated amino acids (yields 51-97%) and is particularly successful for the esterification of tertiary acetylenic... 

The latter were found to undergo the usual photo-Fries rearrangement. N-acetyl amino acids have been studied and... 

The enantioselective hydrolysis of racemic N-acetylated a-amino acids d,l-1 at De-gussa represents a long established large-scale process for the production of L-ami-no acids, l-2 [4]. This enzymatic resolution requires an L-aminoacylase as the biocatalyst. The starting materials for this process are readily available, since racemic N-acetyl amino acids d,l-1 can be economically synthesized by acetylation of racemic a-amino acids with acetyl chloride or acetic anhydride under alkaline conditions via the so-called Schotten-Baumann reaction [5]. The enzymatic resolution reaction of N-acetyl d,L-amino acids, d,l-1, is achieved by a stereospecific L-aminoacylase which hydrolyzes only the L-enantiomer and produces a mixture of the corresponding L-amino acid, l-2, acetate, and N-acetyl D-amino acid, d-1 (Fig. 4) [6],... 

Fig. 4 Enzymatic resolution of racemic N-acetyl amino acids using an L-aminoacylase.

Table 3.1 displays rate constants for the hydrogen abstraction by cysteamine thiyi radicals from several N-acetyl-amino acid amides and diketopiperazines [84]. 

Fauchere JL, Pliska VE (1983) Hydrophobic parameters of amino-acid side-chains from the partitioning of N-acetyl-amino-acid amide. Eur J Med Chem 18 369-375... 

Acylase I has not only been used for the enantioselective resolution of N-acetyl-D,L-amino acids to the corresponding L-amino acids but also for substrate-selective resolution of N-acetyl amino acids using the different activity of the enzymatic... 

Table 12.3-5. Thermodynamics of the N-acetyl amino acid reaction 69. Conditions. pH 7.5, T -25 C Xeq calculated at [So] of 0.5 M, Keq-data determined from synthesis and hydrolysis reaction.

The hydrolysis reaction of N-acetyl amino acids is equilibrium-limited, however, the equilibrium is well on the side of the hydrolysis so that at low substrate concentrations conversion is almost quantitative. For the case of N-acetyl methionine, Wandrey and Flaschel determined the equilibrium constant K defined as in Eq. (1)... 

As with the D-aminoacylases from Streptomyces sp. the enzymes from Alcaligenes strains have a preference for hydrophobic N-acetyl-amino acids. In this respect, they are similar to the L-specific acylase I from kidney preparations and Aspergillus sp. The Alcaligenesfaecalis enzyme prefers the N-acyl-D-amino acid derivatives from Met, Phe and Leu[951. If a high-affinity substrate residue occupies the hydrophobic side-chain pocket the enzyme even deacylates D-Met methyl esters or N-Ac-D-Met-Xaa dipeptide derivatives. 

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