Hydrolysis of amino acid amides

Table 6w4 Effect of Cobalt(lll) Coordination on Rate Constants for Base Hydrolysis of Amino Acid Amides...

Although the above discussion has concentrated upon the hydrolysis of amino acid esters, very similar mechanisms have been demonstrated for the hydrolysis of amino acid amides. A very wide range of intramolecular reactions of this type are now known to occur by intramolecular attack by hydroxide, with most having been demonstrated at non-labile... 

I. A. Yamskov, T. V. Tichonova, V. A. Davankov, Pronase-catalyzed hydrolysis of amino acid amides, Enzyme Microb. Tech., 8 (1986), 241-244. 

The conversion of amino add amides into chiral amino acids has been the subject of a large number of monographs and reviews 21 29L In this section information will be given on amidases and aminopeptidases that have been reported for the stereoselective hydrolysis of amino acid amides. 

One reason for an otherwise apparently excessive interest in Co(trien)X2+ systems is the use of ds-Co(OH)(trien)(OH2)2+ in the hydrolysis of amino acid esters, amino acid amides and peptides785 to form cis-px- and cis-/J2-Co(trien)(aa)2+ (aa = amino acid) complexes.16 In principle, a peptide could be degraded in a stepwise manner and each amino acid residue successively characterized. By the introduction of a chiral center into the backbone of the trien moiety, it was hoped to make such reactions stereoselective. Consequently, while fully A-alkylated trien systems have been widely investigated for M11 central ions, the C-alkylated trien systems have been almost exclusively the reserve of the Co111 chemist (Table 11). 

THE HYDROLYSIS OF AMINO ACID ESTERS, AMIDES AND PEPTIDES 414... 

The metal-accelerated hydrolysis of amino acid esters or amides comprises one of the best investigated types of metal-mediated reaction (Fig. 3-7). One of the reasons for this is the involvement of chelating ligands, which allows chemical characterisation of intermediates and products in favourable cases, and allows detailed mechanistic studies to be made. The reactions have obvious biological relevance and may provide good working models for the role of metals in metalloproteins. 

The rates of hydrolysis of amino acid esters or amides are often accelerated a million times or so by the addition of simple metal salts. Salts of nickel(n), copper(n), zinc(n) and cobalt(m) have proved to be particularly effective for this. The last ion is non-labile and reactions are sufficiently slow to allow both detailed mechanistic studies and the isolation of intermediates, whereas in the case of the other ions ligand exchange processes are sufficiently rapid that numerous solution species are often present. Over the past thirty years the interactions of metal ions with amino acid derivatives have been investigated intensively, and the interested reader is referred to the suggestions for further reading at the end of the book for more comprehensive treatments of this interesting and important area. 

Figure 3-7. Hydrolysis of amino acid esters and amides.

Similar mechanisms may be proposed for the hydrolysis of amino acid esters and amides co-ordinated to labile metal centres such as copper(n) or nickel(n), although mechanistic studies at these centres are much more difficult to perform in view of the rapidity of ligand exchange processes. Further complications arise from the formation of insoluble or colloidal suspensions of metal oxides and hydroxides at higher pH values. In general,... 

Co(trien)(NH3)2] + has been isolated, and only the meso trans isomers (197-198) with two different axial ligands, remain to be distinguished. There is also an extensive chemistry of N and C-alkylated derivatives of (178 180) as cA-[Co(OH)(trien)(OH2)] assists the hydrolysis of amino acid esters, amino acid amides, and peptides to form cis-fi (194) and cA-jS2-[Co(OA0(trien)] + (195)(( A = amino acid) complexes. Chiral alkylated trien ligands have the potential for chiral stereospecificity in such reactions. 

When amines are snbstitnted for hydroxide as the base, there is no change in the rate of hydrolysis. This is becanse amines do not catalyze the hydrolysis of amino acid esters instead they prefer to add directly to the carbonyl carbon to form the corresponding amides. In this reaction the rate-limiting step is not the addition of amine, bnt rather the deprotonation of the coordinated amine by another base (eqnation 8). The of the tetrahedral intermediate is approximately 7 since almost any nonsterically hindered base with a pTTa above 7 can deprotonate it. " ... 

The cobalt(III)-promoted hydrolysis of amino acid esters and peptides and the application of cobalt(III) complexes to the synthesis of small peptides has been reviewed. The ability of a metal ion to cooperate with various inter- and intramolecular acids and bases and promote amide hydrolysis has been investigated. The cobalt complexes (5-10) were prepared as potential substrates for amide hydrolysis. Phenolic and carboxylic functional groups were placed within the vicinity of cobalt(III) chelated amides, to provide models for zinc-containing peptidases such as carboxypeplidase A. The incorporation of a phenol group as in (5) and (6) enhanced the rate of base hydrolysis of the amide function by a factor of 10 -fold above that due to the metal alone. Intramolecular catalysis by the carboxyl group in the complexes (5) and (8) was not observed. The results are interpreted in terms of a bifunctional mechanism for tetrahedral intermediate breakdown by phenol. 

In addition, Asano et al. have reported that DKR of amino acid amides, such as alanine amide, could be performed by using a combination of two enzymes, o-aminopeptidase and a-amino-s-caprolactam racemase, yielding the corresponding o-amino acids. A lipase-catalysed hydrolysis has constituted the key step of a total synthesis of roxifiban, a potent antagonist of the platelet glycoprotein Ilb/IIIa receptor. The DKR of an isobutyl ester to form the corresponding acid in both high yield and enantioselectivity is depicted in Scheme 3.46. 

The classification outlined here is presented mainly for convenience in arranging and relating a wealth of material. Enzymes display an amazing specificity in the reactions each will catalyze. Despite this, these classifications do not represent absolutes in specificity. Some proteinases will, for instance, catalyze the hydrolysis of amino acid esters and amides as well as peptides. 

Hydrolysis of Amino Acid Esters and Amides and Peptides... 

The kinetic resolution of racemic amino acid amides is performed with permeabilized whole cells of P. putida ATCC 12633 with a nearly 100% stereoselectivity for hydrolysis of the L-amide (enantiomeric ratio E > 200 [20]). Thus, both the L-acid and the D-amide can be obtained in nearly 100% e.e. at 50% conversion. The biocatalyst accepts a broad structural variety of amino acid amides, from alanine amide to, for example, p-naphthyl-glycine amide or lupinic acid amide (Scheme 4). So far more than 100 different amino acid amides have been successfully resolved. 

The industrial process for preparing the reagent usually permits a little hydrolysis to occur, and the product may contain a little free calcium hydroxide or basic chloride. It cannot therefore be employed for drying acids or acidic liquids. Calcium chloride combines with alcohols, phenols, amines, amino-acids, amides, ketones, and some aldehydes and esters, and thus cannot be used with these classes of compounds. 

An alternative approach to peptide sequencing uses a dry method in which the whole sequence is obtained from a mass spectrum, thereby obviating the need for multiple reactions. Mass spec-trometrically, a chain of amino acids breaks down predominantly through cleavage of the amide bonds, similar to the result of chemical hydrolysis. From the mass spectrum, identification of the molecular ion, which gives the total molecular mass, followed by examination of the spectrum for characteristic fragment ions representing successive amino acid residues allows the sequence to be read off in the most favorable cases. 

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