Glutamine esterification

A second method of activating the acid for esterification (see Section 7.6) is as the mixed anhydride. The mixed-anhydride reaction had been employed decades ago for preparing activated esters. However, it was never adopted because of its unreliability and the modest yields obtained. The method was fine-tuned (Figure 7.12), after reliable information on the properties of mixed anhydrides was acquired (see Section 2.8). Tertiary amine is required for esterification of the mixed anhydride to occur. The method is generally applicable, except for derivatives of asparagine, glutamine, and serine with unprotected side chains. The base also prevents decomposition that occurs when the activated derivative is a Boc-amino acid (see... 

Amino acid methyl esters are fairly simple to prepare. The classical method uses a suspension of amino acid hydrochloride in MeOH and gaseous HCl as catalyst b° l di-carboxylic acids yield the dimethyl esters.Hydrolytic and alcoholytic cleavage of side-chain amide groups of glutamine and asparagine as well as peptide bond cleavage prevents the use of this method for the esterification of peptide fragments. 

The carboxamide groups of asparagine and glutamine derivatives 26 are susceptible to dehydration during carbodiimide-mediated esterification at the a-carboxy groups (Scheme 4).[64] reaction probably involves formation of aspartyl- or glutamylisoimide 28 by attack... 

A complex procedure for determining the content of asparagine and glutamine separately in proteins has been described (see Chibnall et al. 1958). This procedure makes use of esterification of the protein, reduction of the resulting esters of aspartic acid and glutamic acid with lithium borohydride and hydrolysis of asparagine and glutamine to the acids in which form they are analyzed. Despite the side reactions for which corrections must be made, this method, in conjunction with total enzymic hydrolysis, may be useful to those who must quantitatively estimate these 4 amino acids. 

In the GC analysis of amino acids, derivatization processes are necessary and can be a disadvantage All reactive groups of the ammo acids have to be denvatized for a satisfactory GC analysis. Esterification of the carboxylic group is acid-catalyzed, which causes the degradation of glutamine (the most abundant ammo acid in the CNS) and asparagine to their dicarboxylic acid counterparts (glutamate and aspartate). An alternative derivatization ap-... 

Of these derivahzation procedures, the one that we have found the most sensitive for dilute samples is the PAM procedure. However, as with all the procedures utilizing alcohol esters, this one has a drawback. Dunng the esterification reaction, glutamine is hydrolyzed to glutamate and asparagine to aspartate. Therefore, these dicarboxylic acids must be isolated from the carboxamides prior to esterification. Ion exchange on Dowex AG1-X8 is extremely useful for this procedure (Corradetti et al., 1983) ... 

The general approach for carboxyl protection is esterification. The simplest solution, the use of methyl or ethyl esters, is suitable for semipermanent blocking, although the commonly applied process of unmasking, alkaline hydrolysis, is far from unequivocal. It is accompanied by racemization, partial hydrolysis of carboxamide groups in the side chain of asparagine and glutamine residues and by several other side reactions which are initiated by proton abstraction (Cf. Chapter VII). Nevertheless, perhaps because of the attractively simple esterification of amino acids... 

Likewise the acid or base catalyzed reaction of an alcohol and an acid anhydride is a regular method of esterification [4]. For many acids the most convenient experimental procedure is to form a mixed anhydride in situ by reaction with trifluoroacetic anhydride [23, 24] or the reagents listed above (method A). Cyclic anhydrides of dibasic acids yield monoesters with alcohols and this reaction is utilized in the preparation of glutamine (11) from glutamic acid (10) and in a similar synthesis of asparagine [35, 36]. 

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