Carboxyl groups, amino acids

Activation of the amino acid carboxyl groups is the only set of clearly reversible reactions catalysed by NRPS. So far these reactions have been studied in the case of ACV synthetases only by the amino acid-dependent ATP-PPi exchange reaction [reaction (11)] ... 

This breakthrough method uses 1,2-trans peracetates in the presence of Lewis acids as a promoter to glycosylate (V -I inoc amino acids with an unprotected a-carboxyl group. The deprotection of that glycosylated amino acid carboxyl group prior to incorporation into a peptide can be avoided (62). In addition, the advantage of this method is that the starting materials are readily available... 

Amino acyl adenylates are obtained in the reaction of adenosine 5-phosphate (ASP) with free amino acids in aqueous pyridine, mediated by DCC. The linkage is an anhydride between the amino acid carboxyl group and the phosphate in A5P." ... 

Titration and buffer calculations involving peptides are done exactly as shown earlier for polyprotic acids. We must remember that the amino acid carboxyl groups used in forming the peptide bond are no longer available for titration. The structure of the fully protonated glutamylserylglutamylvaline is shown below. It is assumed that the ct-COOH group retains a pKa of — 2.5... 

The computer-controUed method of titration involved was developed by Moisio and Heikonen [44]. Its apphcation to silage press-juice provides for the determination of lactic acid, acetic acid, reducing sugars, amino acid carboxyl groups and the protein degradation products -NH2 and -NH- groups. The general method follows. 

Because of the inductive effect of the electron-withdrawing —NHj+ group, amino acid carboxyl groups are more acidic than acetic add. 

Synthesis of aminoacyl-tRNA (HI) from amino acids (I) requires activation of the amino acid carboxyl group with formation of an intermediate enzyme-bound amino-acyladenylate (II). 

During this reaction, 0 from the amino acid carboxyl group is transferred to the phosphate of AMP. 

In the case of DNB amino acids, the relevance of the mechanism was established by the following results (i) Methyl esterification of the amino acid carboxylic group cancels all chiral recognition making the docking approach impossible (ii) A relationship between log a, the enanfioselecfivity factor, and log 2. the retention factor of the most retained enantiomer, was found depending on the amino acid side chain size. The retention factors, k, of the first eluting DNB amino acids were similar [41] (iii) The enanfioselecfivity factors and elution order, respectively, obtained on the quinine and quinidine CSPs are similar and opposite for the same enantiomeric pairs (iv) Native amino acid enantiomers are not separated (no n-n interaction) [41]. 

Table 23.5 Amino acid carboxyl group vibrations...

Amino acids have basic amino, acidic carboxyl groups these affect their chromatographic behavior. Changing the pH of the mobile phase changes the amount of protonation or deprotonation of amino groups and the extent of dissociation of carboxyl group. It should be stressed that the chiral stationary phase also contains one enantiomer of an amino acid molecule, so variation of the pH of the mobile phase can also change the properties of the chiral stationary phase. 

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