Schiff base formation from amino acids

Schiff base formation can have a considerable effect on both the position and degree of activation of the coordinated amino acid. The Schiff bases derived from amino acids and pyridoxal have attracted considerable attention due to the biochemical significance of vitamin B6 and the realization that many of the enzymic reactions involving B6 could be brought about in the absence of enzyme by using pyridoxal and various metal ions.444,445,461 4 2,342... 

This method results in regioselective carboxymethylation of the amino group, so the product of reaction is a well-defined derivative. Several N-carboxyalkylated chitosans were prepared via Schiff base formation from carboxylic acids having aldehyde or keto groups [47-48]. The resulting... 

In the case of non-protein amino acid-derived alkaloids, the second obligatory intermedia is derived from the obligatory intermedia enzymatically and by the Schiff base formation as, for example, in the hygrine pathway. The second obligatory intermedia is, in this case, the A-methyl-A pyrrolinium cation. 

NMR studies have been carried out on Schiff bases derived from pyridoxal phosphate and amino acids, since they have been proposed as intermediates in many important biological reactions such as transamination, decarboxylation, etc.90 The pK.d values of a series of Schiff bases derived from pyridoxal phosphate and a-amino adds, most of which are fluorinated (Figure 11), have been derived from H and19F titration curves.91 The imine N atom was found to be more basic and more sensitive to the electron-withdrawing effect of fluorine than the pyridine N atom. Pyridoxal and its phosphate derivative are shown in Figure 12a. The Schiff base formation by condensation of both with octopamine (Figure 12b) in water or methanol solution was studied by 13C NMR. The enolimine form is favoured in methanol, while the ketoamine form predominates in water.92... 

Since the Schiff base formation is reversible, it should be reduced by sodium borohydride for the fixation of the label. The rate of the reduction of the Schiff base becomes slow as the number of the phosphate groups of the label increases. However, except for adenylate kinase, the NP -PL bound to the proteins were easily fixed by borohydride reduction. After reductive fixation, labeled proteins are cleaved by appropriate methods. The labeled lysine is cleaved by neither trypsin nor lysyl endopeptidase. There are at least three ways to detect the labeled peptide during isolation 1) use of radioactive reagent, 2) use of radioactive sodium borohydride for reduction of the Schiff base, and 3) use of fluorescence derived from the pyridoxyl moiety of the reagent (excitation at 295 nm and emission at 390 nm at acidic pH). The labeled lysyl residue is not positively identified in the amino acid sequence analysis. However, the presence of the label in the peptide isolated can be confirmed by the presence of pyridoxyl lysine in the amino acid analysis. 

In the absence of the amino acid substrate, pyridoxal phosphate is bound to enzymes by the formation of a Schiff base to the -amino group of a lysine residue at the active site. As shown in Figure 9.2, the first reaction between the substrate and the coenzyme is transfer of the aldimine linkage from the e-amino group of the lysine residue to the a-amino group of the substrate. 

ALAS is the initial enzyme of the pathway and catalyzes the formation of ALA from succinyl-CoA and glycine. The enzyme is mitochondrial and requires pyridoxal phosphate as a cofactor, which forms a Schiff base with the amino group of glycine at the enzyme surface. The carbanion of the Schiff base displaces Co enzyme A from succinyl-CoA with the formation of a-amino-P-ketoadipic acid, which is then... 

The stereochemistry of Schiff base formation between fructose 1,6-bisphosphate and the aldolase from liver has also been addressed (777). Suggestive evidence for the intermediate formation of a (27 )-carbinolamine is based on the observation that BH4 reduction of substrate on the enzyme followed by acid hydrolysis of the protein gives exclusively glucitollysine and not mannitollysine. This indicates that the re face of the ketimine is exposed to solvent, and it implies that OH left from the same direction in other to form the ketimine. On this basis, the e-amino of the lysine must add to the si face of the substrate carbonyl lEq. (33)1 ... 

Rate constants for the formation and hydrolysis of Schiff bases derived from pyri-doxal 5 -phosphate and co-polypeptides have been determined in the pH range 4-11 at 25 °C. The co-polypeptides contain L-lysine, and aromatic L-amino acids, and... 

While it may be surprising that the above diverse reactions require the same cofactor, this will be readily understood when it is realized that these reactions have certain common features. All require imine (Schiff base) formation between the aldehyde carbonyl of the cofactor and the amino group of the substrate. The pyridoxal phosphate becomes an electrophilic catalyst or electron sink, as electrons may be delocalized from the amino acid into the ring structure. It is the direction of this delocalization that dictates the reaction type and in model systems more than one reaction pathway is often observed. Thus the enzyme both enhances the rate of reaction and gives direction to that reaction (see page 428). 

Evidence for the formation of a Schift base between glycine or serine and pyridoxal phosphate in neutral aqueous solution is provided by spectro-photometric data. Addition of either of these amino acids to a solution of pyridoxal phosphate caused a shift in the absorption peak from 388 to 413 m (55) and a new absorption band appeared with a maximum at 279 m. Schiff base formation causes the shift of the absorption band towards the visible by providing a double bond conjugated with the unsaturated ring system. 

Only a few esters of dehydroamino acids are stable and these only as hydrochlorides, so that all the reactions at the amino group studied to date are electrophilic. As the enamino group is less nucleophilic than an amino group, enamino acid esters can only be acylated by the most reactive acid derivatives, such as acid chlorides (368, 373) and mixed anhydrides (69, 257, 313). Attempted peptide formation from acylamino acids and enamino acid esters by the DCCD or DCCD/hydroxysuccini-mide methods is unsuccessful. The formation of Schiff bases at the enamino group proceeds rather slowly, but can be achieved in the case of the stable dehydrovaline methyl ester by direct condensation with aromatic aldehydes (287, 352). 

The 6/3-amino group of 6-APA may be alkylated either with diazoalkanes <67LA(702)163) or by the reduction of Schiff bases (Scheme 50) (65JCS3616). Two special cases of N-alkylation are also shown in Scheme 50 the formation of an imidazolidinone ring upon treating ampicillin with acetone (66JOC897), and the formation of a 6/3-amidinopenicillanic acid from 6-APA (77MI51105). 

As has been outlined for the Strecker synthesis, the Ugi reaction also proceeds via initial formation of a Schiff base from an aldehyde and an amine. The imine intermediate is attacked by the isocyanidc, a process which is supported by protonation of the imine by the carboxylic acid component. The resulting a-amino nitrilium intermediate is immediately trapped by the carboxylate to give an 6>-acyl imidiate. All steps up to this stage are reversible. Only the final oxygen to nitrogen acyl shift is irreversible and delivers the A-acyl-a-amino amide as the thermodynamically favored product which contains two amide groups. 

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