Amino acids reactions involving

A nice example of the chemical similarity between imines and carbonyl compounds is the Strecker synthesis of amino acids. This involves reaction of an aldehyde with ammonia and HCN (usually in the form of ammonium chloride plus KCN) to give an intermediate a-aminonitrile. Hydrolysis of the a-aminonitrile then produces the a-amino acid. 

Another possible mechanism for the racemization of amino acid esters involves the in situ, transient, formation of Schiff s bases by reaction of the amine group of an amino acid ester with an aldehyde. Using this approach, DKR of the methyl esters of proline 5 and pipecolic acid 6 was achieved using lipase A from C. ant-arclica as the enantioselective hydrolytic enzyme and acetaldehyde as the racemiz-ing agent (Scheme 2.4). Interestingly, the acetaldehyde was released in situ from vinyl butanoate, which acted as the acyl donor, in the presence of triethylamine. The use of other reaction additives was also investigated. Yields of up to 97% and up to 97% e.e. were obtained [6]. 

Piperazine-2,5-diones can be symmetric or asymmetric. Symmetric DKPs are readily obtained by heating amino acid esters,1179-181 whereas asymmetric DKPs are obtained directly from the related dipeptides under basic or, more properly, acid catalysis, or by cyclocondensation of dipeptide esters.1182-185 As an alternative procedure hexafluoroacetone can be used to protect/activate the amino acid for the synthesis of symmetric DKPs or of the second amino acid residue for synthesis of the dipeptide ester and subsequent direct cyclocondensation to DKPs.1186 The use of active esters for the cyclocondensation is less appropriate since it may lead to epimerization when a chiral amino acid is involved as the carboxy component in the cyclization reaction. Resin-bound DKPs as scaffolds for further on-resin transformations are readily prepared using the backbone amide linker (BAL) approach, where the amino acid ester is attached to the BAL resin by its a-amino group and then acylated with a Fmoc-protected amino acid by the HATU procedure, N -deprotection leads to on-resin DKP formation1172 (see Section 6.8.3.2.2.3). 

These chelates are structurally similar to that postulated above for the metal ion-catalyzed hydrolysis of oj-amino esters the position of the protons in the transition state is different, but this is a completely arbitrary distinction. A further distinction is that the metal ion is facilitating attack in this instance not by a polarization of the substrate molecule, but rather by the positioning and fixation of the hydroxide ion at the reaction site. It is not clear which of these two representations—for the amino acid esters involving polarization or for the carboxylate esters involving fixation of the hydroxide ion—is the correct interpretation. It is conceivable that both are correct. A similar explanation will account for the large effect of calcium ions on the alkaline hydro ysis of acetylcitric and benzoyl-citric acids (53). 

Most syntheses of phosphorus amino acid analogues involve the conversion of a nucleophilic, trivalent phosphorus species into a pentavalent adduct. The classical Arbuzov-Michaelis reaction is a well-known transformation that demonstrates this principle.1 In the case of a phosphite diester, the stable form of the reagent is the P-H derivative, depicted in Scheme 2 19 this tautomer is converted into a nucleophilic form by deprotonation or by silylation, which favors the trivalent P—O—Si isomer. 

The earliest approach to amino acid analysis involved postcolumn reaction. This scheme offers several advantages compared to precolumn reaction. First, it simplifies the sample preparation necessary. Often, precolumn derivatizations require sample cleanup steps to eliminate sample... 

A modification of the pyridoxal—amino acid reaction (mentioned above) has been made for automatic analysis of amino acids by ligand-exchange chromatography [95]. This technique involves separation of the amino acids prior to fluorimetric reaction and determination. As the amino acids are eluted from the column, they are mixed with the pyridoxal-zinc(II) reagent to produce a highly fluorescent zinc chelate. Amounts of as low as 1 nmole of amino acid may be detected. The first reaction involved is the formation of the pyridoxyl-amino acid (Schiff base) as in Fig.4.46. The zinc then forms a chelate which probably has the structure shown in Fig. 4.48. 

Some authors have indicated that the sugar-amino acid reactions of the Maillard type are of minor importance in citrus juices because of the high acidities involved. Studies in our laboratories (42-44) would tend to indicate that, to the contrary, the amino acids and sugars are of more than just minor importance in the darkening of citrus juices. Huffman (42) treated citrus juices with cationic ion-exchange resins to remove amino acids, proteins, and the mineral cations, then restored the cations. 

Amino Acid Conjugation. In the second type of acylation reaction, exogenous carboxylic acids are activated to form S-CoA derivative in a reaction involving ATP and CoA. These CoA derivatives then acylate the amino group of a variety of amino acids. Glycine and glutamate appear to be the most common acceptor of amino acids in mammals in other organisms, other amino acids are involved. These include ornithine in reptiles and birds and taurine in fish. 

In our studies the ninhydrin-reaction- revealed a number of new fractions, one of which is fluorescent. There are two fractions with a mobility greater than albumin, and two with a mobility lower than y-globulin (Fig. 62b). Some of these are present in many sera, some are rare. They are present in the ultrafiltrate of serum where also a central fraction can be seen, which has the mobility of the y-globulin. They are also present in normal cerebrospinal fluid and were found in hemoglobin preparations. It is not clear if only amino acids are involved, because larger molecules also react with ninhydrin. 

During the processing of foods, several chemical reactions affect the structure of some amino acids reactions with reducing sugars and polyphenols, interaction of the side chains of the amino acids, oxidation, etc. In some cases, these chemical modifications do not seem to change the nutritional availability of the amino acids involved, while in other cases, they do. In all cases, the presence of the newly formed molecules induces biochemical and physiological changes. 

Pyridoxal is the reagent in other reactions of amino acids, all involving the inline as intermediate. The simplest is the racemization of amino acids by loss of a proton and its replacement on the other face of the enamine. The enamine, in the middle of the diagram below, can be reprotonated on either face of the prochiral inline (shown in green). Protonation on the bottom face would take us back to the natural amino acid from which the enamine was made in the first place. Protonation on the top face leads to the unnatural amino acid after hydrolysis of the inline (really transfer of pyridoxal to a lysine residue of the enzyme). 

The acid fluorides are generally available via reaction of the acid with cyanuric fluoride in the presence of pyridine according to the general technique of Olah.t An alternative method for the synthesis of Fmoc amino acid fluorides involves the use of DAST (N,N-diethylaminosulfur trifluoride), which is advantageous relative to cyanuric fluoride owing to the formation of only water-soluble compounds as byproducts, thus, facilitating the isolation of the acid fluorides.Examples can be found in Table 3. 

The copper metalloenzymes are involved in oxygen-using reactions. These enzymes include cytochrome c oxidase (respiratory chain), lysyl oxidase (collagen synthesis), and dopamine [3-hydroxylase (neurotransmitter synthesis). Lysyl oxidase is a small protein with a molecular weight of 32 kDa. This enzyme contains an unusual modification, namely cross-linking between two different parts of its polypeptide chain. The cross-linked region consists of a structure called lysine tyrosylquinone (Klinman, 1996). Two amino acids are involved in this cross-linked structure, and these are Lys 314 and Tyr 349. Lysine tyrosylquinone is used as a cofactor and is necessary for the catalytic activity of the enzyme. Other copper metalloenzymes contain a related cofactor, namely 2,4,5-tiihydrox5q5henylalanine (topaquinone, TPQ). Serum amino oxidase is a copper metalloenzyme that contains TPQ. TPQ consists of a modified residue of phenylalanine. The copper in the active site of the enzyme occurs immediately adjacent to the TPQ cofactor. 

Several other amino acids arc involved in the conjugation ofcarbo.xylic acids, but these reactions occur only occasionally and appear to be highly substrate and species depen-denl. " Ornithine (in birds), aspartic acid and serine (in rals), alanine (in mouse and hamster), taurine iH NCH CH2SO]H) (in mammals and pigeon.s), and histidine (in African bats) are among these amino acids. ... 

Catalytic—the amino acid is involved in the mechanism of the reaction. 

It has been proposed that penicillin has a conformation which is similar to the transition-state conformation taken up by D-Ala-D-Ala—the portion of the amino acid chain involved in the cross-linking reaction (Fig. 10.61). Since this is the reaction centre for the transpeptidase enzyme, it is quite an attractive theory to postulate that the enzyme mistakes the penicillin molecule for the D-Ala-D-Ala moiety and accepts the penicillin into its active site. Once penicillin is in the active site, the normal enzymatic reaction would be carried out on the penicillin. 

Fig. 1.8 Asaccharolytic fermentation produces ammonia and short-chain fatty acids. This group of fermentations by oral bacteria utilizes proteins, which are converted to peptides and amino acids. The free amino acids are then deaminated to ammonia in a reaction that converts nicotinamide adenine dinucleotide (NAD) to NADH. For example, alanine is converted to pyruvate and ammonia. The pyruvate is reduced to lactate, and ammonium lactate is excreted into the environment. Unlike lactate from glucose, ammonium lactate is a neutral salt. The common end products in from plaque are ammonium acetate, ammonium propionate, and ammonium butyrate, ammonium salts of short chain fatty acids. For example, glycine is reduced to acetate and ammonia. Cysteine is reduced to propionate, hydrogen sulfide, and ammonia alanine to propionate, water, and ammonia and aspartate to propionate, carbon dioxide, and ammonia. Threonine is reduced to butyrate, water, and ammonia and glutamate is reduced to butyrate, carbon dioxide, and ammonia. Other amino acids are involved in more complicated metabolic reactions that give rise to these short-chain amino acids, sometimes with succinate, another common end product in plaque.

Reaction of an a-amino acid with ninhydrin, Two molecules of ninhydrin and the nitrogen atom of the amino acid are involved in the production of the purple product. 

The decarboxylation of amino acids generally involves pyridoxal 5 -phosphate (PLP) as a cofactor (101-104) (Editor s note For additional features of PLP-dependent decarboxylation, see Chapter 7 by Kluger.). The general reaction is shown in Eq. (17),... 

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