A-Amino group

Amino substituents also behave normally with electrophiles, being acylated and converted to benzylidene derivatives. A-Amino groups have also been converted to benzylidene derivatives and are removed on treatment with A-nitrosodiphenylamine (79JHC249). 

The efficiency of this method was demonstrated by the elegant two-step synthesis of aspartame [87], Protection of the a-amino group and activation of the a-carboxylic group are accomplished in only one step Deprotection of the amino functionality occurs during aminolysis, such as with methyl phenylalaninate (H-Phe-OMe in equation 15)... 

N terminus (Section 27.7) The amino acid at the end of a peptide or protein chain that has its a-amino group intact that is, the a-amino group is not part of a peptide bond. 

Typical values for pAlg are in the range of 9.0 to 9.8. At physiological pH, the a-carboxyl group of a simple amino acid (with no ionizable side chains) is completely dissociated, whereas the a-amino group has not really begun its dissociation. The titration curve for such an amino acid is shown in Figure 4.7. 

If the a-amino group is one-third dissociated, there is one part Gly for every two parts Gly°. The important is the for the amino group. The glycine a-amino group has a of 9.6. The result is... 

Note that the dissociation constants of both the a-carboxyl and a-amino groups are affected by the presence of the other group. The adjacent a-amino group makes the a-COOH group more acidic (that is, it lowers the pAl, ) so... 

Myristic acid may be linked via an amide bond to the a-amino group of the N-terminal glycine residue of selected proteins (Figure 9.18). The reaction is referred to as A -myristoylation and is catalyzed by myristoyl—CoAtprolein N-myris-toyltransferase, known simply as NMT. A -Myristoyl-anchored proteins include the catalytic subunit of cAMP-dependent protein kinase, the ppSff tyrosine kinase, the phosphatase known as calcineurin B, the a-subunit of G proteins (involved in GTP-dependent transmembrane signaling events), and the gag proteins of certain retroviruses, including the FHV-l virus that causes AIDS. 

In addition, some COg is directly transported by hemoglobin in the form of carbamate (—NHCOO ). Free a-amino groups of Hb react with COg reversibly ... 

While the first 20-30 residues of a peptide can readily be determined by the Edman method, most polypeptides contain several hundred amino acids. Consequently, most polypeptides must first be cleaved into smaller peptides prior to Edman sequencing. Cleavage also may be necessary to circumvent posttranslational modifications that render a protein s a-amino group blocked , or unreactive with the Edman reagent. 

Transamination is not restricted to a-amino groups. The 5-amino group of ornithine—but not the e-amino group of lysine—readily undergoes transamination. Serum levels of aminotransferases are elevated in some disease states (see Figure 7-11). 

The a-amino group of the new aminoacyl-tRNA in the A site carries out a nucleophilic attack on the esterified carboxyl group of the peptidyl-tRNA occupying the P site (peptidyl or polypeptide site). At initiation, this site is occupied by aminoacyl-tRNA mef. This reaction is catalyzed by a peptidyltransferase, a component of the 285 RNA of the 605 ribosomal subunit. This is another example of ribozyme activity and indicates an important—and previously unsuspected—direct role for RNA in protein synthesis (Table 38-3). Because the amino acid on the aminoacyl-tRNA is already activated, no further energy source is required for this reaction. The reaction results in attachment of the growing peptide chain to the tRNA in the A site. 

One obvious reaction that proteins may undergo when exposed to nitrite is with the a-amino group (Van Slyke reaction). 

Formation of a 2 1 L-histidine Au(III) complex is suggested on the basis of multiinstrumental techniques the N1 of the imidazole ring and the nitrogen of the a-amino group are likely involved in the coordination [66]. 

In contrast to the lability of certain dN adducts formed by the BHT metabolite above, amino acid and protein adducts formed by this metabolite were relatively stable.28,29 The thiol of cysteine reacted most rapidly in accord with its nucleophilic strength and was followed in reactivity by the a-amine common to all amino acids. This type of amine even reacted preferentially over the e-amine of lysine.28 In proteins, however, the e-amine of lysine and thiol of cysteine dominate reaction since the vast majority of a-amino groups are involved in peptide bonds. Other nucleophilic side chains such as the carboxylate of aspartate and glutamate and the imidazole of histidine may react as well, but their adducts are likely to be too labile to detect as suggested by the relative stability of QMs and the leaving group ability of the carboxylate and imidazole groups (see Section 9.2.3). 

Hansen, P., Andersson, L., and Lindeberg, G., Purification of cysteine-containing synthetic peptides via the selective binding of the a-amino group to immobilised Cu2+ and Ni2+ ions, /. Chromatogr. A, 723, 51, 1996. 

Proteins may be covalently attached to the latex particle by a reaction of the chloromethyl group with a-amino groups of lysine residues. We studied this process (17) using bovine serum albumin as a model protein - the reaction is of considerable interest because latex-bound antigens or antibodies may be used for highly sensitive immunoassays. The temperature dependence of the rate of protein attachment to the latex particle was unusually small - this rate increased only by 27% when the temperature was raised from 25°C to 35°C. This suggests that non-covalent protein adsorption on the polymer is rate determining. On the other hand. the rate of chloride release increases in this temperature interval by a factor of 17 and while the protein is bound to the latex particle by only 2 bonds at 25°C, 22 bonds are formed at 35°C. 

D-Gal — hydroxy-L-histidine,4950 d-G1cA — hydroxy-L-tryptophan,51 d-GlcA — hydroxy-L-phenylalanine,51 d-G1cA — L-Ser,51 and carbohydrates N-glycosylated to the a-amino group of the N-terminal portion of proteins.51-53 Most of these compounds will be discussed in more depth later in this article, in terms of model compounds for oligosaccharide linkages to proteins. 

Dipeptidyl aminopeptidase IV hydrolyzes substrates with free a-amino groups. Peptide bonds involving the carboxy group of either Pro or Ala are cleaved by this enzyme to X-Pro or X-Ala, where X may be any amino acid. It has been shown that peptides with the X-Pro moiety are hydrolyzed more completely than those with X-Ala [79],... 

The first four facets are rotationally equivalent to each other as are the final four. The two sets are related by reflectional symmetry to each other. When a chiral adsorbate, for example, S-lysine, is used, the reflectional symmetry is no longer valid and only rotationally equivalent facets should be formed. This was demonstrated elegantly by Zhao with STM [53], The driving force for facet formation is proposed to be a three-point interaction involving the carboxylate group, the a-amino group, and the amino-terminated side chain. The simultaneous optimization of adsorbate-adsorbate and adsorbate-substrate interactions determines the stereochemistry of the facet. 

Detection powders and fingerprint development kits commonly contain cream- or yellow-colored ninhydrin crystals or a solution of dissolved ninhydrin. Ninhydrin (also known as 1,2,3-indantrione, monohydrate 2,2-dihydroxy-1,3-indandione triketohydrindene, monohydrate and triketohydrinden hydrate) has the structure presented in Fig. 13.3.1. Ninhydrin will react with a free a-amino group, -NH2. This group is contained in all amino acids, and analysis with ninhydrin is often performed to verify the presence of amino acids. When a-amino acids (i.e., amino acids with the structure NH2-CHR-COOH) react with ninhydrin, a characteris-... 

A succinylated casein derivative that has nearly all its amines blocked can be used as a substrate in protease assays (Hatakeyama et al., 1992). As the casein is degraded by a protease, free amines are created from a-chain cleavage and release of a-amino groups. The creation of... 

Kinetic studies of stepwise hydration reactions of aikenes. This work has shown that carbocations with labile jff-CH bond(s) that are stabilized by an a-amino group,35-37 or by two a-thiol groups38 0 undergo preferential deprotonation to form the products of an elimination reaction (kp > ks, Scheme 1). 

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