Amino acid general derivatives

A GC analysis of amino acids requires a derivatisation step to increase the volatility of the amino acids. Generally, norleucine and/or norvaline are the internal standards added to the hydrolysate to check the derivatisation yield. According to the experimental method applied, the limits of detection (LOD) vary in the range 10 100 pg for each amino acid. Regarding the chromatographic columns, as most of the derivatives are esters barely polar compounds the most commonly used are fused-silica capillary columns with a low... 

Stereosequences in polymers having the general formula — CHR—X—Y— [e.g., poly-a-amino acids, polymers derived from substituted oxiranes, thi-iranes, aziridines, lactones or lactams, and 1,4 polymers of 1- (or 4) monosub-stituted butadienes] are not affected by the degeneracy phenomena existing in vinyl polymers. Their number is indicated in the first column of Table 2. 

Shift differences range from 6 = 0.01 to 0.09, easily distinguishable with a 300 MHz NMR and frequently with a 90 MHz instrument. In all cases, the chemical shift of the DBTA derivative of the. S -enantiomer of the substrate is downfield relative to that of the / -enantiomer. Absolute configuration may be assigned to substrates. Note that in some cases the center of stereogenic-ity may be quite a distance from those of the reagent. HPLC separation of the derivatives is possible (normal phase) DBTA derivatives of D-amino acids generally elute first. 

A. General description Bivalirudin, a 20 amino-acid synthetic peptide, is a direct thrombin inhibitor. It is an analogue of recombinant hirudin (Refludan), a 65 amino-acid anticoagulant derived from the leech. The molecular weight of bivalirudin is about 2.2kDa (anhydrous free base peptide). 

P,y-Unsaturated a-amino esters. y-Phenylseleno-a, (3-unsaturated esters on treatment with NCS (3 equiv.), N,N-diisopropylethylamine (6 equiv.), and an alkyl carbamate (3 equiv.) in methanol at 25° rearrange to derivatives of p,y-unsaturated a-amino acids, generally in 60-80% yield.2... 

Some complexes which satisfy this definition are dealt with elsewhere, however. Those with X or Y as the Group V donors P, As or Sb are dealt with in Chapter 14 of this volume. Those with X or Y = Hg are described in Chapter 11 and the Group IV donors Si, Ge, Sn and Pb are included in Chapter 12.2, With very few exceptions this means that the relevant X,Y combinations are pairs derived from nitrogen, oxygen, sulfur or selenium donor groups. However, some specific examples of these combinations also occur elsewhere in this work. Amino acids (generally N—O bidentates) are covered in Chapter 20.2, Schiff base ligands (N—O or N—S) in Chapter 20.1, and several S—O... 

Thin-layer chromatography of amino acid-phenylthiohydantoin derivatives on silica gel plates, (a) Separation is done in a 98 2 mixture of chloroform and ethanol. (b) This is followed by further separation using an 88 2 10 mixture of chloroform, ethanol, and methanol. More sophisticated procedures, using column chromatography, give superior resolution and improved sensitivity. Automated sequencers always use such procedures. A general description of the use of columns is given in chapter 6. 

General formulae of natural peptides (a) and of the unnatural p- and y-peptides consisting of homologated a-amino acids are shown in Figure 5, in which an outline of the preparation of p- and y- amino acids is also given. As can be seen, they are available by classical organic name reactions. The p - [50] and the y -amino acid [51] derivatives are... 

Contrary to the case of free a-amino acids, deamination-substitution of esters of a-amino acids generally proceeds with racemization with excess inversion about the a-carbon atom. However, reaction of the ethyl ester of phenylalanine and its derivatives with sodium nitrite in trifluoroacetic acid affords substitution products with retention of configuration and migration products with inversion of configuration. This result may be explained by assuming initial formation of the phenonium intermediate (2 Scheme 6). ... 

Finally, several of the amino acids listed have partial lability during acid hydrolysis and require appropriate corrections, usually derived from kinetic analyses (Smith and Stockell, 1954 Wilcox et al, 1957). One amino acid generally considered to be extremely labile to acid is tryptophan. However, its destruction is actually a function of other contaminants present in the sample (Olcott and Fraenkel-Conrat, 1947). Pure samples of this amino acid are fully stable to the standard conditions of acid hydrolysis. Nonetheless, the observed lability is encountered sufficiently often that the preferred determination of this amino acid is carried out by spectrophotometric analysis (Edelhoch, 1967) or colorimetric methods (Koshland et a/., 1964 Scoffone et al, 1968) on unhydrolyzed samples or by carefully controlled alkaline hydrolysis (Hugh and Moore, 1972) (vide infra). 

Acyclic chiral a-amino acids or derivatives are also effleient eatalysts for the asymmetric two- or three-components Mannich reaction. Protected or free 2-hydroxyketones reacted with 16 in the presence of L-tiyptophan, O-protected-L-threonine or L-serine and generally led to the a ft-adduct 17, explained by the formation of a (Z)-enamine stabilised by an intramolecular hydrogen bond (Scheme 12.5). Threonine surfactant 14 was equally effective for the three-component reaction with hydro gracetone, benzalde-hyde derivatives and anilines. ... 

Nonprotein amino acids usually occur free in plants, or as their 4-glutamyl derivatives. These amino acids generally are extracted with ethanol or with alcohol-water mixtures (Rosenthal, 1982, 1991). Most amino acids can be detected by the characteristic reaction with ninhydrin, which produces blue to purple colored chromophores (Rosenthal, 1991). Most of the early work with nonprotein amino acids was done using paper-chromatographic and thin-layer techniques... 

Many labeling systems have been used for dalbaheptide stmctures. The one used herein, where each of the seven amino acids is identified by a number (see Table 2) and each atom by a letter, is widely appHed because it permits easy comparison of and nmr data (31). The lUPAC system, utilised in Chemicaly hstracts and generally in the description of semisynthetic derivatives, requires decodification for comparison of different dalbaheptides (83). 

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