Tryptophan reactions

Mellet, P. O., Noel, P. R., and Goutefongea, R. (1986). Nitrite-tryptophan reaction Evidence for an equilibrium between tryptophan and its nitrosated form.7. Agric. Food Chem. 34, 892-895. 

Cohen66 introduced the tryptophan reaction for the detection and estimation of desoxyribonucleic acids. Since the reaction is given equally well by 2-desoxyribose, the test obviously depends on this component of the nucleic acid. It is likely that this test is given generally by 2-desoxypentoses. In the recommended procedure the suspected nucleic acid is heated at 100° for 10 minutes with tryptophan and 30 percent (final concentration) perchloric acid, and in a positive test there is a rapid development of a red color. Quantitative estimations can be carried out... 

In the tryptophan reaction, aldehydes and fructose derivatives are among the most important interfering substances. Proteins interfere with the test, but Cohen66 eliminated complications due to such interference, and hence increased the value of the test, by developing a method for extracting the colored reaction-product of desoxyribose and tryptophan without simultaneous extraction of other reaction-products. 

Cohen66 has forwarded a hypothesis to describe the observed effects in this test. Any hypothesis must account for the specificity of the reaction for desoxyribose as contrasted to ribose, the roles of tryptophan and perchloric acid in the reaction, and the shift in absorption maximum observed between the desoxyribose and furfural condensates. (This latter observation indicates that the tryptophan reaction is not due t,o... 

Tryptophane reactions. There are several tryptophane reactions one of these consists in the development of a violet color when a solution of protein which contains tryptophane is treated with glacial acetic acid and concentrated sulfuric acid. It is explained by the fact that the ordinary acetic acid contains glyoxylic acid, HOC—COOH, in small amounts, and this aldehyde-acid reacts with the indol group of tryptophane. The same color is given by glyoxylic acid. Proteins which do not yield tryptophane do not give this color. 

Tryptophane Reactions. To 2 ml of protein solution add 2 ml glacial acetic acid, and mix. Add this mixture very carefully to 5 ml of concentrated sulfuric acid, pouring it slowly down the side of the tube so that the two liquids do not mix. If the proteins contain tryptophane, a violet ring will form at the interface of the two liquids. In the same manner test solutions of gelatin, casein, and egg albumen. 

Total globulin may be determined directly by dye binding, by a tryptophan reaction, and by turbidimetry. 

Saifer, A., Gerstenfeld, S., and Vecsler, F., Photometric microdetermination of total serum globulins by means of a tryptophan reaction. Clin. Chem. 7, 626-636 (1961). 

Figure 5. Conversion of the ferric enzyme to cyanide complex in the presence and absence of tryptophan. Reaction mixture contained tryptophan pyrrolase, 0.47 mg. (specific activity 2.5) potassium phosphate buffer, pH 7.0, 1.1. mmoles i.-tryptophan, 0.28 mmoles in a jfinal volume of 28 ml. Cyanide was added in the presence (O) and absence ( X) of tryptophan as indicated...

Radical cations formed from OH + tryptophane reaction. 

Interestingly, G jrey et al.", employing a similar tryptophan-derived catalyst (3.4), observed a 99% enantiomeric excess (ee) in the Diels-Alder reaction of 2-bromoacrolein with cyclopentadiene... 

In contrast, investigation of the effect of ligands on the endo-exo selectivity of the Diels-Alder reaction of 3.8c with 3.9 demonstrated that this selectivity is not significantly influenced by the presence of ligands. The effects of ethylenediamine, 2,2 -bipyridine, 1,10-phenanthroline, glycine, L-tryptophan and L-abrine have been studied. The endo-exo ratio observed for the copper(II)-catalysed reaction in the presence of these ligands never deviated more than 2% from the endo-exo ratio of 93-7 obtained for catalysis by copper aquo ion. 

First, the pH-dependence of the enantioselectivity of the reaction between 3.8c and 3.9 catalysed by the copper(L-tryptophan) complex has been studied. Above pH 5 the enantioselectivity reaches a plateau value (Figure 3.3). The diminished enantioselectivities observed at lower pH most likely... 

Table 3.3. Influence of temperature and ethanol content on the enantiomeric excess of the Diels-Alder reaction between 3.8c and 3.9 catalysed by [Cu(L-tryptophan)] in aqueous...

Likewise, the influence of the ligand catalyst ratio has been investigated. Increase of this ratio up to 1.75 1 resulted in a slight improvement of the enantioselectivity of the copper(L-tryptophan)-catalysed Diels-Alder reaction. Interestingly, reducing the ligand catalyst ratio from 1 1 to 0.5 1 resulted in a drop of the enantiomeric excess from 25 to 18 % instead of the expected 12.5 %. Hence, as anticipated, ligand accelerated catalysis is operative. 

Finally the influence of the temperature and addition of ethanol on the enantioselectivity of the Diels-Alder reaction was studied. Table 3.3 summarises the results for different aqueous media. Apparently, changes in temperature as well as the presence of varying amounts of ethanol have only a modest influence on the selectivity of the Cu(tryptophan)-catalysed Diels-Alder reaction in aqueous solution. However, reaction times tend to increase significantly at lower temperatures. Also increasing the alcohol content induces an increase of the reaction times. 

Pyrrole derivatives are prepared by the coupling and annulation of o-iodoa-nilines with internal alkynes[291]. The 4-amino-5-iodopyrimidine 428 reacts with the TMS-substituted propargyl alcohol 429 to form the heterocondensed pyrrole 430, and the TMS is removed[292]. Similarly, the tryptophane 434 is obtained by the reaction of o-iodoaniline (431) with the internal alkyne 432 and deprotection of the coupled product 433(293]. As an alternative method, the 2,3-disubstituted indole 436 is obtained directly by the coupling of the o-alky-nyltrifluoroacetanilide 435 with aryl and alkenyl halides or triflates(294]. 

Chapters 9, 10 and 11 describe methods for substitution directly on the ring with successive attention to Nl, C2 and C3. Chapters 12 and 13 are devoted to substituent modification as C3. Chapter 12 is a general discussion of these methods, while Chapter 13 covers the important special cases of the synthesis of 2-aminoethyl (tryptaminc) and 2-aminopropanoic acid (tryptophan) side-chains. Chapter 14 deals with methods for effecting carbo cyclic substitution. Chapter 15 describes synthetically important oxidation and reduction reactions which are characteristic of indoles. Chapter 16 illustrates methods for elaboration of indoles via cycloaddition reactions. 

Lewis acids such as zinc triflate[16] and BF3[17] have been used to effect the reaction of indole with jV-proiected aziridine-2-carboxylate esters. These alkylations by aziridines constitute a potential method for the enantioselective introduction of tryptophan side-chains in a single step. (See Chapter 13 for other methods of synthesis of tryptophans.)... 

The best procedures for 3-vinylation or 3-arylation of the indole ring involve palladium intermediates. Vinylations can be done by Heck reactions starting with 3-halo or 3-sulfonyloxyindoles. Under the standard conditions the active catalyst is a Pd(0) species which reacts with the indole by oxidative addition. A major con.sideration is the stability of the 3-halo or 3-sulfonyloxyindoles and usually an EW substituent is required on nitrogen. The range of alkenes which have been used successfully is quite broad and includes examples with both ER and EW substituents. Examples are given in Table 11.3. An alkene which has received special attention is methyl a-acetamidoacrylate which is useful for introduction of the tryptophan side-chain. This reaction will be discussed further in Chapter 13. 

One effective method for synthesis of tryptophan derivatives involves alkylation of formamido- or acetamido- malonate diesters by gramine[l,2]. Conversion to tryptophans is completed by hydrolysis and decarboxylation. These reactions were discussed in Chapter 12. An enolate of an a-nitro ester is an alternative nucleophile. The products can be converted to tryptophans by rcduction[3,4],... 

Tryptophans can also be prepared by reduction of a,(3-dehydrotryptophans. These can be obtained by a classical azlactone type synthesis from derivatives of indole-3-carboxaldehyde. These reactions usually rquire an iV-EW substituent and the yields are modest[15]. 

Standard Heck conditions were used to introduce the dchydroalanine side-chain with 4-bromo-3-iodo-l-(4-methylphenylsulfonyl)indole[12]. Using 4-fluoro-3-iodo-l-(4-methylphenylsulfonyl)indole as the reactant, Merlic and Semmelhack found that addition of 2 eq, of LiCl or KCl improved yields in reactions carried out with 10% Pd/C as the catalyst[13]. The addition of the dehyroalanine side chain can also be done by stoichiometric Pd-mediated vinylation (see Section 11.2). A series of C-subslituled dehydro tryptophans was prepared in 40-60% yield by this method[14]. 

Enzymatic Process. Chemically synthesized substrates can be converted to the corresponding amino acids by the catalytic action of an enzyme or the microbial cells as an enzyme source, t - Alanine production from L-aspartic acid, L-aspartic acid production from fumaric acid, L-cysteine production from DL-2-aminothiazoline-4-catboxyhc acid, D-phenylglycine (and D-/> -hydtoxyphenylglycine) production from DL-phenyUiydantoin (and DL-/)-hydroxyphenylhydantoin), and L-tryptophan production from indole and DL-serine have been in operation as commercial processes. Some of the other processes shown in Table 10 are at a technical level high enough to be useful for commercial production (24). Representative chemical reactions used ia the enzymatic process are shown ia Figure 6. 

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