Tryptophan selective cleavage

Another commonly used enzyme for selective cleavage of peptides is chymotrypsin, which hydrolyzes peptide bonds on the carboxyl side of aromatic residues (phenylalanine, tyrosine, and tryptophan) ... 

Recently, 2-(2-nitrophenylsulfenyI)-3-methyl-3-bromoindolenine (BNPS-skatole) has replaced N-bromosuccinimide, which had been used frequently in earlier studies. At low reagent to protein tryptophan ratios, in 50% aqueous acetic acid, BNPS-skatole reacts selectively with tryptophan residues converting these to the oxindole derivative. Methionine is concommitantly converted to the sulfoxide. At high concentrations of reagent, slow selective cleavage (to the extent of 15-60%) of the peptide bonds involving tryptophanyl residues is... 

The concept of naked anions, i.e. anions solubilized in non-solvating media by the crown complexation of their counter-cations, has been applied further this year. Naked fluoride ion has been used as a catalyst for Michael additions,such as the cyanoethylation of active methylene compounds, and also as a base to mediate the acylation (and protection) of the indole nitrogen of tryptophan in peptides. The selective cleavage of protected amino-acids from oxyacyl resins [equation (18)] is a... 

D.xi.a. Hydrolysis and Hydration. The electrophilicity of cationic palladium(II) complexes has been taken advantage of in an interesting biomimetic method of selective cleavage of peptide bonds at histidine, methionine, or tryptophane residues. Further discrimination is possible as peptides with two former amino acids are cleaved in aqueous solution but not cleaved in acetone, while those with tryptophane are cleaved only in acetone in the presence of water. The mechanism of cleavage at tryptophane is particularly interesting as it involves the electrophilic ip o-paUadation of indole ring, while the remaining coordination site of Pd in the ip/ro-cyclopalladated complex is used for the electrophilic activation of carbonyl (Scheme 91). 

Conditions for the selective cleavage of the kyneurenyl bond in oxidized hen egg-white lysozyme in aqueous hydrazine solution have been established. Further derivatization and peptide analysis showed that the iV -formylky-nurenine is present at position 62 in place of L-tryptophan in the native protein. 

The reaction is used for the selective cleavage of peptide chains and the spectrophotometric determination of tryptophan. 

Selective hydrolysis of esters is a well-established procedure for the resolution of chiral carboxylic acids. Enzymes such as hydrolases, lipases and proteases are utilized. Due to their high selectivity for (5)-amino acids, proteases have been widely used in the selective transformations of amino acids and their derivatives. a-Chymotrypsin- a serine protease -catalyzes not only the hydrolysis of amide bonds, but also the cleavage of various esters, including a-alkyl-a-amino acid esters. One application is the synthesis of (5)-a-[ C]-methyltryptophan The anion synthesized by LDA-deprotonation of M-benzylidene tryptophan methyl ester (T) was alkylated with [ CJmethyl iodide to obtain the methyl A-benzylidene derivative 2, which was hydrolyzed under acidic conditions. Subsequent selective cleavage of the ester group with a-chymotrypsin provided the enantiomerically pure (5)-amino acid 3 in 33% radiochemical yield. 

In the reaction with Boc20/DMAP, A"-Boc-tryptophan methyl ester affords the A/ ,A -di-Boc derivative in almost quantitative yield. This product has been used in peptide synthesis. Moreover, selective cleavage of the A"-Boc group is possible in the presence of the A "-Boc function by using 2.7 N HCl in dioxane. ... 

As anticipated from the action of NBS on tryptophan peptides (295), BNPS-skatole was proved useful also for selective cleavage of tryptophanyl peptide bonds (289). With a variety of tryptophan containing peptides, 10-fold excess of reagent in 20 hours gave cleavage yields (30-59%) comparable to those obtained with NBS. However, BNPS-skatole is a selective reagent and does not cleave at sites other than tryptophanyl peptide bonds (119, 289). 

Tyrosyl and hystidyl peptide bonds, which are usually cleaved by other brominating agents such as NBS, are sensitive to oxidation also but are not cleaved by TBC. Tyrosine was converted by TBC to 3,5-dibromotyrosine, cysteine to cysteic acid, methionine to methionine sulfoxide and tryptophan most probably converted to a bromooxindole derivative. Optimal conditions for the cleavage of the tryptophanyl peptide bond were found to be 3 equiv. of TBC at pH 3 for 5-15 min at room temperature. The model peptide N-carbobenzoxy-tryptophanyl-glycine was cleaved in about 50% yield NBS also cleaved this peptide to the same extent. In lysozyme, selective cleavage of the expected tryptophanyl peptide bonds (yields 5-60%) was obtained. 

Chymotrypsin (Section 27 10) A digestive enzyme that cat alyzes the hydrolysis of proteins Chymotrypsin selectively catalyzes the cleavage of the peptide bond between the car boxyl group of phenylalanine tyrosine or tryptophan and some other ammo acid... 

Chemical cleavage at selective amino acid residues is an alternative method for scission of peptide bonds in target molecules. Cleavage at the Met-Xaa bond is achieved by CNBr, at the Trp-Xaa bond by the tryptophan-directed reagent 3-bromo-3-methyl-2-[(2-nitro-phenyl)sulfanyl]-3//-indole (BNPS-skatole) and at the Asp-Xaa bond by 2% formic acid.123 24 All these reactions are carried out at acidic pH under standard conditions for each reagent. 

Racemic haptens have the advantage of synthetic expedience, but they do not always yield (R)- and (S)-specific catalysts in a single experiment. The eighteen catalytic antibodies obtained with the tripeptide phosphonate 6,14 for example, only catalyzed the hydrolysis of substrates (7) containing D-phenylalanine at the cleavage site. Depsipeptides containing leucine or tryptophan in place of phenylalanine were not hydrolyzed. The selectivity for d- over... 

The selectivity of these tryptophan cleavages in the albumins amounts to splitting of one or two peptide bonds among about 560. 

The oxidation of trypsin and trypsinogen was carried out in aqueous 0.1 M acetate buffer solutions at room temperature. In this particular case and under these conditions no significant cleavage of peptide bonds next to tryptophan residues occurred. Careful analysis of hydrolyzates of NBS-oxidized trypsinogen and trypsin confirmed the selectivity of the oxidative modification of the protein, as Table XXIV shows. There is no significant loss of tyrosine, histidine, serine, threonine, or cystine, although all of these amino acids will react with NBS but considerably less rapidly than tryptophan. 

L-Tryptophan is degraded via oxidative cleavage of the C-2,C-3 bond, to give A-formyl-kynurenine, as shown in Figure 25. Tryptophan 2,3-dioxygenase (TDO), found in mammals and bacteria, is selective for cleavage of tryptophan, whereas indoleamine 2,3-dioxygenase (IDO), found only in mammals, is able to cleave other... 

Of course the tyrosine side-chain is not inert toward N-bromosuccinimide and to some extent also histidine residues are affected. A more selective reagent for the cleavage at tryptophan was sought and found in the skatol derivative... 

A cyclizative cleavage approach has been used to efficiently and selectively release final products from solid-supported multistep reactions. This approach eliminates side products produced by incomplete reactions as they do not possess the chemical functionalities in a proper arrangement to undergo the cyclizative cleavage reaction. This has been demonstrated in the synthesis of small molecule libraries (Scheme 6.38). A collection of p-carbolines were prepared on the solid support via immobilization of tryptophan 157 on a... 

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