Tryptophans, brominated

The contryphan family of cyclic peptides, isolated from various species of cone snails, have the conserved sequence motif H3N" -X COD-WX PWC-NH2, where X is either Gly or absent, O is 4-trtf j--hydroxypro-line, and X is Glu, Asp, or Gin. The contryphans possess a distinctive number of PTMs that include tryptophan bromination, proline hydroxylation, glutamate carboxylation (7-carboxyglutamate), C-terminal amidation, and leucine and tryptophan, l to d, isomerization. ... 

Liu, R. Zhang, P. Gan, T. Cook, J. M. Regiospecific bromination of 3-methylindoles with NBS and its application to the concise synthesis of optically active unusual tryptophans present in marine cyclic peptides. J. Org. Chem. 1997, 62, 7447-7456. 

The bromine water test is somewhat more satisfactory for pure tryptophane than the glyoxylic acid test. Hydrogen sulfide may interfere (owing to sulfur formation) and must be boiled out first. The solution to be tested must be acid with acetic acid. 

Segraves, N. and Crews, P. 2005. Investigation of brominated tryptophan alkaloids from two Thorectidae sponges Thorectandra and Smenospongia. Journal of Natural Products, 68 1484-1488. 

A new method of determining the presence of tyrosine by bromina-tion was introduced by Horace Brown and employed by Adrian Brown and Millar in 1906 for estimating the rate at which tyrosine is split off from proteins by the action of trypsin. This method might be used for the estimation of tyrosine in proteins its non-employment may be due to the fact that tryptophane and also histidine react with bromine and might thus vitiate the result for tyrosine. 

Tryptophane is not obtained in any large amount by the hydrolysis of proteins by acids and is best prepared by the action of trypsin. According to Hopkins and Cole, the protein is digested in alkaline solution by trypsin, until the solution gives a maximal coloration when tested with bromine water the solution is then acidified, boiled and... 

Neuberg and Popowsky, as also Abderhalden and Kempe, have introduced a few alterations in the procedure, such as evaporation in vacuo, and Levene and Rouiller suggested in 1906 that the tryptophane, on account of its proneness to decompose on evaporation of its solution with consequent loss, be estimated colorimetrically the mercury sulphate precipitate is decomposed, and the solution, freed from hydrogen sulphide, is titrated with bromine water in presence of amyl alcohol. Both cystine and tyrosine react with bromine water the latter can, however, be removed, but for the former a correction has to be made. Up to the present no values concerning the amount of tryptophane in various proteins have appeared, and it will be of interest to see if the values so obtained are very much higher than those obtained by crystallisation of the tryptophane. 

Several new brominated tryptamines and tryptophans have been described. The Tasmanian bryozoan Amathia convoluta contains convolutindole A (1342) (1319), and the North Sea bryozoan Flustra foliacea (Fig. 3.19) has afforded deformylflustrabromine B (1343) (1320), 1344, 1345, and deformylflustrabromine... 

Craig AG, Jimenez EC, Dykert J, Nielsen DB, Gulyas J, Abogadie FC, Porter J, Rivier JE, Cruz LJ, Olivera BM, McIntosh JM (1997) A Novel Post-translational Modification Involving Bromination of Tryptophan. J Biol Chem 272 4689... 

Segraves NL, Crews P (2005) Investigation of Brominated Tryptophan Alkaloids from Two Thorectidae Sponges Thorectandra and Smenospongia. J Nat Prod 68 1484... 

The families Catenicellidae and Flustridae produce alkaloids derived from tryptophan. The majority of these contain bromine at carbon 6 of the indole ring, although some are more extensively brominated. The convolutamydines (brominated tryptophan derivates isolated from the genus Amathia in the order Ctenostomata)128 are similarly brominated at carbon 6, suggesting a common biosynthetic pathway or bacterial symbiont. 

Under free-radical conditions (e.g. in CC14) the cyclic tryptophan tautomer 15 undergoes bromination with NBS at the 3a-position, but under electrophilic conditions, aromatic bromination occurs at C-5 as reported by Crich and coworkers (equation 42)61. Their... 

Another large group of brominated tryptophan-derived marine alkaloids are the aplysinopsins and several new examples have been reported. The sponge Hyrtios erecta has furnished 20 and 21[26], while a New Zealand asddian is the source of kottamides A-E, for example A (22) [27] and E (23) [28], The stony coral Tubastraea sp. contains the structurally complex and unprecedented bisindole tubastrindole A (24) [29]. 

Oxidative laetonization. A recently discovered group of mycotoxins known as tryptoquivalineS has a novel spirolactone unit derived from N-acyltryptophans. A typical member of the group, tryptoquivaline G (1), has now been synthesized by Biichi and co-workers and has been shown to be derived from D-tryptophan. A key step involves oxidation of a derivative (2) of L-tryptophan with DMSO and methanesulfonic anhydride (2 equiv.) to 3 and 4. NBS has been used for such a reaction, but it can also effect undesired nuclear bromination. The main product 3 was converted in several steps into tryptoquivaline L, the C,9 epimer of 1. Contra-thermodynamic epimerization (KH, then protonation) converted this isomer into 1 and thus proved the absolute configuration. 

The combination NaBrOa-HBr in a ratio that produces 1 mole of bromine per mole of tryptophan peptide may prove to be useful for studies where only oxidative modification but not cleavage of tryptophan bound in a protein is required. 

Af -2,2-Bis(ethoxycarbonyl)vinyl-protected amino acids are prepared by reaction of commercially available diethyl 2-(ethoxymethylene)malonate (127) with the respective amino acid in methanolic KOH. This rapid reaction is complete within 5 minutes and leads to the potassium salts. Subsequent acidification with 1M HCl yields the amino acid derivative in 75-90% yield.f This intermediate enamine-type N-protection is of particular interest in chemistry to be performed on the carboxy groups of the amino acids such as esterification with alkyl bromides in the presence of a base. Since cleavage of the enamine entity is achieved by treatment with bromine in chloroform at room temperature, it cannot be used for amino acids sensitive to halogenation such as tyrosine, tryptophan, and methionine (Scheme 61). Based on the experience gained with the enamine-type protection the Al-2-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl (Dde) and N-2-(4,4-dimethyl-2,6-dioxocyclohex-ylidene)isovaleryl derivatives were developed as specific side-chain protecting groups (see Section 2.1.2.2.5.2). 

Modification by performic acid oxidation Treatment of proteins with performic acid leads to the oxidation of cysteine and cystine residues to cysteic acid residues (Sanger 1949). Methionine residues are quantitatively converted to the sulfone (Hirs 1956), and tryptophan undergoes oxidative destruction (Toennies and Homiller 1942 Benassi et al. 1965). Other amino acids are not modified, provided that precautions are taken to avoid chlorination (Thompson 1954 Hirs 1956), or bromination (Sanger and Thompson 1963) of tyrosine residues. Cleavage of peptide bonds does not occur on performic acid oxidation at low temperature. 

Succinyl-L-tryptophan-L-tryptophan (STT, 8) and p-bromobenzyl-oxyacetic acid (BBA, 9) also bind to deoxyhemoglobin at several sites. In the case of STT, four different sites are involved. The major portion of binding in the case comes from hydrophobic contacts in which 21 molecules of water are displaced. Each STT is bound to one a. subunit by three hydrogen bonds and several van der Waals interactions (Figure 18). STT binds to the protein with a Kd of 3.0 mM. BBA is held in place by a salt bridge between its carboxylate and Lys-40. There are also van der Waals contacts between its bromine and the aliphatic protion of the side chain of Glu-30 (Figure 19). 

The recently detected FADH2-dependent halogenases are substrate specific. Tryptophan 7-halogenase catalyzes the chlorination and bromination of d- and L-trypto-phan to 7-chloro- or 7-bromotryptophan, respectively1201. This enzyme also accepts a number of other indole derivatives such as tryptamine, indole-3-acetonitrile, 3-me-thylindole and 5-methylindole as substrates (Fig. 16.9-3a) 43L In addition to indoles, aminophenylpyrrole derivatives are also chlorinated by tryptophan 7-halogenase (Fig. 16.9-3b) 43L... 

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