Dehydroalanine peptides

Selective cleavage of the polypeptide chain at the thiocyanatoalanine residues may be achieved by incubation at alkaline pH, e.g. pH 9.0 for 24 hr at 37°C. Under these conditions some conversion to an uncleaved dehydroalanine peptide also occurs. 

Patchornik and coworkers studied the elimination reactions of S-2,4-dinitrophenyl-cysteine compounds 390, 391). The method proved suitable for the conversion of cysteine peptides into dehydroalanine peptides, in which the double bond can then be exploited as the site of attack in the specific (hydrolytic or oxidative) cleavage of the peptide 298—300). 

Srinivasan, A., R. W. Stephenson, and R. K. Olsen Synthesis of Dehydroalanine Peptides from P-Chloroalanine Peptide Derivatives. J. Org. Chem. 42, 2253 (1977). 

A number of nonprotein amino acids with unsaturated side chains have been isolated. Many of these contain alkene side chains, but some alkyne side chains containing amino acids have also been identified. Nonprotein dehydroamino acids do not have an a-stereocenter these amino acids are still classified under this category. Dehydroamino acids are generally biosynthesized by the enzymatic elimination of a leaving group at the /3-carbon. For example, serine and threonine are enzymatically dehydrated to give dehydroalanine and dehydrobutyrine, respectively. A similar biosynthetic pathway is hypothesized for dehydroamino acids found in nonribosomal peptides, such as nodularins and microcystins. ... 

The cosmopolitan cyanobacterium Microcystis aeruginosa is frequently the major component of freshwater cyanobacterial blooms. These blooms can cause serious water management problems and are occasionally associated with animal poisoning. The aeruginosa toxins are potent lethal peptides which contain three invariant D-amino acids (Ala, erythro-3-methyl Asp, and Glu), two variant L-amino acids, N-methyl dehydroalanine, and a 3 amino acid (1-3). Multiple toxins have been purified from clonal isolates (1,4). The toxic peptide described in this chapter is denoted toxin-LR using the standard one-letter abbreviations for its two variant amino acids, leucine and arginine. 

Lantibiotics. Certain antimicrobial peptides have been identified which contain "unusual" amino acids such as lanthionine, /3-methyllanthionine, dehydroalanine, and jS-methyldehydroalanine. Due to the predominance of lanthionine they have been collectively referred to as "lantibiotics" (26). Among the lactic acid bacteria, two bacteriocins have been identified as lantibiotics, nisin and lacticin 481. Nisin, the first... 

As a consequence of dehydroalanine and B-methyldehydroalanine formation specific bond cleavage can occur. Ebert et al. (56) have shown that addition of cysteine to the double bonds of polydehydroalanine and copolymers of dehydroalanine results in increased solubility and decrease in molecular weight because of peptide bond cleavage caused by formation of a thiazolidine. 

This reaction can be used for selective peptide chain cleavage of cysteine-containing polypeptides and proteins under rather mild conditions. Mild acid treatment of dehydroalanine-containing polypeptides and proteins leads to specific peptide bond cleavage with formation of pyruvate and ammonia (57-59)). 

Lantibiotics contain several unusual amino acids, including the thioether lanthionine (Lan) hnkage and its methyl substituted analog methyUanthionine (MeLan) (Fig. la) that unifies all members of the class and accounts for their family name. In addition to Lan, lantibiotics commonly contain 2,3-dehydroalanine (Dha) and (Z)-2,3-dehydrobutyrine (Dhb). In all, no less than 15 different posttranslational modifications have been documented in lantibiotics (for a selection see Fig. la), and up to 58% of their amino acids are modified. These extensive structmal alterations overcome the constraints imposed by the use of 20 amino acids in ribosomally synthesized peptides. Some less common, posttranslationally crafted residues in lantibiotics are fS-hydroxy aspartate, lysinoalanine, aminovinyl cysteine (AviCys), D-alanine, 2-oxobutyrate, 2-oxopropionate, and 2-hydroxypropionate. The presence of these unusual residues is thought to be important for the biological activity of lantibiotics. 

Figure 3 Different approaches for the introduction of lipid functionalities, here exemplified via the farnesyl group, into peptides. (A) Lipidated amino acid building blocks. (B) Substitution of bromoalanine with a nucleophile. (C) Alkylation or acylation of a free thiol functionality of a cysteine. (D) Conjugate addition of a nucleophile (e.g., farnesylthiolate) to a dehydroalanine. (E) Conjugate addition of a nucleophile to aziridine-2-carboxylic acid containing...

Zhu Y, van der Donk WA. Convergent synthesis of peptide conjugates using dehydroalanines for chemoselective ligations. Org. Lett. 2001 3 1189-1192. 

The mechanism of disulfide reduction by phosphines is hypothesized to involve a stable intermediate containing a sulphur-phosphorous bond (6). Beta elimination would yield the phosphine sulfide and dehydroalanine. The formation of relatively stable adducts between cystine-containing peptides and the reagent was confirmed by mass spectrometry for several peptides with the major adduct representing one reagent molecule per cystine residue. 

The formation of dehydroalanine would also increase the likelihood of lanthionine formation which would be resistant to peptide bond cleavage. It is likely that dehydroalanine formation itself is responsible for the final peptide bond cleavage through migration of the double bond and subsequent hydrolysis. 

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