Methionine peptides

Brunelle P, Schoneich C, Rank A. (2006) One-electron oxidation of methionine peptides — Stability of the three-electron S-N(amide) bond. Can J Chem 84 893-904. 

In view of these many precedents it is surprising that the intramolecular displacement of the sulfur function of methionine derivatives has not been utilized for selective cleavage of methionine peptides. The method has only recently been applied to a number of methionine peptides which according to the following scheme (CXX-CXXIII) were first converted to sulfonium salts and then subjected to intramolecular (imino) lactonization and hydrolysis by short heating in water (Lawson et al., 1961). Table XV shows the strong influence of the nature of the added alkyl group on the yield in... 

To determine whether the sulfonium acetates and/or nitrates lead to better yields than the iodides in peptide cleavages such salts of the methionine peptides were prepared and cleaved concurrently with the corresponding iodides. The iodides, acetates, and nitrates with and without heating before analysis gave comparable cleavage yields (Table XVII) (Lawson et al, 1961). 

A marked improvement in the cleavage of methionine peptides was achieved by the use of cyanogen bromide (Gross and Witkop, 1961). Pro-... 

The new selective cleavage of methionine peptide bonds with cyanogen bromide offers more promise of success. There are about twenty methionine residues in 7-globulin and three to four methionines in the 50,000 molecular-weight fraction which has retained the power to bind antigen (Porter, 1957). 

Independently whether partial oxidation of Met residues occurs during the synthesis or whether Met(O) derivatives are purposely used, the sulfoxide has to be quantitatively reduced in the final steps to regenerate the methionine peptides. For this purpose, various reagents were proposed for the reduction in aqueous solution upon final deprotection and in organic solvents generally prior to the deprotection step in the presence or absence of cystine disulfides. 

E. Gross and B. Witkop used this reaction for the selective cleavage of methionine peptides, as illustrated in the formulation ... 

Gross and Witkop found gel filtration over Sephadex G-25 invaluable for the separation of fragments resulting from the cleavage of methionine-peptide bonds in ribonuclease by reaction with cyanogen bromide. In this case 0.2 N acetic acid was used as solvent. 

II] Bobrowski K, Holcman J., Formation and stability of intramolecular three-electron S.. N, S.-.S, and S.. O bonds in one-electron-oxidized simple methionine peptides. Pulse radiolysis study, J. Phys. Chem., 1990,93,6381-6387. 

Purified K-casein is attacked by chymosin at pH 6.7 in the absence of calcium ion to form insoluble para-tc-casein and a soluble macropeptide with the specific cleavage of a phenylalanyl-methionine peptide bond. This serves as an example of how the cleavage of a single peptide bond by an enzyme can drastically alter the functionality of a protein system. This system is discussed in more detail in the subsequent section on functionality of proteins in the milk system. 

The carboxymethylation of horse heart cytochrome c by [2-i C] bromo-acetate has been monitored by NMR spectroscopy (77). In the absence of cyanide ion position 65 of the cytochrome c is the only reactive methionine. Carboxymethylation gives as expected a single resonance at 48.4 ppm (similar to carboxymethylated methionine peptides). In the presence of cyanide ion the methionines at positions 65 and 80 are both reactive and two resonances for the carboxymethylated product were observed 48.4 ppm position 65 48.2 ppm position 80. Upon reduction of the iron and carboxymethylation, the position 80 carboxymethylmethionine moves to 48.4 ppm, suggesting that the carboxymethylmethionine-80 feels the oxidation state of the metal (Fe +-Fe +). At long reaction times further carboxymethylations were detected. 

Methionine peptides. An aq. suspension of L-prolyl-L-tyrosyl-N -tosyl-L-lysyl-L-methionine d,1-sulfoxide treated with thioglycolic acid, and kept 20 hrs. at 50° under Ng with occasional shaking L-prolyl-L-tyrosyl-N -tosyl-L-lysyl-L-methionine. Y 86%.—Side reactions in the synthesis of methionine containing peptides can be eliminated by temporary conversion of the thioether function of methionine into the sulfoxide. F. e. s. B. Iselin, Helv. 44, 61 (1961). 

The use of BNPS-skatole for the selective cleavage of the tryptophanyl peptide bond has been proved useful in sequence studies. The cleavage yields so far obtained (about 50-70%) with peptides (289, 371) and proteins (119) indicate that the reagent is the best so far available for the cleavage of the tryptophanyl peptide bond and approaches in utility the cyanogen bromide cleavage of methionine peptide bonds (375). The reagent has been successfully applied to staphylococcal nuclease... 

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