Side reaction during the deprotection of (S-acetamidomethyl)cysteine in a peptide with a high serine and threonine content.
The acetamidomethyl (Acm) group is a widely used protecting group for the thiol of cysteine during the SPPS process. We prepared the amino terminal loop of the snake alpha-neurotoxin, [Cys3,Cys23, Ser17](1-24) amide, from the linear peptide [Cys(Acm)3,23,Ser17](1-24) amide obtained by SPPS. Three different methods of deprotection of Cys(Acm) and disulfide bond formation were used: iodine, thallium(III) trifluoroacetate and mercuric acetate/potassium ferricyanide. The iodine method failed to yield the expected peptide, and gave instead the mono-iodinated tyrosine analog. The disulfide cyclized peptide obtained by thallium (III) or Hg(II) procedures displayed a MW value observed by mass spectrometry that was higher than the calculated value. The difference (MWobs-MWcalc) corresponded to a multiple of the Acm moiety, which is shifted intra- and/or intermoleculary. Furthermore, we observed, in addition to the Acm shift in the disulfide cyclized decapeptide with a highSer and Thr content (model peptide II), the dimerization phenomenon in the Tl(TFA)3 process. Therefore we conclude that a side reaction, a S--O(Ser,Thr) Acm shift, occurred during the Cys(Acm) deprotection. This shift was supported by the demonstration of Ser(O-Acm) formation in the reaction of Boc-(L)-Cys(Acm) with Tl(TFA)3 in the presence of an equimolar amount of (L)Ser. We report here the efficiency of a trivalent alcohol, glycerol, as scavenger in the both Tl(TFA)3 and mercuric/ferricyanide methods, in an attempt to circumvent this side-reaction during the disulfide bond formation step starting from a bis-Cys(Acm) peptide with a high Ser and Thr content, such as the N-terminal loop of neurotoxin, model peptide II or a similar peptide.
Lamthanh
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Department of Engineering and Protein Research, CE Saclay, Gif sur Yvette, France.
Roumestand
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Deprun
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Ménez
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D000111:Acetylcysteine / Q000031:analogs & derivatives* / Q000737:chemistry; D000595:Amino Acid Sequence; D000818:Animals; D002625:Chemistry, Organic / Q000379:methods; D003039:Cobra Neurotoxin Proteins / Q000737:chemistry*; D003553:Cystine / Q000737:chemistry*; D005292:Ferricyanides / Q000737:chemistry; D005463:Fluoroacetates; D007455:Iodine / Q000737:chemistry; D008628:Mercury / Q000737:chemistry; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry*; D012694:Serine / Q000737:chemistry*; D012911:Snakes; D013793:Thallium; D013912:Threonine / Q000737:chemistry; D014269:Trifluoroacetic Acid / Q000737:chemistry
D003039:Cobra Neurotoxin Proteins; D005292:Ferricyanides; D005463:Fluoroacetates; D010446:Peptide Fragments; C079739:alpha-cobratoxin (1-24) amide; C079740:(S-acetamidomethyl)cysteine; C054051:thallium trifluoroacetate; D013912:Threonine; D012694:Serine; D003553:Cystine; D007455:Iodine; D013793:Thallium; D014269:Trifluoroacetic Acid; D008628:Mercury; C029823:mercuric acetate; C028033:potassium ferricyanide; D000111:Acetylcysteine