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 amide, from the linear peptide Cys(Acm)3,23,Ser17 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.