Cyclization of N-TerminalS-Carbamoylmethylcysteine Causing Loss of 17 Da from Peptides and Extra Peaks in Peptide Maps

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Abstract
Enzymatic digests of proteins S-alkylated with iodoacetamide may contain peptides with N-terminal S-carbamoylmethylcysteine. These can be partly converted to a form with 17 Da lower mass and increased HPLC retention. Proof by synthesis supported by MS/MS and NMR spectroscopy was used to show that N-terminal S-carbamoylmethyl-l-cysteine can cyclize, losing NH3 to form an N-terminal residue of (R)-5-oxoperhydro-1,4-thiazine-3-carboxylic acid. The abbreviation Otc is proposed for the (R)-5-oxoperhydro-1,4-thiazine-3-carbonyl residue. The rate of cyclization is significant in 0.1 M NH4HCO3 at 37 °C, with the half-life of the acyclic form being 10−12 h for several peptides tested. This is similar to the rate at which N-terminal pyroglutamate forms from N-terminal glutamine. Keywords: iodoacetamide • peptide mapping • cyclization • mass spectrometry • S-carbamoylmethylcysteine