SIDE REACTION OF S-TO-N ACETAMIDOMETHYL SHIFT DURING DISULFIDE BOND FORMATION BY IODINE OXIDATION OF S-ACETA-MIDOMETHYL-CYSTEINE IN A GLUTAMINE-CONTAINING PEPTIDE

During the time course of disulfide bond formation by iodine oxidation (in a methanolic and hydrochloric acid solution) of a cysteinyl(S-ace tamidomethyl)-glutaminyl tridecapeptide, we observed by ESI, FAB mass spectrometry (pseudo-molecular ion and ion-fragments) and H-1-NMR a si de reaction due to a shift of the Acm leaving group from cysteine to t he carboxamide side chain of glutamine. This type of Acm-shift at low level was described previously by L. W. Mendelson et al. (Int. J. Pept . Protein Res. 35:249-257) for an aspariginyl-cysteinyl(S-acetamidomet hyl) peptide in an anhydrous hydrochloric solution. We report here the efficiency of glutamine as a scavenger to suppress the S-->N shift of the acetamidomethyl group daring S-acetamidomethyl cleavage and sulfh ydryl oxidation with iodine, as the folded tridecapeptide was obtained with the expected molecular weight.