Homolysis of C[sbnd]ON bonds during MS/MS of oligo(alkoxyamine amide) protomers

Abstract

Information could be stored in the backbone of sequence-defined oligo(alkoxyamine amide)s based on coded amide moieties, and further retrieved by MS/MS sequencing since their collision-induced dissociation only proceeded via competitive homolysis of alkoxyamine linkages. However, when occurring at chain ends, CON bond homolysis did not yield the set of complementary product ions expected from such a charge-remote reaction. The actual role of the adducted proton on the dissociation mechanism was then investigated. Using a set of tri(alkoxyamine amide)s that each contained two CON bonds, theoretical calculations first showed that protonation most favorably occurred at nitroxide moieties or at the central amide group compared to terminal amides. This result explained why the fragment at the left-hand side of the first CON bond was always released as a radical and never detected as a charged product. These calculations also showed that tri(alkoxyamine amide)s would exist as different protomers that could be experimentally evidenced for some of the studied species by traveling wave ion mobility spectrometry (TWIMS). TWIMS-MS/MS experiments were then performed for individual study of the IMS-resolved protomer. So-obtained data suggested that CON bond cleavage did not occur when the nitroxide nitrogen was protonated. However, calculations performed on optimized structures evidenced that, prior to alkoxyamine homolysis, the proton adducted to the nitroxide could be transferred onto the nearby amide oxygen atom, hence permitting to rationalize TWIMS-MS/MS data.