Tandem mass spectrometry sequencing in the negative ion mode to read binary information encoded in sequence-defined poly(alkoxyamine amide)s
Résumé
RationaleDigitally encoded oligomers composed of two distinct amide coding units spaced by a nitroxide moiety were recently decrypted using a tandem mass spectrometry (MS/MS) sequencing approach developed for protonated oligomers. Here, the MS/MS behavior of deprotonated oligomers was explored in the negative ion mode to provide both structural and mechanistic complementary information. MethodsBinary-encoded oligo(alkoxyamine)amides, containing coding 0/1 amide units spaced by a TEMPO nitroxide moiety, were ionized in negative ion mode electrospray thanks to their end-group containing a carboxylic acid function. Deprotonated molecules were subjected to collision-induced dissociation in MS/MS and MS3 experiments, combined with accurate mass measurements, for a thorough investigation of their dissociation behavior. ResultsDeprotonated oligomers readily dissociated upon collisional activation via competitive homolytic cleavages of all fragile alkoxyamine linkages between any coding 0 or 1 monomers and a nitroxide moiety. As expected, only product ions holding the deprotonated end-group were detected while complementary moieties containing the termination were released as radicals. The so-formed distonic radical anions were observed to further depolymerize according to a radical-induced process, as evidenced by MS3 experiments. ConclusionsMessages encoded in oligo(alkoxyamine)amides were readily decrypted by MS/MS sequencing performed in the negative ion mode. When compared with results obtained in positive ion mode ESI-MS/MS, these data provided further evidence regarding the influence of adducted proton on the charge-remote homolytic cleavage of alkoxyamine linkages. Copyright (c) 2015 John Wiley & Sons, Ltd.