Optimal conditions for tandem mass spectrometric sequencing of information-containing nitrogen-substituted...

To prevent solubility issues faced with sequence-defined polyurethanes, a new family of digital polyurethanes was conceived with the alkyl coding chain held by the carbamate nitrogen (N) atom and CH₃ instead of OH as the ϖ termination. This led to different dissociation mechanisms that were explored prior to optimizing tandem mass spectrometric (MS/MS) sequencing.

N-Substituted polyurethanes (N-R PUs) were dissolved in methanol and subjected to collision-induced dissociation (CID) as deprotonated chains in the negative ion mode, and as ammonium and sodium adducts in the positive ion mode, using electrospray ionization (ESI) as the ionization technique. Their dissociation behavior was thoroughly investigated using a quadrupole time-of-flight (QTOF) instrument in order to provide accurate mass measurements to support proposed fragmentation mechanisms.

While O-(CO) bonds were broken in N-H PUs, the CH₂ -O linkage between repeating units was cleaved upon CID of N-R PUs. This main process occurred either from deprotonated molecules or from cationized chains but was followed by different rearrangements, producing up to four product ion series. Yet, MS/MS spectra could be drastically simplified for precursor ions having their acidic α group methylated, as was found to spontaneously occur upon storage in methanol.

Using experimental conditions aimed at avoiding any reactive proton in precursor ions (no acidic end-groups and alkali adduction), full coverage sequence of N-R PUs was successfully achieved with the single ion series observed in MS/MS, opening a promising perspective for reading large amounts of information stored in these dyad-encoded polymers.