Promoting carboxylate salts in the ESI source to simplify positive mode MS/MS sequencing of acid-terminated encoded polyurethanes

Abstract

Synthetic polymers prepared by solid-phase processes are often released from the solid support by hydrolysis, leading to an acidic α chain-end which can usefully be employed to perform mass spectrometry experiments in the negative ion mode. This is the case for sequence-defined polyurethanes, which exhibit very simple MS/MS pattern as deprotonated species in great contrast to data obtained in the positive ion mode. Indeed, after deprotonation of their acidic end-group, collision induced dissociation (CID) of these polymers proceeds via competitive cleavages of all carbamate bonds. This leads to a unique series of anionic fragments spaced by the mass of one of the other coding co-monomer, enabling their sequence to be readily deciphered. However, as their size increases, polyurethanes are best ionized in the positive mode but their dissociation pattern is far more complicated, with four to five fragmentation routes depending on the adducted cation. This spectral complexity could however be highly reduced when selecting precursor ions that have experienced H/Na (or any other alkali) exchange in their acidic α end-group: in these conditions, only one reaction is observed and yields pairs of complementary products. Mechanisms could be proposed to explain how, although part of an end-group, the acidic proton had a key role in the dissociation processes of polyurethane chains. Exchange of this proton was then further optimized by proper selection of the Xanion in the NaX salt supplemented to the electrosprayed solution. By allowing simplification of MS/MS data while avoiding signal dilution over multiple ion series, CID of these [M -H + zNa] (z-1)+ permitted to envisage reliable decoding of sequence-defined polyurethanes storing large amount of information.

Optimal conditions for tandem mass spectrometric sequencing of information‐containing nitrogen‐substituted polyurethanes

“…In the case of N–H PUs, the carboxylate form of α was promoted by H/Na exchange in solution (Figure S2C, supporting information), but requires the use of NaOH for optimal exchange 17 . While this approach also works for N–R PUs, the exchanged Na + does not always remain in the α carboxylate function, i.e., in the form [(P – H + Na) + 3Na] 3+ , but exhibits a certain mobility to generate, at least in the gas phase, the distonic species [(P – H) + 4Na] 3+ in which the deprotonated oligomer is adducted with four Na + .…”

“…Sequence‐defined N–H PUs were easily ionized as cationic adducts but exhibited complex MS/MS spectra, unless activated as [M – H + (z + 1)Na] z+ precursor ions, where the acidic proton of the α end‐group has been exchanged with an alkali 17 . The high mobility of this proton was indeed demonstrated to assist bond cleavages, yielding up to five series of product ions.…”

“…In addition to re‐engineering structures, fragmentation of synthetic polymers can also be manipulated by appropriate adjustments of experimental parameters in the ionization step. This was recently shown for polyurethanes, with sodium salt supplemented to the electrosprayed solution, permitting their acidic α termination to be changed into a carboxylate moiety and hence simplify their dissociation pattern in positive ion mode 17 …”

Applications of Discrete Synthetic Macromolecules in Life and Materials Science: Recent and Future Trends

“…In this context, Lutz and co‐workers showed that the MS/MS read‐out of acid‐terminated encoded polyurethanes was significantly simplified when a H/Na exchange occurred at the acid‐end, especially for structures with a high DP. [ 66 ] On the other hand, the same group tuned the synthesis of a uniform poly(alkoxyamine phosphodiester) in such a way that the encoded data could not be unambiguously retrieved anymore when making use of MS/MS techniques. [ 67 ] Instead, the data could only be revealed when making use of ion mobility spectrometry measurements, which revealed a key that was necessary to allow actual decryption via MS/MS, opening possibilities in the area of anti‐counterfeiting applications (vide infra).…”

Reading mixtures of uniform sequence-defined macromolecules to increase data storage capacity

“…When synthetic molecules are used for data storage, the successful read-out of the encoded data is the most difficult step 4 . Tandem mass spectrometry seems to be a promising tool in this context and has already been successfully applied several times 4,8,[32][33][34][35] . Also, other analytic methods are discussed for a possible read-out.…”