Chain-end and backbone analysis of poly(N-isopropylacrylamide)s using sequential electron transfer dissociation and collisionally activated dissociation

Abstract

Electrospray ionization multistage mass spectrometry (ESI–MSn) was employed for the structural characterization of poly(N-isopropylacrylamide) (pNIPAM) compounds, a widely used class of thermoresponsive materials. Experiments were performed on singly and doubly sodiated pNIPAMs terminated with thiopropionic acid or thiopropyl triethoxysilane substituents, using collisionally activated dissociation (CAD) and electron transfer dissociation (ETD) techniques. Single stages of CAD or ETD only caused small neutral losses diagnostic of the side chains and end groups. A more thorough structural characterization of the pNIPAMs was possible by applying a sequential stage of CAD on fragments generated by ETD, as no surviving charge-reduced precursor was present in the ETD spectra. The ETD fragments were selected such that they preserved differentiation of the two chain ends and contained features that led to random backbone cleavages. ETD fragments with an unpaired electron or with a particularly stable charge site fulfilled this requirement, giving rise to backbone fragments that made it possible to characterize the chain connectivity in addition to the end group and side chain structures. Whether a radical ion or an ion with a well stabilized charge site is used to obtain structurally diagnostic fragments by ETD-CAD depends on the end groups, the functional group being reduced in the ETD step and the structure of the ETD fragment chosen for consecutive CAD.