Polymer Topology Revealed by Ion Mobility Coupled with Mass Spectrometry Denis Morsa, † Thomas Defize, ‡ Dominique Dehareng, § Christine Je ́ rôme, ‡ and Edwin De Pauw* ,† † Mass Spectrometry Laboratory, University of Liege, B6c Sart-Tilman, B-4000 Liege, Belgium ‡ Center for Education and Research on Macromolecules, University of Liege, B6a Sart-Tilman, B-4000 Liege, Belgium § Center for Protein Engineering, University of Liege, B6a Sart-Tilman, B-4000 Liege, Belgium * S Supporting Information ABSTRACT: Hyperbranched and star shaped polymers have raised tremendous interest because of their unusual structural and photochemical properties, which provide them potent applications in various domains, namely in the biomedical field. In this context, the development of adequate tools aiming to probe particular three-dimensional features of such polymers is of crucial importance. In this present work, ion mobility coupled with mass spectrometry was used to experimentally derive structural information related to cationized linear and star shaped poly-ε- caprolactones as a function of their charge state and chain length. Two major conformations were observed and identified using theoretical modeling: (1) near spherical conformations whose sizes are invariant with the polymer topology for long and lightly charged chains and (2) elongated conformations whose sizes vary with the polymer topology for short and highly charged chains. These conformations were further confirmed by collisional activation experiments based on the ejection thresholds of the coordinated cations that vary according to the elongation amplitude of the polymer chains. Finally, a comparison between solution and gas-phase conformations highlights a compaction of the structure with a loss of specific chain arrangements during the ionization and desolvation steps of the electrospray process, fueling the long-time debated question related to the preservation of the analyte structure during the transfer into the mass spectrometer. B ranched polymers such as dendrimers, hyperbranched or star shaped polymers raise a tremendous interest in macromolecular chemistry because of their attractive proper- ties. 1,2 These polymers exhibit not only interesting rheological and mechanical properties but also a lower melt viscosity than their linear counterparts, allowing processing at a lower temperature. 3 This point is of particular interest for low- stability polymers such as polylactones, which have attracted increasing attention because of their biodegradability and biocompatibility properties required for biomedicine applica- tions. 4,5 Separation and characterization of branched polymers usually rely on size exclusion chromatography coupled with a static light scattering detector. 6,7 This technique allows for the determination of the mean-square radius of gyration (R g ) of a polymer chain, 8 which is related to its volume and namely depends on its topology. 9 Recent studies show that ion mobility coupled with mass spectrometry (IM-MS) is another valuable tool to study polymer samples. 10,11 This technique consists of the separation of ions according to their gas-phase size and shape, 12-14 gathered under the term collision cross section (ccs) Ω. 15 In this context, topology-based separations of cyclic and branched polymers have been reported. 16,17 IM-MS has also been employed for conformational investigations of multicharged linear chains of poly(ethylene glycol) (PEG) 18-20 and polylactide (PLA) 21 produced by electrospray (ESI). 22 The authors reported progressive transitions from spherical to elongated conformations as the polymer chains shorten and the electrostatic repulsions increase. The corresponding 3D structures were assigned by matching experimental and theoretically modeled ccs. 14,23,24 In this present work, we analyzed the conformations of the linear and three star-branched poly-ε-caprolactones (PCL) in the gas phase using IM-MS with several objectives in mind. The first one is to probe specific structural properties that are dependent on the polymer topology and check the adequacy of IM-MS to resolve the four topoisomers. The second one is to probe the stability of the different conformations previously identified using collisional activation and to investigate the fate of charged vibrationally excited polymer chains in the gas phase. In the last part, we probed the structural evolution of a polymer chain following the ionization and desolvation steps of Received: June 18, 2014 Accepted: August 31, 2014 Published: September 4, 2014 Article pubs.acs.org/ac © 2014 American Chemical Society 9693 dx.doi.org/10.1021/ac502246g | Anal. Chem. 2014, 86, 9693-9700