Promoteur de thèse: Pascal Gerbaux
Co-promoteur: Jérôme Cornil
Résumé de la dissertation: An increasing number of studies use ion mobility spectrometry (IMS) coupled to mass spectrometry (IMS-MS) to obtain direct information about the spatial structure of gaseous ions. IMS separates ions according to their ion mobility K, a physical quantity related to the ion charge (z) and collisional cross section (CCS). As a first approximation, the CCS directly reflects the ion spatial structure. Synthetic polymers occupy a unique place in the field of IMS-MS. Due to their intrinsic dispersity, synthetic polymers have the asset to offer a broad range of homologous ions with different lengths that can be detected in several charge states. In addition, the gas-phase structure of polymer ions mostly depends on their z/L ratio, with z the charge state and L the backbone length. In other words, depending on their ability to stabilize the adducted charges, polymer ions with different lengths, charge states and backbone natures will adopt more extended or more compact conformations in the gas phase.
Based on the postulate that the secondary structure of a gaseous polymer ion depends on its primary structure, i.e., its chemical structure, ion mobility spectrometry (IMS) appears as an attractive analytical dimension to indirectly resolve the structure of polymer ions. The principal objective of this thesis is to evaluate whether a relationship between primary and secondary structures can be established for synthetic polymers. To do so, IMS-MS experiments are complemented with molecular dynamics (MD) simulations to decipher the gas-phase structure and the conformational dynamics of polymer ions.
In the first part of this thesis, the gas-phase structure of Electrospray (ESI)-produced poly(lactide) (PLA) ions is probed. This polyester is characterized by a peculiar IMS behavior. Indeed, several non-interconverting conformers are detected for ions with crucial z/L ratios. Experiments performed using various in-source conditions strongly suggest that the proportions between these conformers originate from in-source desolvation and ionization processes. ESI mechanisms leading to the formation of gaseous polymer ions are further studied through MD simulations. A key finding is that selected PLA ions appear to inherit their structural features from their formation processes. In other words, the structures of the gas-phase conformers detected by IMS-MS report on the ESI mechanistic pathways. The second part of this thesis is focused on more concrete applications of IMS-MS for polymer structural elucidation. First, a critical look is given at current polymer IMS-MS data fitting methodologies that aim to derive physicochemical data and structural interpretations. Finally, the postulate that a direct relationship exists between the secondary and primary macroion structures is assessed by investigating isomeric homo and copolymers. It appears that the analysis of multiply charged ions is mandatory to spot significant differences between these species. Differences in structural transition patterns for macroions with critical z/L ratios are indeed inherent to the efficient gas-phase separation of polymer ions with different primary structures.
Overall, because of the need to screen the charges carried by the polymer backbone, the relation between primary and secondary structures is not a rule of thumb. Yet, IMS reveals as a powerful analytical dimension complementary to MS and to other solution-phase techniques for the structural characterization of synthetic polymers.