Ion mobility mass spectrometry coupled with molecular dynamics simulations: in-depth structural analysis of polystyrene-based Au-containing copolymers

Abstract

Artificial enzymes based on polystyrene copolymers featuring a metal complex within their structure, so-called single-chain nanoparticles (SCNPs), are being explored as hybrid heterogeneous/homogeneous catalysts. Using styrene (derivative) building blocks, SCNP precursor copolymers decorated with pendent triphenylphosphine ligands complexed with catalytically active gold motifs have recently been reported. It is highly challenging to determine the location and orientation of the functional groups – including the catalytic center – the coil geometry, and even the macromolecular architecture within these complex precursors via conventional analytical techniques. The use of ion mobility mass spectrometry (IMS-MS) combined with molecular dynamics (MD) simulations is emerging as a way to establish the structure of gaseous ions, including the description of the secondary interactions responsible for the folding. IMS-MS is used to separate intricate polymer mixtures, while providing structural information through collisional cross section (CCS) determination. MD simulations are used to assign a detailed internal structure to the conformations sampled by IMS-MS by comparing the experimental CCS with the theoretical values computed for the MD structures. In the present contribution, we provide an in-depth investigation of the conformation of gaseous Au-functionalized copolymer ions composed of three different monomer units, i.e., styrene, styrene-CH₂-OH and styrene-PPh₂-AuCl, and those bearing a TEMPO unit as the initiator end group. For the styrene/styrene-CH₂-OH copolymer ions, an H-bond between protonated TEMPO and a styrene-CH₂-OH unit is responsible for the ultimate folding of the polymer ions with the charge settled at the center of the globular ions. When incorporating the triphenylphosphine-AuCl unit, a strong H-bond between the chlorine atom and protonated TEMPO is detected. However, the steric hindrance around the triphenylphosphine ligand prevents the charge from being incorporated into the core of the globular ions.