Gas-phase conformations of cationized poly(styrene) oligomers

The gas-phase conformations of poly(styrene) oligomers cationized by Li⁺, Na⁺, Cu⁺, and Ag⁺ (M⁺PSn) were examined using ion mobility experiments and molecular mechanics/dynamics calculations. M⁺PSn ions were formed by MALDI and their ion-He collision cross-sections were measured by ion mobility methods. The experimental collision cross-sections of each M⁺PS n-mer were similar for all four metal cations and increased linearly with n. Molecular modeling of selected M⁺PS oligomers cationized by Li⁺ and Na⁺ yielded quasi-linear structures with the metal cation sandwiched between two phenyl groups. The relative energies of the structures were ∼2-3 kcal/mol more stable when the metal cation was sandwiched near the middle of the oligomer chain than when it was near the ends of the oligomer. The cross-sections of these theoretical structures agree well with the experimental values with deviations typically around 1-2%. The calculations also show that the metal cation tends to align the phenyl groups on the same side of the -CH₂-CH- backbone. Calculations on neutral poly(styrene), on the other hand, showed structures in which the phenyl groups were more randomly positioned about the oligomer backbone. The conformations and metal-oligomer binding energies of M⁺PS are also used to help explain CID product distributions and fragmentation mechanisms of cationized PS oligomers.