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 similar to2-3 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. (J Am Soc Mass Spectrom 2002, 13, 499-505) (C) 2002 American Society for Mass Spectrometry.