INCLUSION OF A MALDI ION-SOURCE IN THE ION CHROMATOGRAPHY TECHNIQUE -CONFORMATIONAL INFORMATION ON POLYMER AND BIOMOLECULAR IONS

A matrix-assisted laser desorption ionization (MALDI) source has been coupled to the ion chromatography instrument developed at UCSB. The so urce produces a strong, consistent signal for several hours on a singl e sample. In this paper we report the application of this method to a series of poly(ethylene glycol) (PEG) polymers cationized by sodium. D ata have been taken for Na(+)PEG5 to Na(+)PEG19. The temperature depen dence of the ion mobility (collision cross-section) in He gas for Na()PEG9, Na(+)PEG13 and Na(+)PEG17 has been measured from 80 to 580 K. A detailed analysis of these three systems has been accomplished in ord er to extract the conformations of the ion and how they vary with temp erature. This analysis included several significant changes from metho ds used previously. Molecular mechanics methods were used both to obta in the lowest energy 0 K structures and to predict how these structure s would change as temperature increases. In order to account for the o bserved low temperature results, a 12-6-4 potential was incorporated i n place of the hard-sphere potential used previously. For all three sy stems studied in detail, the oxygen atoms on the PEG units solvated th e Na+ ion, forming a crown ether type ring of five oxygens surrounding Na+ and several others above and below this ring. The molecular mecha nics model was also applied to neutral PEG13. In this instance a quite compact structure is obtained for T less than or equal to 200 K but a sudden melting type transition occurs between 200 and 300 K and chaot ic motion dominates at and above 300 K. Data are also reported on the temperature dependence of the ion mobility of C-60(+). This ion is exp ected to change shape only slightly over the temperature range reporte d here. Consequently it provided an excellent set of calibration data for evaluating the intramolecular interaction potentials used to descr ibe the collision process.