Poly (Ethylene terephthalate) oligomers cationized by alkali ions: Structures, energetics, and their effect on mass spectra and the matrix-assisted laser desorption/ionization process

In this article the folding dynamics and energetics for a set of poly(ethyl ene terephthalate) (PET) oligomers cationized by various alkali ions are st udied: M(+)PETn for n = 2 to 7 and M = Li, Na, and K. Experimental cross se ctions were determined for matrix-assisted laser desorption/ionization (MAL DI) generated ions using the ion mobility based ion chromatography method. Very good agreement was obtained with cross sections generated by the AMBER 4.0 suite of molecular dynamics programs. For n = 2 and 4 the benzene ring s of the oligomers pi stack with the metal ion coordinated to both terminal hydroxyl oxygen atoms and several of the nearby carbonyl oxygen atoms. For n = 3, two isomers are both observed and predicted by theory: an open form where the third PET monomer attaches to the dimer and extends into space a nd a closed form where the third PET moiety bends back and coordinates its hydroxyl oxygen with the metal ion. For Na(+)PET3, equilibrium is observed between 100 and 180 K with an Arrhenius analysis yielding an open to closed form isomerization barrier of 1.6 kcal/mol. For this same system the two i someric forms are frozen out at 80 K and coupling the observed isomeric dis tribution with an RRKM analysis indicates the closed form is more stable by 0.5 kcal/mol. For K(+)PET3 the barrier to isomerization is too low to obse rve (<1.0 kcal/mol), whereas for Li(+)PET3 a temperature independent isomer distribution is observed (80 to 55 degrees K). Using methods developed for determining isomerization barriers in carbon clusters it was possible to o btain an open to closed form isomerization barrier of 7 +/- 2 kcal/mol for Li(+)PET3. In this system, the open and closed form isomer populations were observed to be strong functions of the laser power in the MALDI source. Th is allowed a detailed description of the formation mechanism to be formulat ed and indicated alkali ion attachment to the polymer during expansion of t he plume emanating from the surface. Finally, the mass spectrum of a PET ol igomer sample has been shown to strongly depend on the cationizing alkali m etal ion. It is qualitatively shown that M+-PETn binding energies and struc tures are responsible. (J Am Soc Mass Spectrom 1999, 10, 883- 895) (C) 1999 American Society for Mass Spectrometry.