CONFORMATIONS OF ALKALI ION CATIONIZED POLYETHERS IN THE GAS-PHASE - POLYETHYLENE-GLYCOL AND BIS[(BENZO-15-CROWN-5)-15-YLMETHYL] PIMELATE

Ion chromatography/ion mobility methods are applied to two polyether s ystems cationized by alkali ions. MALDI is used to generate the ions i n all cases. Polyethylene glycol (PEG) 600 is cationized by Li+ and Cs + and compared to earlier work by von Helden et al, on Na+ cationized PEG 600. A very similar distribution is obtained in the MALDI spectrum of all three metals suggesting the true PEG 600 distribution is being observed. Each cation efficiently wraps the polymer around it, howeve r, the detailed structure of the inner coordination sphere of ether ox ygens around the metal ion is metal dependent. Both Li+ and Na+ prefer a near planar five-fold coordination sphere capped on top and bottom leading to seven-fold coordination (Li+) and eight-fold coordination ( Na+). The much larger Cs+ prefers 11-fold coordination if sufficient o xygen centers are available. The second system studied is the interest ing dicrown bis[(benzo-15-crown-5)-15-ylmethyl] pimelate, or BBP. This tweezers-like molecule coordinates the metal ions primarily in folded , or sandwich, complexes where both rings are involved. Other structur es that include metal coordination to one ring and a carbonyl oxygen ( partially folded) and to one ring and no additional oxygens (unfolded) are observed in elevated temperature molecular dynamics simulations a nd are more important for small alkali ions (Li+) and essentially not important for large ions (Cs+). The AMBER 4.0 suite of programs is use d for all calculations which in general are in excellent agreement wit h experiment. A quite detailed discussion of the folding dynamics of c ationized BBP is presented. (C) 1997 Elsevier Science B.V.