While electrospray ionization mass spectrometry (ESI-MS) has become a powerful technique for analyzing many types of host-guest complexation, questions remain as to just how accurately ion abundances generated by ESI reflect the true distribution of species at equilibrium in solution. To better understand this relationship, an equilibrium partitioning model was developed to explain the various interactions that dictate how much of a particular host-guest complex is transferred from solution into the gas phase in the ESI process. By evaluating the simultaneous equilibria of the complexation reaction and the partitioning of species between the surface and interior of the ESI droplets, one can estimate the ion abundances generated. The predictions of this new model were evaluated and experimentally confirmed through the analysis of the complexes of 18-crown-6 with alkali metal cations in an ESI quadrupole ion trap mass spectrometer, and it was determined that binding constants alone may not give accurate predictions about the observed ESI-MS response to different host-guest complexes.

No abstract

[reaction: see text] Synthesis and assembly studies of a guanosine-cytidine dinucleoside 1 that self-assembles into a trimeric supramolecule (I) are presented. Dinucleoside 1 was obtained by utilizing two consecutive palladium-catalyzed cross-coupling reactions. Ensemble I was analyzed by ESI-MS, NMR spectroscopies, size exclusion chromatography (SEC), and vapor pressure osmometry (VPO).

Mathematical models based on equilibrium partitioning theory were developed to relate ion abundances produced by electrospray ionization mass spectrometry with solution concentrations of complexes resulting from competitive host-guest binding. Through modeling the possible equilibria in the electrospray droplets, including the partitioning between the droplet surface and interior that dictate what ions are generated by an electrospray, the factors responsible for distorting the distribution of ions from their solution concentrations were evaluated. Experiments with crown ether-alkali metal complexes confirm the validity of the models and yield a greater understanding of the behavior of host-guest complexes in ESI-MS, allowing for more accurate and less trouble-prone measurements of solution binding interactions.