General Approach to Direct Measurement of the Hydration State of Coordination Complexes in the Gas Phase: Variable Temperature Mass Spectrometry.

The formation of ternary aqua complexes of metal-based diagnostics and therapeutics is closely correlated to their in vivo efficacy but approaches to quantify the presence of coordinated water ligands are limited. We introduce a general and high-throughput method for characterizing the hydration state of para- and diamagnetic coordination complexes in the gas phase based on variable-temperature ion trap tandem mass spectrometry. Ternary aqua complexes are directly observed in the mass spectrum and quantified as a function of ion trap temperature. We recover expected periodic trends for hydration across the lanthanides and distinguish complexes with several inner-sphere water ligands by inspection of temperature-dependent speciation curves. We derive gas-phase thermodynamic parameters for discernible inner- and second-sphere hydration events, and discuss their application to predict solution-phase behavior. The differences in temperature at which water binds in the inner and outer spheres arise primarily from entropic effects. The broad applicability of this method allows us to estimate the hydration states of Ga, Sc, and Zr complexes under active preclinical and clinical study with as-yet undetermined hydration number. Variable-temperature mass spectrometry emerges as a general tool to characterize and quantitate trends in inner-sphere hydration across the periodic table.

The Interplay Between Hydrogen Bonding and Coulombic Forces in Determining the Structure of Sulfuric Acid-Amine Clusters.

Acid-base cluster chemistry drives atmospheric new particle formation (NPF), but the details of the growth mechanisms are difficult to experimentally probe. Clusters of ammonia, alkylamines, and sulfuric acid, species fundamental to NPF, are probed by infrared spectroscopy. These spectra show that substitution of amines for ammonia, which is linked to accelerated growth, induces profound structural rearrangement in clusters with initial compositions (NH4+) n+1(HSO4-) n (1 ≤ n ≤ 3). This rearrangement is driven by the loss of N-H hydrogen bond donors, yielding direct bisulfate-bisulfate hydrogen bonds, and its onset with respect to cluster composition indicates that more substituted amines induce rearrangement at smaller sizes. A simple model counting hydrogen bond donors and acceptors explains these observations. The presence of direct hydrogen bonds between formal anions shows that hydrogen bonding can compete with Coulombic forces in determining cluster structure. These results suggest that NPF mechanisms may be highly dependent on amine identity.