Proton transfer reactivity of isolated charge states of the protein hen egg-white lysozyme shows that multiple distinct conformations of this protein are stable in the gas phase. The reactivities of the 9+ and 10+ charge state ions, formed by electrospray ionization of "native" (disulfide-intact) and "denatured" (disulfide-reduced) (22). We show here that, by comparison of the measured proton transfer reactivity of individual charge states to those calculated using a simple model, relatively unambiguous information about the conformation of several gas-phase lysozyme ions can be obtained. MATERIALS AND METHODSAll experiments were performed on an external electrospray ion source Fourier-transform mass spectrometer equipped with a 2.7-T superconducting magnet (Fig. 1 Abbreviations: GB, gas-phase basicity (basicities); GBaPP, apparent gas-phase basicity (basicities).

The evaporation of water from hydrated alkaline earth metal ions, produced by electrospray ionization, was studied in a Fourier transform mass spectrometer. Zero-pressure-limit dissociation rate constants for loss of a single water molecule from the hydrated divalent metal ions, M 2+ (H 2 O) n (M = Mg, Ca, and Sr for n = 5-7, and M = Ba for n = 4-7), are measured as a function of temperature using blackbody infrared radiative dissociation. From these values, zero-pressurelimit Arrhenius parameters are obtained. By modeling the dissociation kinetics using a master equation formalism, threshold dissociation energies (E o ) are determined. These reactions should have a negligible reverse activation barrier; therefore, E o values should be approximately equal to the binding energy or hydration enthalpy at 0 K. For the hepta-and hexahydrated ions at low temperature, binding energies follow the trend expected on the basis of ionic radii: Mg > Ca > Sr > Ba. For the hexahydrated ions at high temperature, binding energies follow the order Ca > Mg > Sr > Ba. The same order is observed for the pentahydrated ions. Collisional dissociation experiments on the tetrahydrated species result in relative dissociation rates that directly correlate with the size of the metals. These results indicate the presence of two isomers for hexahydrated magnesium ions: a lowtemperature isomer in which the six water molecules are located in the first solvation shell, and a high-temperature isomer with the most likely structure corresponding to four water molecules in the inner shell and two water molecules in the second shell. These results also indicate that the pentahydrated magnesium ions have a structure with four water molecules in the first solvation shell and one in the outer shell. The dissociation kinetics for the hexa-and pentahydrated clusters of Ca 2+ , Sr 2+ , and Ba 2+ are consistent with structures in which all the water molecules are located in the first solvation shell.

Metal ions play an important role in the function of many metalloenzymes. For example, the interactions of regulatory proteins such as calmodulin with other proteins depend on whether Ca 2+ is specifically bound. 1 Studies of metal ion hydration provide information not only about the metal ion chemistry in solution, but can lead to an improved understanding of the structure and function of many biomolecules in which metal ion interactions play a role. The hydration enthalpies of singly charged metal ions in the gas phase have been extensively investigated by using a variety of experimental techniques. 2 In contrast, gas-phase hydration studies of di-and trivalent metal ions are significantly more limited. 3-5 Kebarle and co-workers measured free energies of hydration for a series of divalent metal ions bound to between 7 and 13 water molecules. 4b,d From these values, hydration enthalpies for water molecules in the second solvation shell were estimated. Recently, calculations of successive hydration enthalpies of both inner and outer solvent shell water molecules for several divalent metal ions have been reported. 6,7 Here, binding energies of water molecules to both divalent calcium and nickel ions are measured by using blackbody infrared radiative dissociation (BIRD) and master equation modeling. These values are considerably lower than MP2 hydration enthalpies reported previously. 6 They are in better agreement with recently reported B3LYP values calculated with large basis sets. 7 To our knowledge, these are the first experimental measurements of the binding energy of individual inner shell water molecules to divalent metal ions.Experimental measurements are performed with an external electrospray ionization source Fourier transform mass spectrometer. This instrument and the BIRD experiments have been described previously. 8 Hydrated metal ions are generated from ~10 −4 M chloride salt solutions with nanoelectrospray ionization. The hydrated ions are trapped and thermalized by using a pulse of N 2 gas (10 −6 Torr). The ion of interest is mass selected and dissociated for times ranging from 10 to 500 s. At the low pressures during the reaction times (<10 −8 Torr), dissociation occurs by absorption of blackbody photons generated by the heated vacuum chamber walls. 8-11 For measurements with hydrated nickel ions, the pressure was <3 × 10 −8 Torr. At this pressure, collisions with background gas (primarily residual N 2 ) may affect the measured rate constants. Dissociation rate constants are measured as a function of temperature. Under these experimental conditions, the ion population is non-Boltzmann. The threshold dissociation energy (E o ), the thermochemical value of interest, is derived from the measured Arrhenius parameters by master equation modeling. 9,10 Microcanonical radiative rate constants used in the modeling are calculated from ab initio derived values 12 and varied over a 9-fold range. Microcanonical dissociation rate constants are calculated from RRKM theory and adjusted to provide rapid energy ...

The recently developed technique of desorption electrospray ionization (DESI) has been applied to the rapid analysis of controlled substances. Experiments have been performed using a commercial ThermoFinnigan LCQ Advantage MAX ion-trap mass spectrometer with limited modifications. Results from the ambient sampling of licit and illicit tablets demonstrate the ability of the DESI technique to detect the main active ingredient(s) or controlled substance(s), even in the presence of other higher-concentration components. Full-scan mass spectrometry data provide preliminary identification by molecular weight determination, while rapid analysis using the tandem mass spectrometry (MS/MS) mode provides fragmentation data which, when compared to the laboratory-generated ESI-MS/MS spectral library, provide structural information and final identification of the active ingredient(s). The consecutive analysis of tablets containing different active components indicates there is no cross-contamination or interference from tablet to tablet, demonstrating the reliability of the DESI technique for rapid sampling (one tablet/min or better). Active ingredients have been detected for tablets in which the active component represents less than 1% of the total tablet weight, demonstrating the sensitivity of the technique. The real-time sampling of cannabis plant material is also presented.

Secretory vesicles obtained from the atrial gland of the gastropod mollusk Aplysia californica were chemically analyzed individually with a combination of optical trapping, capillary electrophoresis separation, and a laser-induced fluorescence detection. With the use of optical trapping, a single vesicle that had attoliters (10(-18) liters) of volume was introduced into the tapered inlet of a separation capillary. Once the vesicle was injected, it was lysed, and its components were fluorescently labeled with naphthalene-2, 3-dicarboxaldehyde before separation. The resultant electropherograms indicated distinct variations in the contents of single vesicles.