IR Vaporization Mass Spectrometry of Aerosol Particles with Ionic Solutions: The Problem of Ion−Ion Recombination

A novel approach to analyze membrane proteins by laser mass spectrometry: from protein subunits to the integral complex

A novel laser-based mass spectrometry method termed LILBID (laser-induced liquid bead ion desorption) is applied to analyze large integral membrane protein complexes and their subunits. In this method the ions are IR-laser desorbed from aqueous microdroplets containing the hydrophobic protein complexes solubilized by detergent. The method is highly sensitive, very efficient in sample handling, relatively tolerant to various buffers, and detects the ions in narrow, mainly low-charge state distributions. The crucial experimental parameter determining whether the integral complex or its subunits are observed is the laser intensity: At very low intensity level corresponding to an ultrasoft desorption, the intact complexes, together with few detergent molecules, are transferred into vacuum. Under these conditions the oligomerization state of the complex (i.e., its quaternary structure) may be analyzed. At higher laser intensity, complexes are thermolyzed into subunits, with any residual detergent being stripped off to yield the true mass of the polypeptides. The model complexes studied are derived from the respiratory chain of the soil bacterium Paracoccus denitrificans and include complexes III (cytochrome bc(1) complex) and IV (cytochrome c oxidase). These are well characterized multi-subunit membrane proteins, with the individual hydrophobic subunits being composed of up to 12 transmembrane helices.

Product Screening of Fast Reactions in IR-Laser-Heated Liquid Water Filaments in a Vacuum by Mass Spectrometry

In the present article a novel approach for rapid product screening of fast reactions in IR-laser-heated liquid microbeams in a vacuum is highlighted. From absorbed energies, a shock wave analysis, high-speed laser stroboscopy, and thermodynamic data of high-temperature water the enthalpy, temperature, density, pressure, and the reaction time window for the hot water filament could be characterized. The experimental conditions (30 kbar, 1750 K, density approximately 1 g/cm3) present during the lifetime of the filament (20-30 ns) were extreme and provided a unique environment for high-temperature water chemistry. For the probe of the reaction products liquid beam desorption mass spectrometry was employed. A decisive feature of the technique is that ionic species, as well as neutral products and intermediates may be detected (neutrals as protonated aggregates) via time-of-flight mass spectrometry without any additional ionization laser. After the explosive disintegration of the superheated beam, high-temperature water reactions are efficiently quenched via expansion and evaporative cooling. For first exploratory experiments for chemistry in ultrahigh-temperature, -pressure and -density water, we have chosen resorcinol as a benchmark system, simple enough and well studied in high-temperature water environments much below 1000 K. Contrary to oxidation reactions usually present under less extreme and dense supercritical conditions, we have observed hydration and little H-atom abstraction during the narrow time window of the experiment. Small amounts of radicals but no ionic intermediates other than simple proton adducts were detected. The experimental findings are discussed in terms of the energetic and dense environment and the small time window for reaction, and they provide firm evidence for additional thermal reaction channels in extreme molecular environments.

A New Way To Detect Noncovalently Bonded Complexes of Biomolecules from Liquid Micro-Droplets by Laser Mass Spectrometry

A new version of laser mass-spectrometry is presented, which allows the quantitative analysis of specific biocomplexes in native solution. On-demand micro droplets, injected into vacuum, are irradiated by mid IR-laser pulses. Above a certain intensity threshold they explode due to the transmitted energy, setting free a fraction of the charged biomolecules which are then mass-analyzed. Amounts of analyte in the attomolar range may be detected with the ion intensity being linear over a wide range of molarity. Evidence is given that this method is soft, tolerant against various buffers, reflects properties of the liquid phase, and suitable for studying noncovalently bonded specific complexes. This is highlighted by results from antibiotics specifically binding into the minor groove of duplex DNA.

Laser Ablation of Imidazolium Based Ionic Liquids

Time-of-flight mass spectrometry has been used to investigate the IR ablation of several ionic liquid imidazolium salts of the form R(1)R(2)Iium X (R(1) = methyl; R(2) = methyl, ethyl, butyl, and hexyl; X = Cl(-), NO(3)(-), and CH(3)SO(4)(-)). The ablated ionic species were analyzed by time-of-flight mass spectrometry using pulsed extraction, and neutral species were detected using vacuum UV photoionization at 10.5 eV. The results demonstrate that at least 99% of the ablated material is removed in the form of nano- or microdroplets consisting of intact ionic liquid. Approximately 1% is ejected as imidazole molecules (R(1)R(2)Im) produced through the elimination of HCl, and about 0.1% of the material is ejected in the form of single salt molecules of R(1)R(2)Iium X. A chemical thermometer was used to measure the internal temperature (475 +/- 25 K) of the ablated vapor plume.

Infrared laser post-ionization of large biomolecules from an IR-MALD(I) plume

A two-infrared laser desorption/ionization method is described. A first laser, which was either an Er:YAG laser or an optical parametric oscillator (OPO), served for ablation/vaporization of small volumes of analyte/matrix sample at fluences below the ion detection threshold for direct matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). A second IR-laser, whose beam intersected the expanding ablation plume at a variable distance and time delay, was used to generate biomolecular ions out of the matrix-assisted laser desorption (MALD) plume. Either one of the two above lasers or an Er:YSGG laser was used for post-ionization. Glycerol was used as IR-MALDI matrix, and mass spectra of peptides, proteins, as well as nucleic acids, some of which in excess of 10(5) u in molecular weight, were recorded with a time-of-flight mass spectrometer. A mass spectrum of cytochrome c from a water ice matrix is also presented. The MALD plume expansion was investigated by varying the position of the post-ionization laser beam above the glycerol sample surface and its delay time relative to the desorption laser. Comparison between the OPO (pulse duration, tau(L) = 6 ns) and the Er:YAG laser (tau(L) approximately 120 ns) as primary excitation laser demonstrates a significant effect of the laser pulse duration on the MALD process.