Visible and ultraviolet spectroscopy of gas phase protein ions

Protomer-Dependent Electronic Spectroscopy and Photochemistry of the Model Flavin Chromophore Alloxazine

Flavin chromophores play key roles in a wide range of photoactive proteins, but key questions exist in relation to their fundamental spectroscopic and photochemical properties. In this work, we report the first gas-phase spectroscopy study of protonated alloxazine (AL∙H⁺), a model flavin chromophore. Laser photodissociation is employed across a wide range (2.34⁻5.64 eV) to obtain the electronic spectrum and characterize the photofragmentation pathways. By comparison to TDDFT quantum chemical calculations, the spectrum is assigned to two AL∙H⁺ protomers; an N5 (dominant) and O4 (minor) form. The protomers have distinctly different spectral profiles in the region above 4.8 eV due to the presence of a strong electronic transition for the O4 protomer corresponding to an electron-density shift from the benzene to uracil moiety. AL∙H⁺ photoexcitation leads to fragmentation via loss of HCN and HNCO (along with small molecules such as CO₂ and H₂O), but the photofragmentation patterns differ dramatically from those observed upon collision excitation of the ground electronic state. This reveals that fragmentation is occurring during the excited state lifetime. Finally, our results show that the N5 protomer is associated primarily with HNCO loss while the O4 protomer is associated with HCN loss, indicating that the ring-opening dynamics are dependent on the location of protonation in the ground-state molecule.

Tailored photocleavable peptides: fragmentation and neutralization pathways in high vacuum

Photocleavable tags (PCTs) have the potential for excellent spatio-temporal control over the release of subunits of complex molecules. Here, we show that electrosprayed oligopeptides, functionalized by a tailored ortho-nitroarylether can undergo site-specific photo-activated cleavage under UV irradiation (266 nm) in high vacuum. The comparison of UV photodissociation (UVPD) and collision-induced dissociation (CID) points to the thermal nature of the cleavage mechanism, a picture corroborated by the temperature dependence of the process. Two competing photodissociation pathways can be identified. In one case a phenolate anion is separated from a neutral zwitterion. In the other case a neutral phenol derivative leaves a negatively charged peptide behind. To understand the factors favoring one channel over the other, we investigate the influence of the peptide length, the nature of the phenolic group and the position of the nitro-group (ortho vs. para). The observed gas phase cleavage of a para-nitro benzylic ether markedly differs from the established behavior in solution.

Infrared laser dissociation of single megadalton polymer ions in a gated electrostatic ion trap: the added value of statistical analysis of individual events

In this study, we report the unimolecular dissociation mechanism of megadalton SO₃-containing poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) polymer cations and anions with the aid of infrared multiphoton dissociation coupled to charge detection ion trap mass spectrometry. A gated electrostatic ion trap ("Benner trap") is used to store and detect single gaseous polymer ions generated by positive and negative polarity in an electrospray ionization source. The trapped ions are then fragmented due to the sequential absorption of multiple infrared photons produced from a continuous-wave CO₂ laser. Several fragmentation pathways having distinct signatures are observed. Highly charged parent ions characteristically adopt a distinctive "stair-case" pattern (assigned to the "fission" process) whereas low charge species take on a "funnel like" shape (assigned to the "evaporation" process). Also, the log-log plot of the dissociation rate constants as a function of laser intensity between PAMPS positive and negative ions is significantly different.

Data-Independent Acquisition Coupled to Visible Laser-Induced Dissociation at 473 nm (DIA-LID) for Peptide-Centric Specific Analysis of Cysteine-Containing Peptide Subset

Thanks to comprehensive and unbiased sampling of all precursor ions, the interest to move toward bottom-up proteomic with data-independent acquisition (DIA) is continuously growing. DIA offers precision and reproducibility performances comparable to true targeted methods but has the advantage of enabling retrospective data testing with the hypothetical presence of new proteins of interest. Nonetheless, the chimeric nature of DIA MS/MS spectra inherent to concomitant transmission of a multiplicity of precursor ions makes the confident identification of peptides often challenging, even with spectral library-based extraction strategy. The introduction of specificity at the fragmentation step upon ultraviolet or visible laser-induced dissociation (LID) range targeting only the subset of cysteine-containing peptides (Cys-peptide) has been proposed as an option to streamline and reduce the search space. Here, we describe the first coupling between DIA and visible LID at 473 nm to test for the presence of Cys-peptides with a peptide-centric approach. As a test run, a spectral library was built for a pool of Cys-synthetic peptides used as surrogates of human kinases (1 peptide per protein). By extracting ion chromatograms of query standard and kinase peptides spiked at different concentration levels in an Escherichia coli proteome lysate, DIA-LID demonstrates a dynamic range of detection of at least 3 decades and coefficients of precision better than 20%. Finally, the spectral library was used to search for endogenous kinases in human cellular extract.

Protein charge transfer absorption spectra: an intrinsic probe to monitor structural and oligomeric transitions in proteins

The utility of ProCharTS as an intrinsic spectral probe to track protein aggregation and monitor conformational changes is reported.

Visible Multiphoton Dissociation of Chromophore-Tagged Peptides

The visible photodissociation mechanisms of QSY7-tagged peptides of increasing size have been investigated by coupling a mass spectrometer and an optical parametric oscillator laser beam. The experiments herein consist of energy resolved collision- and laser-induced dissociation measurements on the chromophore-tagged peptides. The results show that fragmentation occurs by similar channels in both activation methods, but that the branching ratios are vastly different. Observation of a size-dependent minimum laser pulse energy required to induce fragmentation, and collisional cooling rates in time resolved experiments show that laser-induced dissociation occurs through the absorption of multiple photons by the chromophore and the subsequent heating through vibrational energy redistribution. The differences in branching ratio between collision- and laser-induced dissociation can then be understood by the highly anisotropic energy distribution following absorption of a photon. Graphical Abstract ᅟ.

UV-Vis Action Spectroscopy Reveals a Conformational Collapse in Hydrogen-Rich Dinucleotide Cation Radicals

We report the generation of deoxyriboadenosine dinucleotide cation radicals by gas-phase electron transfer to dinucleotide dications and their noncovalent complexes with crown ether ligands. Stable dinucleotide cation radicals of a novel hydrogen-rich type were generated and characterized by tandem mass spectrometry and UV-vis photodissociation (UVPD) action spectroscopy. Electron structure theory analysis indicated that upon electron attachment the dinucleotide dications underwent a conformational collapse followed by intramolecular proton migrations between the nucleobases to give species whose calculated UV-vis absorption spectra matched the UVPD action spectra. Hydrogen-rich cation radicals generated from chimeric riboadenosine 5'-diesters gave UVPD action spectra that pointed to novel zwitterionic structures consisting of aromatic π-electron anion radicals intercalated between stacked positively charged adenine rings. Analogies with DNA ionization are discussed.

Near-UV Photodissociation of Tryptic Peptide Cation Radicals. Scope and Effects of Amino Acid Residues and Radical Sites

Peptide cation-radical fragment ions of the z-type, [^●AXAR⁺], [^●AXAK⁺], and [^●XAR⁺], where X = A, C, D, E, F, G, H, K, L, M, N, P, Y, and W, were generated by electron transfer dissociation of peptide dications and investigated by MS³-near-ultraviolet photodissociation (UVPD) at 355 nm. Laser-pulse dependence measurements indicated that the ion populations were homogeneous for most X residues except phenylalanine. UVPD resulted in dissociations of backbone CO─NH bonds that were accompanied by hydrogen atom transfer, producing fragment ions of the [y_n]⁺ type. Compared with collision-induced dissociation, UVPD yielded less side-chain dissociations even for residues that are sensitive to radical-induced side-chain bond cleavages. The backbone dissociations are triggered by transitions to second (B) excited electronic states in the peptide ion R-CH^●-CONH- chromophores that are resonant with the 355-nm photon energy. Electron promotion increases the polarity of the B excited states, R-CH⁺-C^●(O^-)NH-, and steers the reaction to proceed by transfer of protons from proximate acidic C_α and amide nitrogen positions. Graphical Abstract ᅟ.

Dimerization and conformation-related free energy landscapes of dye-tagged amyloid-β12–28linked to FRET experiments

The free energy landscapes of Aβ-peptide dimer models under different prototype conditions support the hypothesis that the gas-phase action-FRET measurement after electrospray ionization operates under non-equilibrium conditions, with a memory of the solution conditions – even for the dimer of this relatively short peptide.

Where Does the Electron Go? Stable and Metastable Peptide Cation Radicals Formed by Electron Transfer

Electron transfer to doubly and triply charged heptapeptide ions containing polar residues Arg, Lys, and Asp in combination with nonpolar Gly, Ala, and Pro or Leu generates stable and metastable charge-reduced ions, (M + 2H)^+●, in addition to standard electron-transfer dissociation (ETD) fragment ions. The metastable (M + 2H)^+● ions spontaneously dissociate upon resonant ejection from the linear ion trap, giving irregularly shaped peaks with offset m/z values. The fractions of stable and metastable (M + 2H)^+● ions and their mass shifts depend on the presence of Pro-4 and Leu-4 residues in the peptides, with the Pro-4 sequences giving larger fractions of the stable ions while showing smaller mass shifts for the metastables. Conversion of the Asp and C-terminal carboxyl groups to methyl esters further lowers the charge-reduced ion stability. Collisional activation and photodissociation at 355 nm of mass-selected (M + 2H)^+● results in different dissociations that give sequence specific MS³ spectra. With a single exception of charge-reduced (LKGLADR + 2H)^+●, the MS³ spectra do not produce ETD sequence fragments of the c and z type. Hence, these (M + 2H)^+● ions are covalent radicals, not ion-molecule complexes, undergoing dramatically different dissociations in the ground and excited electronic states. The increased stability of the Pro-4 containing (M + 2H)^+● ions is attributed to radicals formed by opening of the Pro ring and undergoing further stabilization by hydrogen atom migrations. UV-VIS photodissociation action spectroscopy and time-dependent density functional theory calculations are used in a case in point study of the stable (LKGPADR + 2H)^+● ion produced by ETD. In contrast to singly-reduced peptide ions, doubly reduced (M + 3H)⁺ ions are stable only when formed from the Pro-4 precursors and show all characteristics of even electron ions regarding no photon absorption at 355 nm or ion-molecule reactions, and exhibiting proton driven collision induced dissociations. Graphical Abstract ᅟ.

Gas-phase protein conformation/multimer ion formation by electrospray ion mobility-mass spectrometry: bovine insulin and ubiquitin

Ion mobility-mass spectrometry (IMMS) is a very attractive method for studies in structural biology because of the ability of rapid isolation by nearly simultaneous m/z characterization and size separation, leading to an emergence of IMMS as a complimentary biochemical tool. Earlier, we developed a method based on varying the protein concentration in solution prior to electrospray ionization (ESI) with subsequent m/z selection and dissociation of protein multimers by IMMS of cytochrome c. The focus of this work will be to correctly distinguish truly different ion conformations formed by ESI versus homomultimeric complexes with the same m/z for well-studied proteins bovine ubiquitin and insulin. These proteins were chosen due to their large difference in solution phase structures: insulin tightly bound by disulfide linkages, and ubiquitin-a protein that may adopt a range of states from compact to extended. Our preliminary results, as with cytochrome c reveal false negatives for protein oligomer formation and false positives for protein conformational states. In addition, these results will be couched in terms of the need for quantification of IMMS analysis of proteins given the total area under IMMS peaks can also distinguish conformation versus aggregation as higher order oligomers have more mass per ion.This article is part of the themed issue 'Quantitative mass spectrometry'.

Charge Transfer Dissociation of Complex Oligosaccharides: Comparison with Collision-Induced Dissociation and Extreme Ultraviolet Dissociative Photoionization

The structural characterization of oligosaccharides still challenges the field of analytical chemistry. Tandem mass spectrometry offers many advantages toward this aim, although the generic fragmentation method (low-energy collision-induced dissociation) shows clear limitations and is often insufficient to retrieve some essential structural information on these molecules. In this work, we present the first application of helium charge transfer dissociation (He-CTD) to characterize the structure of complex oligosaccharides. We compare this method with low-energy collision-induced dissociation and extreme-ultraviolet dissociative photoionization (XUV-DPI), which was shown previously to ensure the successful characterization of complex glycans. Similarly to what could be obtained by XUV-DPI, He-CTD provides a complete description of the investigated structures by producing many informative cross-ring fragments and no ambiguous fragmentation. Unlike XUV-DPI, which is performed at a synchrotron source, He-CTD has the undeniable advantage of being implementable in a conventional benchtop ion trap in a conventional laboratory setting. Graphical Abstract ᅟ.

Optical properties of prodigiosin and obatoclax: action spectroscopy and theoretical calculations

Prodiginine molecules (prodigiosin and obatoclax) are well-known pH-chromic dyes with promising anti-tumor properties. They present multiple tautomeric and rotameric forms. The protonation state and the structure of such flexible ligands in interaction with a protein are crucial to understand and to model the protein's biological activities. The determination of the protonation state via UV/vis absorption is possible if the ligand spectra of the neutral and protonated states are sufficiently different, and also if we can eliminate other factors potentially impacting the spectrum. Upon measuring the absorption spectra of the ligand in solution, varying solvents and pH values, we have determined that the optical properties of prodigiosin and obatoclax depend on the protonation state and not on the solvent permittivity constant. In parallel, action spectroscopy (using tunable lasers coupled to ion traps) in the gas phase of protonated and sodiated prodigiosin and obatoclax molecules has been performed to evaluate the sensitivity of the charge and the conformational state to their optical properties free of solvent. The spectra are interpreted using computational simulations of molecular structures and electronic excitations. The excitation energies are only slightly sensitive to various isomerizations, and may be used to distinguish between protonated and deprotonated states, even in the presence of a sodium counter-ion.

Online coupling of high-resolution chromatography with extreme UV photon activation tandem mass spectrometry: Application to the structural investigation of complex glycans by dissociative photoionization

The activation of ions by extreme-energy photons (XUV) produced by a synchrotron radiation beamline is a powerful method for characterizing complex glycans using tandem mass spectrometry (MS). As previously described, this activation method leads to rich fragmentation spectra with many structurally valuable cross-ring cleavages while maintaining labile modifications on the glycan structures. However, until now, the tandem MS event was too long to be compatible with liquid chromatography elution times. In this work, the duty cycle of the activation and detection of fragments was shortened, and the background signal on the spectra was drastically reduced. Both improvements allowed, for the first time, the successful coupling of a UHPLC system to XUV-activated tandem MS. The approach was used to characterize a complex mixture of oligo-porphyrans, which are a class of highly sulfated oligosaccharides, in a fully automated way. Due to an enhanced dynamic range and an increased sensitivity, some hypothetical structures of low abundance have been unequivocally confirmed in this study and others have been revised. Some previously undescribed species of oligo-porphyrans that exhibit lateral branching have been fully resolved. This work contributes to the scarce knowledge of the structure of porphyrans in red algae and pushes the current capacities of XUV-activation tandem MS by demonstrating the possibility of a direct coupling with UHPLC. This study will considerably broaden the applicability and practicality of this method in many fields of analytical biology.

Ligand-induced substrate steering and reshaping of [Ag2(H)](+) scaffold for selective CO2 extrusion from formic acid

Metalloenzymes preorganize the reaction environment to steer substrate(s) along the required reaction coordinate. Here, we show that phosphine ligands selectively facilitate protonation of binuclear silver hydride cations, [LAg₂(H)]⁺ by optimizing the geometry of the active site. This is a key step in the selective, catalysed extrusion of carbon dioxide from formic acid, HO₂CH, with important applications (for example, hydrogen storage). Gas-phase ion-molecule reactions, collision-induced dissociation (CID), infrared and ultraviolet action spectroscopy and computational chemistry link structure to reactivity and mechanism. [Ag₂(H)]⁺ and [Ph₃PAg₂(H)]⁺ react with formic acid yielding Lewis adducts, while [(Ph₃P)₂Ag₂(H)]⁺ is unreactive. Using bis(diphenylphosphino)methane (dppm) reshapes the geometry of the binuclear Ag₂(H)⁺ scaffold, triggering reactivity towards formic acid, to produce [dppmAg₂(O₂CH)]⁺ and H₂. Decarboxylation of [dppmAg₂(O₂CH)]⁺ via CID regenerates [dppmAg₂(H)]⁺. These gas-phase insights inspired variable temperature NMR studies that show CO₂ and H₂ production at 70 °C from solutions containing dppm, AgBF₄, NaO₂CH and HO₂CH.

Designing catalysts and understanding the influence of ligands for particular transformations remains a highly challenging task. Here, the authors show that bisphosphine ligands can alter the geometry of the active site in silver catalysts, driving protonation and ultimately extrusion of carbon dioxide from formic acid.

Charge, Color, and Conformation: Spectroscopy on Isomer-Selected Peptide Ions

Monitoring the chromism induced by intramolecular hydrogen and charge transfers within proteins as well as the isomerization of both protein and cofactor is essential not only to understand photoactive signaling pathways but also to design targeted opto-switchable proteins. We used a dual-ion mobility drift tube coupled to a tunable picosecond laser to explore the optical and structural properties of a peptide chain bound to a chromophore-a prototype system allowing for a proton transfer coupled to conformational change. With the support of molecular dynamics and DFT calculations, we show how proton transfer between the peptide and its cofactor can dramatically modify the optical properties of the system and demonstrate that these changes can be triggered by collisional activation in the gas phase.

Complexes of Ni(ii) and Cu(ii) with small peptides: deciding whether to deprotonate

Infrared multiple photon dissociation (IRMPD) spectroscopy differentiates two binding modes (iminol versus charge solvated) for Ni(ii) bound to model peptides.

Leveraging ultraviolet photodissociation and spectroscopy to investigate peptide and protein three-dimensional structure with mass spectrometry

Recent advances in mass spectrometry and lasers have facilitated the development of novel experiments combining the benefits of both technologies.

Going clean: structure and dynamics of peptides in the gas phase and paths to solvation

The gas phase is an artificial environment for biomolecules that has gained much attention both experimentally and theoretically due to its unique characteristic of providing a clean room environment for the comparison between theory and experiment. In this review we give an overview mainly on first-principles simulations of isolated peptides and the initial steps of their interactions with ions and solvent molecules: a bottom up approach to the complexity of biological environments. We focus on the accuracy of different methods to explore the conformational space, the connections between theory and experiment regarding collision cross section evaluations and (anharmonic) vibrational spectra, and the challenges faced in this field.

Combining UV photodissociation action spectroscopy with electron transfer dissociation for structure analysis of gas-phase peptide cation-radicals

We report the first example of using ultraviolet (UV) photodissociation action spectroscopy for the investigation of gas-phase peptide cation-radicals produced by electron transfer dissociation. z-Type fragment ions (●) Gly-Gly-Lys(+), coordinated to 18-crown-6-ether (CE), are generated, selected by mass and photodissociated in the 200-400 nm region. The UVPD action spectra indicate the presence of valence-bond isomers differing in the position of the Cα radical defect, (α-Gly)-Gly-Lys(+) (CE), Gly-(α-Gly)-Lys(+) (CE) and Gly-Gly-(α-Lys(+))(CE). The isomers are readily distinguishable by UV absorption spectra obtained by time-dependent density functional theory (TD-DFT) calculations. In contrast, conformational isomers of these radical types are calculated to have similar UV spectra. UV photodissociation action spectroscopy represents a new tool for the investigation of transient intermediates of ion-electron reactions. Specifically, z-type cation radicals are shown to undergo spontaneous hydrogen atom migrations upon electron transfer dissociation.

Probing Protein Structure and Folding in the Gas Phase by Electron Capture Dissociation

The established methods for the study of atom-detailed protein structure in the condensed phases, X-ray crystallography and nuclear magnetic resonance spectroscopy, have recently been complemented by new techniques by which nearly or fully desolvated protein structures are probed in gas-phase experiments. Electron capture dissociation (ECD) is unique among these as it provides residue-specific, although indirect, structural information. In this Critical Insight article, we discuss the development of ECD for the structural probing of gaseous protein ions, its potential, and limitations.

Ion-Induced Dipole Interactions and Fragmentation Times: Cα-Cβ Chromophore Bond Dissociation Channel

The fragmentation times corresponding to the loss of the chromophore (Cα-Cβ bond dissociation channel) after photoexcitation at 263 nm have been investigated for several small peptides containing tryptophan or tyrosine. For tryptophan-containing peptides, the aromatic chromophore is lost as an ionic fragment (m/z 130), and the fragmentation time increases with the mass of the neutral fragment. In contrast, for tyrosine-containing peptides the aromatic chromophore is always lost as a neutral fragment (mass = 107 amu) and the fragmentation time is found to be fast (<20 ns). These different behaviors are explained by the role of the postfragmentation interaction in the complex formed after the Cα-Cβ bond cleavage.

Contribution of synchrotron radiation to photoactivation studies of biomolecular ions in the gas phase

Photon activation of ions in the visible and ultraviolet range attracts a growing interest, partly for its promising applications in tandem mass spectrometry. However, this task is not trivial, as it requires notably high brilliance photon sources. Hence, most of the work in that field has been performed using lasers. Synchrotron radiation is a source continuously tunable over a wide photon energy range and which possesses the necessary characteristics for ion activation. This review focuses on the array of applications of synchrotron radiation in photon activation of ions ranging from near UV to soft X-rays.

Electron photodetachment dissociation for structural characterization of synthetic and bio-polymer anions

Tandem mass spectrometry (MS-MS) is a generic term evoking techniques dedicated to structural analysis, detection or quantification of molecules based on dissociation of a precursor ion into fragments. Searching for the most informative fragmentation patterns has led to the development of a vast array of activation modes that offer complementary ion reactivity and dissociation pathways. Collisional activation of ions using atoms, molecules or surface resulting in unimolecular dissociation of activated ions still plays a key role in tandem mass spectrometry. The discovery of electron capture dissociation (ECD) and then the development of other electron-ion or ion/ion reaction methods, constituted a significant breakthrough, especially for structural analysis of large biomolecules. Similarly, photon activation opened promising new frontiers in ion fragmentation owing to the ability of tightly controlled internal energy deposition and easy implementation on commercial instruments. Ion activation by photons includes slow heating methods such as infrared multiple photon dissociation (IRMPD) and black-body infrared radiative dissociation (BIRD) and higher energy methods like ultra-violet photodissociation (UVPD) and electron photo detachment dissociation (EPD). EPD occurs after UV irradiation of multiply negatively charged ions resulting in the formation of oxidized radical anions. The present paper reviews the hypothesis regarding the mechanisms of electron photo-detachment, radical formation and direct or activated dissociation pathways that support the observation of odd and even electron product ions. Finally, the value of EPD as a complementary structural analysis tool is illustrated through selected examples of synthetic polymers, oligonucleotides, polypeptides, lipids, and polysaccharides.

Resonance Raman Analysis of the Tryptophan Cation Radical

Electron transfer (ET) reactions within proteins are accomplished by a broad set of redox-active molecules, including natural amino acids. Tryptophan participates in ET chemistry as both a cation and a neutral radical. Identification and characterization of the biologically relevant species is essential to understand efficient ET mechanisms in proteins. We present resonance Raman spectra and excitation profiles of the tryptophan cation radical generated by combining a strong oxidant, Ce(IV), with tryptophan model compounds in a fast-flow mixing device. Isotopically modified derivatives, coupled with calculations, allowed the assignment of the normal modes of this radical. Raman bands that are sensitive to protonation state and hydrogen bonding environment of the cation radical were identified. The present findings, along with resonance Raman spectra of the closed-shell and neutral radical counterparts, form a foundation for probing tryptophan-mediated ET reactions in proteins.

Absolute absorption cross sections from photon recoil in a matter-wave interferometer

We measure the absolute absorption cross section of molecules using a matter-wave interferometer. A nanostructured density distribution is imprinted onto a dilute molecular beam through quantum interference. As the beam crosses the light field of a probe laser some molecules will absorb a single photon. These absorption events impart a momentum recoil which shifts the position of the molecule relative to the unperturbed beam. Averaging over the shifted and unshifted components within the beam leads to a reduction of the fringe visibility, enabling the absolute absorption cross section to be extracted with high accuracy. This technique is independent of the molecular density, it is minimally invasive and successfully eliminates many problems related to photon cycling, state mixing, photobleaching, photoinduced heating, fragmentation, and ionization. It can therefore be extended to a wide variety of neutral molecules, clusters, and nanoparticles.

Multiple Electron Ejection from Proteins Resulting from Single-Photon Excitation in the Valence Shell

One-photon multiple ionization is a signature of dynamical electron correlations in atoms and small molecules, as observed in the Auger process when Auger electron emission follows core-shell ionization. In such a process, the high energy needed to remove several electrons is due to the strong Coulombic attraction between the last departing electron(s) and the ionic core. Multiply negatively charged molecules offer the possibility to overcome the Coulombic attraction, opening the way for multielectron photodetachment following valence shell excitation. Here photodetachment studies have been performed on electrosprayed protein polyanions using vacuum ultraviolet synchrotron radiation coupled to a radiofrequency ion trap. Double, triple, and quadruple electron emissions from protein polyanions resulting from single-photon excitation in the valence shell were observed with ionization thresholds below 20 eV photon energy. This suggests the existence of large electronic correlations in proteins between weakly bound electrons standing on distant sites. Besides, the resulting multiradical polyanions appear to be remarkably stable, an important issue in radiobiology.

Photodissociation mass spectrometry: new tools for characterization of biological molecules

Photodissociation mass spectrometry combines the ability to activate and fragment ions using photons with the sensitive detection of the resulting product ions by mass spectrometry. This combination affords a versatile tool for characterization of biological molecules. The scope and breadth of photodissociation mass spectrometry have increased substantially over the past decade as new research groups have entered the field and developed a number of innovative applications that illustrate the ability of photodissociation to produce rich fragmentation patterns, to cleave bonds selectively, and to target specific molecules based on incorporation of chromophores. This review focuses on many of the key developments in photodissociation mass spectrometry over the past decade with a particular emphasis on its applications to biological molecules.

Excited states of multiply-charged anions probed by photoelectron imaging: riding the repulsive Coulomb barrier

Recent progress towards understanding the repulsive Coulomb barrier in multiply-charged anion using photoelectron spectroscopy is discussed.

Combined collision-induced dissociation and photo-selected reaction monitoring mass spectrometry modes for simultaneous analysis of coagulation factors and estrogens

Oral estrogens are directly associated with changes in plasma levels of coagulation proteins. Thus, the detection of any variation in protein concentrations due to estrogen contraceptives, by a simultaneous analysis of both coagulation proteins and estrogens, would be a very informative tool. In the present study, the merit of photo-selected reaction monitoring (SRM), a new analytical tool, was evaluated towards estrogens detection in plasma. Then, SRM and photo-SRM detection modes were combined for the simultaneous analysis of estrogen molecules together with heparin co-factor and factor XIIa, two proteins involved in the coagulation cascade. This study shows that photo-SRM could open new multiplexed analytical routes.

Vacuum ultraviolet action spectroscopy of polysaccharides

We studied the optical properties of gas-phase polysaccharides (maltose, maltotetraose, and maltohexaose) ions by action spectroscopy using the coupling between a quadrupole ion trap and a vacuum ultraviolet (VUV) beamline at the SOLEIL synchrotron radiation facility (France) in the 7 to 18 eV range. The spectra provide unique benchmarks for evaluation of theoretical data on electronic transitions of model carbohydrates in the VUV range. The effects of the nature of the charge held by polysaccharide ions on the relaxation processes were also explored. Finally the effect of isomerization of polysaccharides (with melezitose and raffinose) on their photofragmentation with VUV photons is presented.

UV photodissociation action spectroscopy of haloanilinium ions in a linear quadrupole ion trap mass spectrometer

UV-vis photodissociation action spectroscopy is becoming increasingly prevalent because of advances in, and commercial availability of, ion trapping technologies and tunable laser sources. This study outlines in detail an instrumental arrangement, combining a commercial ion-trap mass spectrometer and tunable nanosecond pulsed laser source, for performing fully automated photodissociation action spectroscopy on gas-phase ions. The components of the instrumentation are outlined, including the optical and electronic interfacing, in addition to the control software for automating the experiment and performing online analysis of the spectra. To demonstrate the utility of this ensemble, the photodissociation action spectra of 4-chloroanilinium, 4-bromoanilinium, and 4-iodoanilinium cations are presented and discussed. Multiple photoproducts are detected in each case and the photoproduct yields are followed as a function of laser wavelength. It is shown that the wavelength-dependent partitioning of the halide loss, H loss, and NH3 loss channels can be broadly rationalized in terms of the relative carbon-halide bond dissociation energies and processes of energy redistribution. The photodissociation action spectrum of (phenyl)Ag2 (+) is compared with a literature spectrum as a further benchmark.