Metal nanoclusters: from fundamental aspects to electronic properties and optical applications

Monolayer-protected noble metal clusters, also called nanoclusters, can be produced with the atomic precision and in large-scale quantity and are playing an increasingly important role in the field of nanoscience. To outline the origin and the perspectives of this new field, we overview the main results obtained on free metal clusters produced in gas phase including mainly electronic properties, the giant atom concept, the optical properties, briefly the role of the metal atom (alkali, divalent, noble metal) and finally the atomic structure of clusters. We also discuss the limitations of the free clusters. Then, we describe the field of monolayer-protected metal clusters, the main results, the new offered perspectives, the added complexity, and the role of the ligand beyond the superatom concept.

High performance {Nb5TaX12}@PVP (X = Cl, Br) cluster-based nanocomposites coatings for solar glazing applications

The development of highly ultraviolet (UV) and near-infrared (NIR) absorbent transparent coatings is an important enabling technology and area of research for environmental sustainability and energy conservation. Different amounts of K₄[{Nb₅TaXⁱ ₁₂}X^a ₆] cluster compounds (X = Cl, Br) dispersed into polyvinylpyrrolidone matrices were prepared by a simple, nontoxic and low-cost wet chemical method. The resulting solutions were used to fabricate visibly transparent, highly UV and NIR absorbent coatings by drop casting. The properties of the solution and films were investigated by complementary techniques (optical absorption, electrospray ionization mass spectrometry and Raman spectroscopy). The UV and NIR absorption of such samples strongly depended on the concentration, dispersion and oxidation state of the [{Nb₅TaXⁱ ₁₂}X^a ₆] nanocluster-based units. By varying and controlling these parameters, a remarkable improvement of the figures of merit T_L/T_E and S_NIR for solar-glazing applications was achieved compared to the previous results on nanocomposite coatings based on metal atom clusters.

Archaeological Mortar Characterization Using Laser-Induced Breakdown Spectroscopy (LIBS) Imaging Microscopy

Lime mortar is a complex mixture resulting from hardening of lime, water, and aggregates. Lime mortar was used from the time of the Roman Empire until the Industrial Revolution. The recipes used differ according to the period, geographical area of preparation, craftsman, or function. This is why the study of archaeological mortars is of such great importance in building archaeology. In this study, we used laser-induced breakdown spectroscopy (LIBS) to characterize the elemental composition of three lime mortar samples with a µ-LIBS instrument, allowing elemental image compilation. These samples originate from three different geographical locations: Angers (France), Dardilly (France), and Pompeii (Italy), and were taken from buildings that had different functions: cathedral, aqueduct, and house, respectively. Thanks to image processing and the creation of masks, it was possible to extract not only the lime signature and nature of the aggregate but also its granulometry and circularity. All this information is essential for cultural heritage research. This study shows the potential of the LIBS technique in archaeometric analysis of archaeological mortars.

Nanoarchitectonics of Glass Coatings for Near-Infrared Shielding: From Solid-State Cluster-Based Niobium Chlorides to the Shaping of Nanocomposite Films

The high potential of [{Nb₆Clⁱ₁₂}L^a₆] cluster-based building blocks as near-infrared radiation blockers for energy saving applications is exposed in the present paper (i = inner edge-bridging ligand, a = apical ligand of the Nb₆; L = H₂O and/or Cl). To do so, a combined experimental and theoretical investigation of edge-bridged [{Nb₆Clⁱ₁₂}Cl^a_6-x(H₂O)_x]^m+/0/n- cluster unit series (x = 0, 4, 6; m = 2, 3, 4; n = 2, 3, 4) has been carried out. By using the K₄[{Nb₆Clⁱ₁₂}Cl^a₆] starting solid-state precursor, we explored the behavior of the [{Nb₆Clⁱ₁₂}Cl^a₆]^4- cluster unit during the different steps of its integration as a building block into a polyvinylpyrrolidone (PVP) matrix to form a glass coating composite denoted {Nb₆Clⁱ₁₂}^m+@PVP (m = 2 or 3). The optical, vibrational and redox properties [{Nb₆Clⁱ₁₂}Cl^a_6-x(H₂O)_x]^m+/0/n- building blocks have been interpreted with the support of electronic structure calculations and simulation of properties. The chemical modifications and oxidation properties have been identified and studied thanks to various techniques in solution. Combining Raman and ultraviolet-visible spectroscopies, electrochemistry, and quantum chemical simulations, we bring new knowledge to the understanding of the evolution of the properties of the [{Nb₆Clⁱ₁₂}Cl^a_6-x(H₂O)_x]^m+/0/n- cluster units as a function of the number of valence electron per cluster (VEC) and the nature of terminal ligands (x = 0, n = 4; x = 4, charge = 0; x = 6, m = 4). The fine understanding of the physical properties and vibrational fingerprints depending on the VEC and chemical modifications in solution are mandatory to master the processing of cluster-based building blocks for the controlled design and shaping of glass coating nanocomposites. On the basis of this acquired knowledge, [{Nb₆Clⁱ₁₂}Cl^a_6-x(H₂O)_x]^m+/0/n- building blocks were embedded in a PVP matrix. The resulting {Nb₆Clⁱ₁₂}²⁺@PVP nanocomposite film shows excellent ultraviolet (UV, 280-380 nm) and near-infrared (NIR, 780-1080 nm) blocking ability (>90%) and a highly visible light transmittance thanks to the controlled integration of the {Nb₆Clⁱ₁₂}²⁺ cluster core. The figures of merit (FOM) value of T_vis/T_sol (T_vis = visible transmittance and T_sol = solar transmittance) as well as the haze, clarity, and the NIR shielding values (S_NIR) were measured. After optimization of the integration process, a {Nb₆Clⁱ₁₂}²⁺@PVP nanocomposite on glass substrate has been obtained with a high FOM equal to 1.29. This high value places the transparent green olive {Nb₆Clⁱ₁₂}²⁺@PVP nanocomposites at the top system in the benchmark in the field of glass coating composites for energy-saving applications.

Photo-control of bimolecular reactions: reactivity of the long-lived Rhodamine 6G triplet excited state with ˙NO

Photo-chemistry provides a non-intuitive but very powerful way to probe kinetically limited, sometimes thermodynamically non-favored reactions and, thus, access highly specific products. However, reactivity in the excited state is difficult to characterize directly, due to short lifetimes and challenges in controlling the reaction medium. Among photo-activatable reagents, rhodamine dyes find widespread uses due to a number of favorable properties including their high absorption coefficient. Their readily adaptable synthesis allows development of tailor-made dyes for specific applications. Remarkably, few studies have directly probed the chemical reactivity of their triplet excited state. Here we present a new conceptual approach to examine the specific chemistry of the triplet excited state. We have developed a pump (488 nm) - probe (600 nm) strategy to examine the gas-phase lifetime and reactivity of the triplet cation of Rhodamine 6G (³Rh6G⁺) in an ion trap mass spectrometer. The confounding effects of solvent, aggregation and formation of other reactive intermediates is thus avoided allowing fundamental reactivity to be explored. In the presence, in the ion trap, of helium seeded with 1% of nitric oxide (˙NO) (∼ 60 ion/˙NO collisions per second), the triplet lifetime is shortened from 1.9 s to 0.7 s. Simultaneously, the reaction products [Rh6G-H]˙⁺ and [Rh6G-H + NO]⁺ are observed. Reaction of ³Rh6G⁺ with ˙NO₂ yields [Rh6G-H]˙⁺, [Rh6G-H + NO₂]⁺ and [Rh6G-2H]⁺. None of these products are observed for the singlet, ¹Rh6G⁺. DFT calculations suggest a stepwise mechanism only allowed from ³Rh6G⁺, in which H atom abstraction by ˙NO_x (x = 1 or 2) yields [Rh6G-H]˙⁺ which, then, reacts with another ˙NO_x molecule. This illustrates the power of light to initiate specific chemical reactions, and the relevance of gas-phase ion-molecule reaction approaches to understand stepwise reaction mechanism from specific excited states.

Revisiting properties of edge-bridged bromide tantalum clusters in the solid-state, in solution and vice versa: an intertwined experimental and modelling approach

Edge-bridged halide tantalum clusters based on the {Ta₆Br₁₂}⁴⁺ core have been the topic of many physicostructural investigations both in solution and in the solid-state. Despite a large number of studies, the fundamental correlations between compositions, local symmetry, electronic structures of [{Ta₆Brⁱ₁₂}L^a₆]^m+/n- cluster units (L = Br or H₂O, in solution and in the solid-state), redox states, and vibrational and absorption properties are still not well established. Using K₄[{Ta₆Brⁱ₁₂}Br^a₆] as a starting precursor (i: inner and a: apical), we have investigated the behavior of the [{Ta₆Brⁱ₁₂}Br^a₆]^4- cluster unit in terms of oxidation properties and chemical modifications both in solution (water and organic solvent) and after recrystallization. A wide range of experimental techniques in combination with quantum chemical simulations afford new data that allow the puzzling behavior of the cluster units in response to changes in their environment to be revealed. Apical ligands undergo changes like modifications of interatomic distances to complete substitutions in solution that modify noticeably the cluster physical properties. Changes in the oxidation state of the cluster units also occur, which modify significantly their physical properties, including optical properties, which can thus be used as fingerprints. A subtle balance exists between the number of substituted apical ligands and the cluster oxidation state. This study provides new information about the exact nature of the species formed during the transition from the solid-state to solutions and vice versa. This shows new perspectives on optimization protocols for the design of Ta₆ cluster-based materials.

Exploring Conformational Landscapes Using Trap and Release Tandem Ion Mobility Spectrometry

The dynamics and thermodynamics of structural changes in isolated glu-fibrinopeptide B (GluFib) were investigated by tandem ion mobility spectrometry (IMS). Doubly protonated GluFib²⁺ ions were first selected by IMS and then stored for a controlled duration in a thermalized ion trap. Temperature-induced conformational changes were finally monitored by IMS as a function of trapping time. Based on this procedure, isomerization rates and equilibrium populations of the different conformers were determined as a function of temperature. We demonstrate that the measured thermodynamic quantities can be directly compared to simulated observables from ensemble molecular modeling based on appropriate order parameters. We obtained good qualitative agreement with replica-exchange molecular dynamics simulations based on the AMOEBA force field and processed using the weighted histogram analysis method. This suggests that the balance between Coulomb repulsion and optimal charge solvation is the main source of the observed conformational bistability. Our results emphasize the differences between the kinetically driven quasi-equilibrium distributions obtained after collisional activation and the thermodynamically driven distributions from the present equilibrium experiments due to entropic effects. As a consequence, our measurements not only allow straightforward determination of Arrhenius activation energies but also yield the relative enthalpy and entropy changes associated to a structural transition.

How Spherical Are Gaseous Low Charged Dendrimer Ions: A Molecular Dynamics/Ion Mobility Study?

The globular shape of gaseous ions, resulting from the ionization of large molecules such as polymers and proteins, is a recurring subject that has undergone a renewed interest with the advent of ion mobility spectrometry (IMS), especially in conjunction with theoretical chemistry techniques such as Molecular Dynamics (MD). Globular conformations result from a fine balance between entropy and enthalpy considerations. For multiply charged ions isolated in the gas phase of a mass spectrometer, the Coulombic repulsion between the different charges tends to prevent the ions from adopting a compact, and folded 3D structure. In the present paper, we closely associate data from IMS experiments and MD simulations to unambiguously access the conformations of dendrimer ions in the gas phase with special attention paid to the dendrimer structure, the generation, and the charge state. By doing so, we here combine a set of structural tools able to evaluate the (non)globular shape of ions based on both experimental and theoretical results.

Kinetic study of azobenzene E/Z isomerization using ion mobility-mass spectrometry and liquid chromatography-UV detection

Z and E azobenzene isomers are molecular switches which can interconvert both photochemically and thermally. Presently, we studied a ketal-substituted bridged azobenzene in which two stable diastereomeric conformers (Z1 and Z2) photochemically interconvert through the transient E isomer. UV-VIS absorption spectroscopy is commonly used to study the relaxation kinetics of azobenzenes, but it does not allow direct quantitation of the process in this case. In the present paper, liquid chromatography coupled to UV detection (LC-UV) and ion mobility-mass spectrometry (IMS-MS) were combined to study the thermal back relaxation kinetics of the E isomer. LC separation of the three isomers was achieved in less than 10 minutes, allowing the characterization of the relatively slow thermal back relaxation kinetics at low temperature through UV detection. In addition, the faster E→Z thermal back relaxation at higher temperature was studied using IMS-MS, which allows shorter timescale separation than LC. Baseline separation of the two Z isomers was achieved in IMS-MS for [Z + Ag]⁺ ions, and their gas-phase conformations were determined by IRMPD experiments. Both IMS-MS and LC-UV methodologies succeeded to study the E→Z thermal back relaxation kinetics, and appeared to be complementary techniques. We show that the combination of the two techniques allows the characterization of the isomerization processes over a broad temperature range, and the determination of the associated thermodynamic observables.

Second harmonic scattering from mass characterized 2D graphene oxide sheets

In this communication, we report the second harmonic scattering from mass characterized 2D graphene oxide sheets dispersed in an aqueous suspension, in the femtosecond regime at 800 nm laser excitation. Charge-detection mass-spectrometry, performing at the single sheet level, allows for an exhaustive molar mass distribution and thus concentration for these 2D nanomaterials samples. The orientation-averaged hyperpolarizability value is (1.36 ± 0.15) × 10^-25 esu as determined by the concentration-dependent harmonic scattering signal. In addition, the multi-photon excited fluorescence spectrum is characterized by a broad band in the visible range between 350 and 700 nm centered at about 500 nm.

Secondary structure effects on internal proton transfer in poly-peptides

A pump-probe approach was designed to determine the internal proton transfer (PT) rate in a series of poly-peptide radical cations containing both histidine and tryptophan. The proton transfer is driven by the gas-phase basicity difference between residues. The fragmentation scheme indicates that the gas-phase basicity of histidine is lower than that of radical tryptophan so that histidine is always pulling the proton away from tryptophan. However, the proton transfer requires the two basic sites to be in close proximity, which is rate limited by the peptide conformational dynamics. PT rate measurements were used to probe and explore the peptide conformational dynamics in several poly-glycines/prolines/alanines. For small and unstructured peptides, the PT rate decreases with the size, as expected from a statistical point of view in a flat conformational space. Conversely, if structured conformations are accessible, the structural flexibility of the peptide is decreased. This slows down the occurrence of conformations favorable to proton transfer. A dramatic decrease in the PT rates was observed for peptides HAnW, when n changes from 5 to 6. This is attributed to the onset of a stable helix for n = 6. No such discontinuity is observed for poly-glycines or poly-prolines. In HAnW, the gas-phase basicity and helix propensity compete for the position of the charge. Interestingly, in this competition between PT and helix formation in HA6W, the energy gain associated with helix formation is large enough to slow down the PT beyond experimental time but does not ultimately prevail over the proton preference for histidine.

Ion mobility resolved photo-fragmentation to discriminate protomers

RATIONALE

Among the sources of structural diversity in biomolecular ions, the co-existence of protomers is particularly difficult to take into account, which in turn complicates structural interpretation of gas-phase data.

METHODS

We investigated the sensitivity of gas-phase photo-fragmentation measurements and ion mobility spectrometry (IMS) to the protonation state of a model peptide derivatized with chromophores. Accessible interconversion pathways between the different identified conformers were probed by tandem ion mobility measurement. Furthermore, the excitation coupling between the chromophores has been probed through photo-fragmentation measurements on mobility-selected ions. All results were interpreted based on molecular dynamics simulations.

RESULTS

We show that protonation can significantly affect the photo-fragmentation yields. Especially, conformers with very close collision cross sections (CCSs) may display dramatically different photo-fragmentation yields in relation with different protonation patterns.

CONCLUSIONS

We show that, even if precise structure assignment based on molecular modeling is in principle difficult for large biomolecular assemblies, the combination of photo-fragmentation and IMS can help to identify the signature of protomer co-existence for a population of biomolecular ions in the gas phase. Such spectroscopic data are particularly suitable to follow conformational changes.

Catenane Structures of Homoleptic Thioglycolic Acid-Protected Gold Nanoclusters Evidenced by Ion Mobility-Mass Spectrometry and DFT Calculations

Thiolate-protected metal nanoclusters have highly size- and structure-dependent physicochemical properties and are a promising class of nanomaterials. As a consequence, for the rationalization of their synthesis and for the design of new clusters with tailored properties, a precise characterization of their composition and structure at the atomic level is required. We report a combined ion mobility-mass spectrometry approach with density functional theory (DFT) calculations for determination of the structural and optical properties of ultra-small gold nanoclusters protected by thioglycolic acid (TGA) as ligand molecules, Au₁₀(TGA)₁₀. Collision cross-section (CCS) measurements are reported for two charge states. DFT optimized geometrical structures are used to compute CCSs. The comparison of the experimentally- and theoretically-determined CCSs allows concluding that such nanoclusters have catenane structures.

Sub-100 nanometer silver doped gold–cysteine supramolecular assemblies with enhanced nonlinear optical properties

The ability of gold(i) thiolates to self-assemble into supramolecular architectures opens the route for a new class of nanomaterials with a unique structure–optical property relationship.

Isomeric Effect of Mercaptobenzoic Acids on the Synthesis, Stability, and Optical Properties of Au25(MBA)18 Nanoclusters

We report a simple size focusing, two-step "bottom-up" protocol to prepare water-soluble Au₂₅(MBA)₁₈ nanoclusters, using the three isomers of mercaptobenzoic acids (p/m/o-MBA) as capping ligands and Me₃NBH₃ as a mild reducing agent. The relative stability of the gas-phase multiply deprotonated Au₂₅(MBA)₁₈ ions was investigated by collision-induced dissociation. This permitted us to evaluate the possible isomeric effect on the Au-S interfacial bond stress. We also investigated their optical properties. The absorption spectra of Au₂₅(MBA)₁₈ isomers were very similar and showed bands at 690, 470, and 430 nm. For all Au₂₅(MBA)₁₈ isomeric clusters, no measurable one-photon excited fluorescence under UV-vis light was found, in neither solid- nor solution-state. The two-photon excited emission spectra and first hyperpolarizabilities of the clusters were also determined. The results are discussed in terms of the possible isomeric effect on excitations within the metal core and the possibility of charge transfer excitations from the ligands to the metal nanocluster.

Isolated Collagen Mimetic Peptide Assemblies Have Stable Triple-Helix Structures

The origin of the triple-helix structure and high stability of collagen has been debated for many years. As models of the triple helix and building blocks for new biomaterials, collagen mimetic peptide (CMP) assemblies have been deeply studied in the condensed phase. In particular, it was found that hydroxylation of proline, an abundant post-translational modification in collagen, increases its stability. Two main hypotheses emerged to account for this behavior: 1) intra-helix stereoelectronic effects, and 2) the role of water molecules H-bound to hydroxyproline side-chains. However, in condensed-phase investigations, the influence of water cannot be fully removed. Therefore, we employed a combination of tandem ion mobility and mass spectrometries to assess the structure and stability of CMP assemblies in the gas phase. These results show a conservation of the structure and stability properties of triple helix models in the absence of solvent, supporting an important role of stereoelectronic effects. Moreover, evidence that small triple helix assemblies with controlled stoichiometry can be studied in the gas phase is given, which opens new perspectives in the understanding of the first steps of collagen fiber growth.

Radical Anions of Oxidized vs. Reduced Oxytocin: Influence of Disulfide Bridges on CID and Vacuum UV Photo-Fragmentation

The nonapeptide oxytocin (OT) is used as a model sulfur-containing peptide to study the damage induced by vacuum UV (VUV) radiations. In particular, the effect of the presence (or absence in reduced OT) of oxytocin's internal disulfide bridge is evaluated in terms of photo-fragmentation yield and nature of the photo-fragments. Intact, as well as reduced, OT is studied as dianions and radical anions. Radical anions are prepared and photo-fragmented in two-color experiments (UV + VUV) in a linear ion trap. VUV photo-fragmentation patterns are analyzed and compared, and radical-induced mechanisms are proposed. The effect of VUV is principally to ionize but secondary fragmentation is also observed. This secondary fragmentation seems to be considerably enabled by the initial position of the radical on the molecule. In particular, the possibility to form a radical on free cysteines seems to increase the susceptibility to VUV fragmentation. Interestingly, disulfide bridges, which are fundamental for protein structure, could also be responsible for an increased resistance to ionizing radiations. Graphical Abstract.

Photo-induced linkage isomerization in the gas phase probed by tandem ion mobility and laser spectroscopy

Ruthenium complexes involving sulfoxide ligands can undergo linkage isomerization upon light absorption, accompanied by dramatic changes in their optical properties. These remarkable photochromic properties are sensitive to the nature of the ligand as well as to that of the solvent. We used tandem ion mobility spectrometry coupled to mass spectrometry to gain direct experimental insight into the isomerization pathways connecting the different linkage isomers of an isolated ruthenium complex with two dimethyl-sulfoxide ligands. We find that the isomerization behavior of the solvent-free complex differs from that previously reported in the solution-phase, which is in line with recent theoretical predictions.

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.

Characterization of foreign materials in paraffin-embedded pathological specimens using in situ multi-elemental imaging with laser spectroscopy

Pathologists typically encounter many disparate exogenous materials in clinical specimens during their routine histopathological examinations, especially within the skin, lymph nodes, and lungs. These foreign substances may be free extracellular deposits or induce several clinical abnormalities or histopathological patterns. However, pathologists almost never investigate or report the chemical nature of exogenous metals in clinical specimens due to a lack of convenient and available technologies. In this paper, a novel strategy based on laser-induced breakdown spectroscopy (LIBS) technology is evaluated for in situ multi-elemental tissue imaging. The improved procedures allow visualization of the presence of chemical elements contained within paraffin-embedded specimens of medical interest with elemental images that are stackable with conventional histology images. We selected relevant medical situations for which the associated pathology reports were limited to the presence of lymphohistiocytic and inflammatory cells containing granules (a granuloma and a pseudolymphoma) or to lymph nodes or skin tissues containing pigments or foreign substances. Exogenous elements such as aluminum, titanium, copper, and tungsten were identified and localized within the tissues. The all-optical LIBS elemental imaging instrument that we developed is fully compatible with conventional optical microscopy used for pathology analysis. When combined with routine histopathological analysis, LIBS is a versatile technology that might help pathologists establish or confirm diagnoses for a wide range of medical applications, particularly when the nature of external agents present in tissues needs to be investigated.

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.

Mass and charge distributions of amyloid fibers involved in neurodegenerative diseases: mapping heterogeneity and polymorphism

Characterization by charge detection mass spectrometry of amyloid fibers involved in neurodegenerative diseases: Aβ peptide, tau and α-synuclein.

Heterogeneity and polymorphism are generic features of amyloid fibers with some important effects on the related disease development. We report here the characterization, by charge detection mass spectrometry, of amyloid fibers made of three polypeptides involved in neurodegenerative diseases: Aβ_1–42 peptide, tau and α-synuclein. Beside the mass of individual fibers, this technique enables to characterize the heterogeneity and the polymorphism of the population. In the case of Aβ_1–42 peptide and tau protein, several coexisting species could be distinguished and characterized. In the case of α-synuclein, we show how the polymorphism affects the mass and charge distributions.

Ultraviolet, Infrared, and High-Low Energy Photodissociation of Post-Translationally Modified Peptides

Mass spectrometry-based methods have made significant progress in characterizing post-translational modifications in peptides and proteins; however, certain aspects regarding fragmentation methods must still be improved. A good technique is expected to provide excellent sequence information, locate PTM sites, and retain the labile PTM groups. To address these issues, we investigate 10.6 μm IRMPD, 213 nm UVPD, and combined UV and IR photodissociation, known as HiLoPD (high-low photodissociation), for phospho-, sulfo-, and glyco-peptide cations. IRMPD shows excellent backbone fragmentation and produces equal numbers of N- and C-terminal ions. The results reveal that 213 nm UVPD and HiLoPD methods can provide diverse backbone fragmentation producing a/x, b/y, and c/z ions with excellent sequence coverage, locate PTM sites, and offer reasonable retention efficiency for phospho- and glyco-peptides. Excellent sequence coverage is achieved for sulfo-peptides and the position of the SO₃ group can be pinpointed; however, widespread SO₃ losses are detected irrespective of the methods used herein. Based on the overall performance achieved, we believe that 213 nm UVPD and HiLoPD can serve as alternative options to collision activation and electron transfer dissociations for phospho- and glyco-proteomics. Graphical Abstract ᅟ.

Supramolecular influence on cis-trans isomerization probed by ion mobility spectrometry

We used tandem ion mobility spectrometry measurements to investigate how the photo-isomerization of a chromophore (a methylpyridinium derivative) is affected by the complexation with a crown ether. A dramatic blue-shift of the photo-isomerization spectrum was observed upon complexation, which could be well reproduced by ab initio calculations. Our results support that the observed changes in the photo-physical properties of the chromophore originate from the charge-solvation of its pyridinium moiety by the host cage.

Combining Structural Probes in the Gas Phase - Ion Mobility-Resolved Action-FRET

In the context of native mass spectrometry, the development of gas-phase structural probes sensitive to the different levels of structuration of biomolecular assemblies is necessary to push forward conformational studies. In this paper, we provide the first example of the combination of ion mobility (IM) and Förster resonance energy transfer (FRET) measurements within the same experimental setup. The possibility to obtain mass- and mobility-resolved FRET measurements is demonstrated on a model peptide and applied to monitor the collision-induced unfolding of ubiquitin. Graphical Abstract ᅟ.

One-pot direct synthesis for multifunctional ultrasmall hybrid silica nanoparticles

A novel and simple route to synthesize ultrasmall silica nanoparticles having hydrodynamic diameters under 10 nm for imaging and therapeutic applications.

Comparison of Different Ion Mobility Setups Using Poly (Ethylene Oxide) PEO Polymers: Drift Tube, TIMS, and T-Wave

Over the years, polymer analyses using ion mobility-mass spectrometry (IM-MS) measurements have been performed on different ion mobility spectrometry (IMS) setups. In order to be able to compare literature data taken on different IM(-MS) instruments, ion heating and ion temperature evaluations have already been explored. Nevertheless, extrapolations to other analytes are difficult and thus straightforward same-sample instrument comparisons seem to be the only reliable way to make sure that the different IM(-MS) setups do not greatly change the gas-phase behavior. We used a large range of degrees of polymerization (DP) of poly(ethylene oxide) PEO homopolymers to measure IMS drift times on three different IM-MS setups: a homemade drift tube (DT), a trapped (TIMS), and a traveling wave (T-Wave) IMS setup. The drift time evolutions were followed for increasing polymer DPs (masses) and charge states, and they are found to be comparable and reproducible on the three instruments. ᅟ.

Fragmentation patterns of chromophore-tagged peptides in visible laser induced dissociation

RATIONALE

Tandem mass spectrometry (MS/MS) is the pivotal tool for protein structural characterization and quantification. Identification relies on the fragmentation step of tryptic peptides in bottom-up strategy. Specificity of fragmentation can be obtained using laser-induced dissociation (LID) in the visible range, after tagging of the targeted peptides with an adequate chromophore. Backbone fragmentation is required to obtain specific fragments and confident identification. We present herein a study of fragmentation patterns of chromophore-tagged peptides in LID, showing the potential of LID methodology to provide the maximum number of fragments for further identification and quantification.

METHODS

A total of 401 cysteine-containing tryptic peptides originating from the human proteome were derivatizated on the thiol group of cysteine with a Dabcyl maleimide chromophore, which has a high photo-absorption cross section at 473 nm. The derivatized peptides were then analyzed by LID at 473 nm on a Q Exactive instrument.

RESULTS

LID spectra present a characteristic fragment at m/z 252.112 for all precursors. This product ion arises from the internal dissociation of the Dabcyl chromophore. Several peptide-backbone fragment ions are also detected. Results show the quasi absence of fragmentation at the cysteine site. This indicates that part of the energy must be redistributed across the entire system despite excitation initially localized at the chromophore. Indeed, the fragmentation mainly occurs at 3 to 5 amino acids from the derivatized cysteine residue.

CONCLUSIONS

LID of derivatized cysteine-containing peptides displays the initial fragmentation of the chromophore. As energy is redistributed all along the peptide sequence, fragmentation of the peptide backbone is also observed. Thus, LID of chromophore-tagged peptides produces adequate fragment ions, allowing both good sequence coverage for a greater confidence of identification, and a large choice of transitions for specific quantification.

Polymers for Traveling Wave Ion Mobility Spectrometry Calibration

One of the main issues when using traveling wave ion mobility spectrometry (TWIMS) for the determination of collisional cross-section (CCS) concerns the need for a robust calibration procedure built from referent ions of known CCS. Here, we implement synthetic polymer ions as CCS calibrants in positive ion mode. Based on their intrinsic polydispersities, polymers offer in a single sample the opportunity to generate, upon electrospray ionization, numerous ions covering a broad mass range and a large CCS window for different charge states at a time. In addition, the key advantage of polymer ions as CCS calibrants lies in the robustness of their gas-phase structure with respect to the instrumental conditions, making them less prone to collisional-induced unfolding (CIU) than protein ions. In this paper, we present a CCS calibration procedure using sodium cationized polylactide and polyethylene glycol, PLA and PEG, as calibrants with reference CCS determined on a home-made drift tube. Our calibration procedure is further validated by testing the polymer calibration to determine CCS of numerous different ions for which CCS are reported in the literature. Graphical Abstract ᅟ.

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.

Hydrogen-Induced Adsorption of Carbon Monoxide on the Gold Dimer Cation: A Joint Experimental and DFT Investigation

It is demonstrated, using tandem mass spectrometry and radio frequency ion trap, that the adsorption of a H atom on the gold dimer cation, Au₂H⁺, prevents its dissociation and allows for adsorption of CO. Reaction kinetics are measured by employing a radio frequency ion trap, where Au₂⁺ and CO interact for a given reaction time. The effect of a hydrogen atom is evaluated by comparing reaction rate constants measured for Au₂⁺ and Au₂H⁺. The theoretical results for the adsorption of CO molecules and their reaction characteristics with Au₂⁺ and Au₂H⁺ are found to agree with the experimental findings. The joint investigations provide insights into hydrogen atom adsorption effects and consequent reaction mechanisms.

Au10(SG)10: A Chiral Gold Catenane Nanocluster with Zero Confined Electrons. Optical Properties and First-Principles Theoretical Analysis

We report facile synthesis of the Au₁₀(SG)₁₀ nanoclusters, where SG stands for glutathione, found to be promising as a new class of radiosensitizers for cancer radiotherapy. The homoleptic catenane structure with two Au₅SG₅ interconnected rings, among different isomer structures, gives the best agreement between theoretical and experimental optical spectra and XRD patterns. This catenane structure exhibits a centrosymmetry-broken structure, resulting in enhanced second harmonic response and new characteristic circular dichroism signals in the spectral region of 250-400 nm. This is the first determination of the nonlinear optical properties of a ligated cluster with an equal Au-to-ligand ratio, thus without a metallic core and therefore zero confined electrons. Insight into the nonlinear and chiroptical efficiencies arising from interplay between structural and electronic properties is provided by the TD-DFT approach.

Correlation between the shape of the ion mobility signals and the stepwise folding process of polylactide ions

In the field of polymer characterization, the use of ion mobility mass spectrometry (IMMS) remains mainly devoted to the temporal separation of cationized oligomers according to their charge states, molecular masses and macromolecular architectures in order to probe the presence of different structures. When analyzing multiply charged polymer ions by IMMS, the most striking feature is the observation of breaking points in the evolution of the average collision cross sections with the number of monomer units. Those breaking points are associated to the folding of the polymer chain around the cationizing agents. Here, we scrutinize the shape of the arrival time distribution (ATD) of polylactide ions and associate the broadening as well as the loss of symmetry of the ATD signals to the coexistence of different populations of ions attributed to the transition from opened to folded stable structures. The observation of distinct distributions reveals the absence of folded/extended structure interconversion on the ion mobility time scale (1-10 ms) and then on the lifetime of ions within the mass spectrometer at room temperature. In order to obtain information on the possible interconversion between the different observed populations upon ion activation, we performed IM-IM-MS experiments (tandem ion mobility measurements). To do so, mobility-selected ions were activated by collisions before a second mobility measurement. Interestingly, the conversion by collisional activation from a globular structure into a (partially) extended structure, i.e. the gas phase unfolding of the ions, was not observed in the energetic regime available with the used experimental setup. The absence of folded/extended interconversion, even upon collisional activation, points to the fact that the polylactide ions are 'frozen' in their specific 3D structure during the desolvation/ionization electrospray processes. Copyright © 2017 John Wiley & Sons, Ltd.

Ligand-core NLO-phores: a combined experimental and theoretical approach to the two-photon absorption and two-photon excited emission properties of small-ligated silver nanoclusters

We report a combined experimental and theoretical study of the two-photon absorption and excited emission properties of monodisperse ligand stabilized Ag₁₁, Ag₁₅ and Ag₃₁ nanoclusters in aqueous solutions. The nanoclusters were synthesized using a cyclic reduction under oxidative conditions and separated by vertical gel electrophoresis. The two-photon absorption cross-sections of these protected noble metal nanoclusters measured within the biologically attractive 750-900 nm window are several orders of magnitude larger than that reported for commercially available standard organic dyes. The two-photon excited fluorescence spectra are also presented for excitation wavelengths within the same excitation spectral window. They exhibit size-tunability. Because the fundamental photophysical mechanisms underlying these multiphoton processes in ligand protected clusters with only a few metal atoms are not fully understood yet, a theoretical model is proposed to identify the key driving elements. Elements that regulate the dipole moments and the nonlinear optical properties are the nanocluster size, its structure and the charge distribution on both the metal core and the bound ligands. We coined this new class of NLO materials as "Ligand-Core" NLO-phores.

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.

Action-FRET of a Gaseous Protein

Mass spectrometry is an extremely powerful technique for analysis of biological molecules, in particular proteins. One aspect that has been contentious is how much native solution-phase structure is preserved upon transposition to the gas phase by soft ionization methods such as electrospray ionization. To address this question-and thus further develop mass spectrometry as a tool for structural biology-structure-sensitive techniques must be developed to probe the gas-phase conformations of proteins. Here, we report Förster resonance energy transfer (FRET) measurements on a ubiquitin mutant using specific photofragmentation as a reporter of the FRET efficiency. The FRET data is interpreted in the context of circular dichroism, molecular dynamics simulation, and ion mobility data. Both the dependence of the FRET efficiency on the charge state-where a systematic decrease is observed-and on methanol concentration are considered. In the latter case, a decrease in FRET efficiency with methanol concentration is taken as evidence that the conformational ensemble of gaseous protein cations retains a memory of the solution phase conformational ensemble upon electrospray ionization. Graphical Abstract ᅟ.

Correction: Supramolecular influence on cis–trans isomerization probed by ion mobility spectrometry

Correction for ‘Supramolecular influence on cis–trans isomerization probed by ion mobility spectrometry’ by Izabella Czerwinska et al., Phys. Chem. Chem. Phys., 2016, 18, 32331–32336.

Action-FRET of β-cyclodextrin inclusion complexes

Action-FRET is introduced as an original method to probe the structure of gaseous non-covalent complexes.