Autocatalysis in Eschenmoser Coupling Reactions.

The Eschenmoser coupling reaction (ECR) of thioamides with electrophiles is believed to proceed via thiirane intermediates. However, little is known about converting the intermediates into ECR products. Previous mechanistic studies involved external thiophiles to remove the sulfur atom from the intermediates. In this work, an ECR proceeding without any thiophilic agent or base is studied by electrospray ionization-mass spectrometry. ESI-MS enables the detection of the so-far elusive polysulfide species Sn , with n ranging from 2 to 16 sulfur atoms, proposed to be the key species leading to product formation. Integrating observations from ion mobility spectrometry, ion spectroscopy, and reaction monitoring via flow chemistry coupled with mass spectrometry provides a comprehensive understanding of the reaction mechanism and uncovers the autocatalytic nature of the ECR reaction.

Kinetic enantio-recognition of chiral viologen guests by planar-chiral porphyrin cages.

The kinetic enantio-recognition of chiral viologen guests by planar-chiral porphyrin cage compounds, measured in terms of ΔΔG‡on, is determined by the planar-chirality of the host and influenced by the size, as measured by ion mobility-mass spectrometry, but not the chirality of its substituents.

Kinetics of ligand exchange in solution: a quantitative mass spectrometry approach.

Complex speciation and exchange kinetics of labile ligands are critical parameters for understanding the reactivity of metal complexes in solution. We present a novel approach to determine ligand exchange parameters based on electrospray ionization mass spectrometry (ESI-MS). The introduction of isotopically labelled ligands to a solution of metal host and unlabelled ligands allows the quantitative investigation of the solution-phase equilibria. Furthermore, ion mobility separation can target individual isomers, such as ligands bound at specific sites. As a proof of concept, we investigate the solution equilibria of labile pyridine ligands coordinated in the cavity of macrocyclic porphyrin cage complexes bearing diamagnetic or paramagnetic metal centres. The effects of solvent, porphyrin coordination sphere, transition metal, and counterion on ligand dissociation are discussed. Rate constants and activation parameters for ligand dissociation in the solution can be derived from our ESI-MS approach, thereby providing mechanistic insights that are not easily obtained from traditional solution-phase techniques.

Gas-phase structure of polymer ions: Tying together theoretical approaches and ion mobility spectrometry.

An increasing number of studies take advantage of ion mobility spectrometry (IMS) coupled to mass spectrometry (IMS-MS) to investigate the spatial structure of gaseous ions. Synthetic polymers occupy a unique place in the field of IMS-MS. Indeed, due to their intrinsic dispersity, they offer a broad range of homologous ions with different lengths. To help rationalize experimental data, various theoretical approaches have been described. First, the study of trend lines is proposed to derive physicochemical and structural parameters. However, the evaluation of data fitting reflects the overall behavior of the ions without reflecting specific information on their conformation. Atomistic simulations constitute another approach that provide accurate information about the ion shape. The overall scope of this review is dedicated to the synergy between IMS-MS and theoretical approaches, including computational chemistry, demonstrating the essential role they play to fully understand/interpret IMS-MS data.

Mechanistic Studies on the Epoxidation of Alkenes by Macrocyclic Manganese Porphyrin Catalysts.

Macrocyclic metal porphyrin complexes can act as shape-selective catalysts mimicking the action of enzymes. To achieve enzyme-like reactivity, a mechanistic understanding of the reaction at the molecular level is needed. We report a mechanistic study of alkene epoxidation by the oxidant iodosylbenzene, mediated by an achiral and a chiral manganese(V)oxo porphyrin cage complex. Both complexes convert a great variety of alkenes into epoxides in yields varying between 20-88 %. We monitored the process of the formation of the manganese(V)oxo complexes by oxygen transfer from iodosylbenzene to manganese(III) complexes and their reactivity by ion mobility mass spectrometry. The results show that in the case of the achiral cage complex the initial iodosylbenzene adduct is formed on the inside of the cage and in the case of the chiral one on the outside of the cage. Its decomposition leads to a manganese complex with the oxo ligand on either the inside or outside of the cage. These experimental results are confirmed by DFT calculations. The oxo ligand on the outside of the cage reacts faster with a substrate molecule than the oxo ligand on the inside. The results indicate how the catalytic activity of the macrocyclic porphyrin complex can be tuned and explain why the chiral porphyrin complex does not catalyze the enantioselective epoxidation of alkenes.

MLCT Excited-State Behavior of Trinuclear Ruthenium(II) 2,2'-Bipyridine Complexes.

Four trinuclear ruthenium(II) polypyridyl complexes were synthesized, and a detailed investigation of their excited-state properties was performed. The tritopic sexi-pyridine bridging ligands were obtained via para or meta substitution of a central 2,2'-bipyridine fragment. A para connection between the 2,2'-bipyridine chelating moieties of the bridging ligand led to a red-shifted MLCT absorption band in the visible part of the spectra, whereas the meta connection induced a broadening of the LC transitions in the UV region. A convergent energy transfer from the two peripheral metal centers to the central Ru(II) moiety was observed for all trinuclear complexes. These complexes were in thermal equilibrium with an upper-lying 3MLCT excited state over the investigated range of temperatures. For all complexes, deactivation via the 3MC excited state was absent at room temperature. Importantly, the connection in the para position for both central and peripheral 2,2'-bipyridines of the bridging ligand resulted in a trinuclear complex (Tpp) that absorbed more visible light, had a longer-lived excited state, and had a higher photoluminescence quantum yield than the parent [Ru(bpy)3]2+, despite its red-shifted photoluminescence. This behavior was attributed to the presence of a highly delocalized excited state for Tpp.

Assessing the Structural Heterogeneity of Isomeric Homo and Copolymers: an Approach Combining Ion Mobility Mass Spectrometry and Molecular Dynamics Simulations.

Synthetic polymers occupy a unique place in the field of ion mobility mass spectrometry (IMS-MS). Indeed, due to their intrinsic dispersity, they have the asset to offer a broad range of homologous ions with different lengths that can be detected in several charge states. In addition, the gas-phase structure of polymer ions mostly depends on their ability to screen the adducted charges. Several works dealing with linear, cyclic, and star-shaped polymers have already shown that the gas-phase structure of polymer ions heavily relies on the polymer architecture, i.e., the primary structure. In the present work, we move a step further by evaluating whether a relationship exists between the primary and secondary structures of synthetic homo and copolymers. The IMS-MS experiments will be further complemented by MD simulations. To highlight the effectiveness of IMS separation, we selected isomeric homo and copolymers made of lactide (LA) and propiolactone (PL) units. In this way, the mass analysis becomes useless since isomeric comonomer sequences can coexist for any given chain length. An UPLC method was implemented in the workflow to successfully separate all PL-LA comonomer sequences before infusion in the IMS-MS instrument. The analysis of doubly charged copolymers showed that the comonomer sequence has an impact on the IMS response. However, this only holds for copolymer ions with precise sizes and charge states, and this is therefore not a rule of thumb.

Efficient Convergent Energy Transfer in a Stereoisomerically Pure Heptanuclear Luminescent Terpyridine-Based Ru(II)-Os(II) Dendrimer.

The stereoisomerically pure synthesis of a novel heptanuclear Ru(II)-Os(II) antenna bearing multitopic terpyridine ligands is reported. An unambiguous structural characterization was obtained by 1H NMR spectroscopy and ion mobility spectrometry (IMS-MS). The heptanuclear complex exhibits large molar absorption coefficients (77900 M-1 cm-1 at 497 nm) and undergoes unitary, downhill, convergent energy transfer from the peripheral Ru(II) subunits to the central Os(II) that displays photoluminescence with a lifetime (τ = 161 ns) competent for diffusional excited-state electron transfer reactivity in solution.

Effects of electrospray mechanisms and structural relaxation on polylactide ion conformations in the gas phase: insights from ion mobility spectrometry and molecular dynamics simulations.

Recent advances in molecular dynamics (MD) simulations have made it possible to examine the behavior of large charged droplets that contain analytes such as proteins or polymers, thereby providing insights into electrospray ionization (ESI) mechanisms. In the present study, we use this approach to investigate the release of polylactide (PLA) ions from water/acetonitrile ESI droplets. We found that cationized gaseous PLA ions can be formed via various competing pathways. Some MD runs showed extrusion and subsequent separation of polymer chains from the droplet, as envisioned by the chain ejection model (CEM). On other occasions the PLA chains remained inside the droplets and were released after solvent evaporation to dryness, consistent with the charge residue model (CRM). Following their release from ESI droplets, the nascent gaseous PLA ions were subjected to structural relaxation for several µs in vacuo. The MD conformations generated in this way for various PLA charge states compared favorably to experimental results obtained by ion mobility spectrometry-mass spectrometry (IMS-MS). The structures of all PLA ions evolved during relaxation in the gas phase. However, some macroion species retained features that resembled their nascent structures. For this subset of ions, the IMS-MS response appears to be strongly correlated with the ESI release mechanism (CEM vs. CRM). The former favored extended structures, whereas the latter preferentially generated compact conformers.

Limitations of ion mobility spectrometry-mass spectrometry for the relative quantification of architectural isomeric polymers: A case study.

Since their discovery, cyclic polymers have attracted great interest because of their unique properties. Today, the preparation of these macrocyclic structures still remains a challenge for polymer chemists, and most of the preparation pathways lead to an inescapable contamination by linear by-products. As the properties of the polymers are closely related to their structure, it is of prime importance to be able to assess the architectural purity of a sample. METHODS: In this work, the suitability of ion mobility spectrometry-mass spectrometry (IMS-MS) for the quantification of two isomers was investigated. A cyclic poly(L-lactide) was prepared through photodimerization of its linear homologue. Since IMS-MS can be used to differentiate cyclic polymer ions from their linear analogues because of their more compact three-dimensional conformation, the present work envisaged the use of IMS-MS for the quantification of residual linear polymers within the cyclic polymer sample. RESULTS: Using the standard addition method to plot calibration curves, the fraction of linear contaminants in the sample was determined. By doing so, unrealistically high values of contamination were measured. CONCLUSIONS: These results were explained by an ionization efficiency issue. This work underlines some intrinsic limitations when using IMS-MS in the context of the relative quantification of isomers having different ionization efficiencies. Nevertheless, the linear-to-cyclic ratio can be roughly estimated by this method.

Charging and supercharging of proteins for mass spectrometry: recent insights into the mechanisms of electrospray ionization.

Electrospray ionization (ESI) is an essential technique for transferring proteins from solution into the gas phase for mass spectrometry and ion mobility spectrometry. The mechanisms whereby [M + zH]z+ protein ions are released from charged nanodroplets during ESI have been controversial for many years. Here we discuss recent computational and experimental studies that have shed light on many of the mysteries in this area. Four types of protein ESI experiments can be distinguished, each of which appears to be associated with a specific mechanism. (i) Native ESI proceeds according to the charged residue model (CRM) that entails droplet evaporation to dryness, generating compact protein ions in low charge states. (ii) Native ESI supercharging is also a CRM process, but the dried-out proteins accumulate additional charge because supercharging agents such as sulfolane interfere with the ejection of small ions (Na+, NH4+, etc.) from the shrinking droplets. (iii) Denaturing ESI follows the chain ejection model (CEM), where protein ions are gradually expelled from the droplet surface. H+ equilibration between the droplets and the protruding chains culminates in highly charged gaseous proteins, analogous to the collision-induced dissociation of multi-protein complexes. (iv) Denatured ESI supercharging also generates protein ions via the CEM. Supercharging agents stabilize protonated sites on the protein tail via charge-dipole interactions, causing the chain to acquire additional charge. There will likely be scenarios that fall outside of these four models, but it appears that the framework outlined here covers most of the experimentally relevant conditions.

A New Class of Rigid Multi(azobenzene) Switches Featuring Electronic Decoupling: Unravelling the Isomerization in Individual Photochromes.

We report a novel class of star-shaped multiazobenzene photoswitches comprising individual photochromes connected to a central trisubstituted 1,3,5-benzene core. The unique design of such C3-symmetric molecules, consisting of conformationally rigid and pseudoplanar scaffolds, made it possible to explore the role of electronic decoupling in the isomerization of the individual azobenzene units. The design of our tris-, bis-, and mono(azobenzene) compounds limits the π-conjugation between the switches belonging to the same molecule, thus enabling the efficient and independent isomerization of each photochrome. An in-depth experimental insight by making use of different complementary techniques such as UV-vis absorption spectroscopy, high performance liquid chromatography, and advanced mass spectrometry methods as ion mobility revealed an almost complete absence of electronic delocalization. Such evidence was further supported by both experimental (electrochemistry, kinetical analysis) and theoretical (DFT calculations) analyses. The electronic decoupling provided by this molecular design guarantees a remarkably efficient photoswitching of all azobenzenes, as evidenced by their photoisomerization quantum yields, as well as by the Z-rich UV photostationary states. Ion mobility mass spectrometry was exploited for the first time to study multiphotochromic compounds revealing the occurrence of a large molecular shape change in such rigid star-shaped azobenzene derivatives. In view of their high structural rigidity and efficient isomerization, our multiazobenzene photoswitches can be used as key components for the fabrication of complex stimuli-responsive porous materials.

One Step Further in the Characterization of Synthetic Polymers by Ion Mobility Mass Spectrometry: Evaluating the Contribution of End-groups.

Several families of polymers possessing various end-groups are characterized by ion mobility mass spectrometry (IMMS). A significant contribution of the end-groups to the ion collision cross section (CCS) is observed, although their role is neglected in current fitting models described in literature. Comparing polymers prepared from different synthetic procedures might thus, be misleading with the current theoretical treatments. We show that this issue is alleviated by comparing the CCS of various polymer ions (polyesters and polyethers) as a function of the number of atoms in the macroion instead of the usual representation involving the degree of polymerization. Finally, we extract the atom number density from the spectra which gives us the possibility to evaluate the compaction of polymer ions, and by extension to discern isomeric polymers.

Electrospray Ionization of Polypropylene Glycol: Rayleigh-Charged Droplets, Competing Pathways, and Charge State-Dependent Conformations.

Recent molecular dynamics (MD) simulations from various laboratories have advanced the general understanding of electrospray ionization (ESI)-related processes. Unfortunately, computational cost has limited most of those previous endeavors to ESI droplets with radii of ∼3 nm or less, which represent the low end of the size distribution in the ESI plume. The current work extends this range by conducting simulations on aqueous ESI droplets with radii of 5.5 nm (∼23 000 water molecules). Considering that computational cost increases with r6, this is a significant step forward. We focused on the ESI process for polypropylene glycol (PPG) which is a common ESI-MS calibrant. Different chain lengths (PPG10, 30, and 60) were tested in droplets that were charged with excess Na+. Solvent evaporation and Na+ ejection, with occasional progeny droplet formation, kept the systems at 80-100% of the Rayleigh limit throughout their life cycle. PPG chains migrated to the droplet surface where they captured Na+ via binding to ether oxygens. Various possible pathways for PPG release into the gas phase were encountered. Some PPG10 runs showed ejection from the droplet surface, consistent with the ion evaporation model (IEM). In other instances, PPG was released after near-complete solvent evaporation, as envisioned by the charged residue model (CRM). A third avenue was the partial separation from the droplet to form double or single-tailed structures, with subsequent chain detachment from the droplet. This last pathway is consistent with the chain ejection model (CEM). Immediately after detachment many chains were electrostatically stretched, but they subsequently collapsed into compact conformers. Extended structures were retained only for the most highly charged ions. Our simulations were complemented by ESI-MS and ion mobility measurements. MD-predicted charge states and collision cross sections were in agreement with these experimental data, supporting the mechanistic insights obtained.

Chain Ejection Model for Electrospray Ionization of Unfolded Proteins: Evidence from Atomistic Simulations and Ion Mobility Spectrometry.

The ion evaporation model (IEM) and the charged residue model (CRM) represent cornerstones of any discussion related to the mechanism of electrospray ionization (ESI). Molecular dynamics (MD) simulations have confirmed that small ions such as Na+ are ejected from the surface of aqueous ESI droplets (IEM), while folded proteins in native ESI are released by water evaporation to dryness (CRM). ESI of unfolded proteins yields [M + zH] z+ ions that are much more highly charged than their folded counterparts. A chain ejection model (CEM) has been proposed to account for the protein ESI behavior under such non-native conditions (Konermann, L., et al. Anal. Chem. 2013, 85, 2-9). The CEM envisions that unfolded proteins are driven to the droplet surface by hydrophobic and electrostatic factors, followed by gradual ejection via intermediates where droplets carry extended protein tails. Thus far, it has not been possible to support the CEM through MD simulations using realistic protein models and atomistic force fields. Such endeavors require much larger droplets than in previous MD studies. Also, the incorporation of CEM-related H+ migration is difficult. This work overcomes these challenges in MD simulations on unfolded apo-myoglobin (aMb) in droplets with a 5.5 nm radius (∼22500 water molecules). We focused on solutions at pH ∼4 where the aMb solution charge coincides with the charge on some of the electrosprayed ions (22+ to 27+), such that H+ migration could be neglected. Na+ ions were added to ensure a droplet charge close to the Rayleigh limit. We found that 16 of 17 MD runs on various protonation patterns produced [M + zH] z+ ions via chain ejection. The predicted stretched-out aMb conformations were consistent with experimental collision cross sections. These results support the view that unfolded proteins follow the CEM. Overall, the IEM/CRM/CEM triad can account for a wide range of ESI scenarios involving various types of analytes.

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.