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

Dynamics of Rayleigh Fission Processes in ∼100 nm Charged Aqueous Nanodrops

Fission of micron-size charged droplets has been observed using optical methods, but little is known about fission dynamics and breakup of smaller nanosize droplets that are important in a variety of natural and industrial processes. Here, spontaneous fission of individual aqueous nanodrops formed by electrospray is investigated using charge detection mass spectrometry. Fission processes ranging from formation of just two progeny droplets in 2 ms to production of dozens of progeny droplets over 100+ ms are observed for nanodrops that are charged above the Rayleigh limit. These results indicate that Rayleigh fission is a continuum of processes that produce progeny droplets that vary widely in charge, mass, and number.

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.

Release of nanodiscs from charged nano-droplets in the electrospray ionization revealed by molecular dynamics simulations

Electrospray ionization (ESI) is essential for application of mass spectrometry in biological systems, as it prevents the analyte being split into fragments. However, due to lack of a clear understanding of the mechanism of ESI, the interpretation of mass spectra is often ambiguous. This is a particular challenge for complex biological systems. Here, we focus on systems that include nanodiscs as membrane environment, which are essential for membrane proteins. We performed microsecond atomistic molecular dynamics simulations to study the release of nanodiscs from highly charged nano-droplets into the gas phase, the late stage of ESI. We observed two distinct major scenarios, highlighting the diversity of morphologies of gaseous product ions. Our simulations are in reasonable agreement with experimental results. Our work provides a detailed atomistic view of the ESI process of a heterogeneous system (lipid nanodisc), which may give insights into the interpretation of mass spectra of all lipid-protein systems.

Investigation of Charge-State-Dependent Compaction of Protein Ions with Native Ion Mobility-Mass Spectrometry and Theory

The precise relationship between native gas-phase protein ion structure, charge, desolvation, and activation remains elusive. Much evidence supports the Charge Residue Model for native protein ions formed by electrospray ionization, but scaling laws derived from it relate only to overall ion size. Closer examination of drift tube CCSs across individual native protein ion charge state distributions (CSDs) reveals deviations from global trends. To investigate whether this is due to structure variation across CSDs or contributions of long-range charge-dipole interactions, we performed in vacuo force field molecular dynamics (MD) simulations of multiple charge conformers of three proteins representing a variety of physical and structural features: β-lactoglobulin, concanavalin A, and glutamate dehydrogenase. Results from these simulated ions indicate subtle structure variation across their native CSDs, although effects of these structural differences and long-range charge-dependent interactions on CCS are small. The structure and CCS of smaller proteins may be more sensitive to charge due to their low surface-to-volume ratios and reduced capacity to compact. Secondary and higher order structure from condensed-phase structures is largely retained in these simulations, supporting the use of the term "native-like" to describe results from native ion mobility-mass spectrometry experiments, although, notably, the most compact structure can be the most different from the condensed-phase structure. Collapse of surface side chains to self-solvate through formation of new hydrogen bonds is a major feature of gas-phase compaction and likely occurs during the desolvation process. Results from these MD simulations provide new insight into the relationship of gas-phase protein ion structure, charge, and CCS.

Electrohydrodynamic processing of natural polymers for active food packaging: A comprehensive review

The active packaging materials fabricated using natural polymers is increasing in recent years. Electrohydrodynamic processing has drawn attention in active food packaging due to its potential in fabricating materials with advanced structural and functional properties. These materials have the significant capability in enhancing food's quality, safety, and shelf-life. Through electrospinning and electrospray, fibers and particles are encapsulated with bioactive compounds for active packaging applications. Understanding the principle behind electrohydrodynamics provides fundamentals in modulating the material's physicochemical properties based on the operating parameters. This review provides a deep understanding of electrospray and electrospinning, along with their advantages and recent innovations, from food packaging perspectives. The natural polymers suitable for developing active packaging films and coatings through electrohydrodynamics are intensely focused. The critical properties of the packaging system are discussed with characterization techniques. Furthermore, the limitations and prospects for natural polymers and electrohydrodynamic processing in active packaging are summarized.

Resolving Isomers of Star-Branched Poly(Ethylene Glycols) by IMS-MS Using Multiply Charged Ions

Ion mobility spectrometry (IMS) mass spectrometry (MS) centers on the ability to separate gaseous structures by size, charge, shape, and followed by mass-to-charge (m/z). For oligomeric structures, improved separation is hypothesized to be related to the ability to extend structures through repulsive forces between cations electrostatically bonded to the oligomers. Here we show the ability to separate differently branched multiply charged ions of star-branched poly(ethylene glycol) oligomers (up to 2000 Da) regardless of whether formed by electrospray ionization (ESI) charged solution droplets or from charged solid particles produced directly from a surface by matrix-assisted ionization. Detailed structural characterization of isomers of the star-branched compositions was first established using a home-built high-resolution ESI IMS-MS instrument. The doubly charged ions have well-resolved drift times, achieving separation of isomers and also allowing differentiation of star-branched versus linear oligomers. An IMS-MS "snapshot" approach allows visualization of architectural dispersity and (im)purity of samples in a straightforward manner. Analyses capabilities are shown for different cations and ionization methods using commercially available traveling wave IMS-MS instruments. Analyses directly from surfaces using the new ionization processes are, because of the multiply charging, not only associated with the benefits of improved gas-phase separations, relative to that of ions produced by matrix-assisted laser desorption/ionization, but also provide the potential for spatially resolved measurements relative to ESI and other ionization methods.

Ion Formation from Rapidly Heated Aqueous Droplets by Droplet-Assisted Ionization

When aqueous droplets travel through a temperature-controlled capillary from atmospheric pressure into a vacuum, they undergo aerodynamic and/or thermal breakup to give charged progeny droplets that subsequently produce gas-phase molecular ions from solutes that were in the original droplets. This phenomenon is the basis of droplet-assisted ionization, a method that was recently developed for online characterization of aerosols by mass spectrometry. The conditions allowing initial droplets to break up into progeny droplets were studied by computational fluid dynamics (CFD) with a droplet evaporation model. The CFD results were then used to interpret experimental measurements of ion current vs capillary wall temperature. For capillary wall temperatures below about 150 °C, the abilities of droplets to undergo either aerodynamic or thermal breakup are strongly temperature dependent. Above this temperature, the mode of initial droplet breakup becomes temperature independent, and the temperature dependence of the ion signal intensity can be explained in relation to ion formation from charged progeny droplets. Activation energies for ion formation fall into two main categories: ∼41 kJ mol^-1 for droplets containing predominantly nonionic solutes, which matches the enthalpy of vaporization for water and suggests a charge residue process for ion formation, and ∼24 kJ mol^-1 for droplets containing salts, which suggests an ion evaporation process where the ion is ejected from the droplet surface within a cluster of solvent molecules.

Weighing synthetic polymers of ultra-high molar mass and polymeric nanomaterials: What can we learn from charge detection mass spectrometry?

Advances in soft ionization techniques for mass spectrometry (MS) of polymeric materials make it possible to determine the masses of intact molecular ions exceeding megadaltons. Interfacing MS with separation and fragmentation methods has additionally led to impressive advances in the ability to structurally characterize polymers. Even if the gap to the megadalton range has been bridged by MS for polymers standards, the MS-based analysis for more complex polymeric materials is still challenging. Charge detection mass spectrometry (CDMS) is a single-molecule method where the mass and the charge of each ion are directly determined from individual measurements. The entire molecular mass distribution of a polymer sample can be thus accurately measured. Described in this perspective paper is how molecular weight distribution as well as charge distribution can provide new insights into the structural and compositional studies of synthetic polymers and polymeric nanomaterials in the megadalton to gigadalton range of molecular weight. The recent multidimensional CDMS studies involving couplings with separation and dissociation techniques will be presented. And, finally, an outlook for the future avenues of the CDMS technique in the field of synthetic polymers of ultra-high molar mass and polymeric nanomaterials will be provided.

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

Gas-phase polymer ions may retain structural features associated with their electrospray formation mechanisms.

Computational Insights into Compaction of Gas-Phase Protein and Protein Complex Ions in Native Ion Mobility-Mass Spectrometry

Native ion mobility-mass spectrometry (IM-MS) is a rapidly growing field for studying the composition and structure of biomolecules and biomolecular complexes using gas-phase methods. Typically, ions are formed in native IM-MS using gentle nanoelectrospray ionization conditions, which in many cases can preserve condensed-phase stoichiometry. Although much evidence shows that large-scale condensed-phase structure, such as quaternary structure and topology, can also be preserved, it is less clear to what extent smaller-scale structure is preserved in native IM-MS. This review surveys computational and experimental efforts aimed at characterizing compaction and structural rearrangements of protein and protein complex ions upon transfer to the gas phase. A brief summary of gas-phase compaction results from molecular dynamics simulations using multiple common force fields and a wide variety of protein ions is presented and compared to literature IM-MS data.

Determination of Gas-Phase Ion Structures of Locally Polar Homopolymers Through High-Resolution Ion Mobility Spectrometry-Mass Spectrometry

The strong synergy arising from coupling two orthogonal analytical techniques such as ion mobility and mass spectrometry can be used to separate complex mixtures and determine structural information of analytes in the gas phase. A tandem study is performed using two systems with different gases and pressures to ascertain gas-phase conformations of homopolymer ions. Aside from spherical and stretched configurations, intermediate configurations formed by a multiply charged globule and a "bead-on-a-string" appendix are confirmed for polyethylene-glycol (PEG), polycaprolactone (PCL), and polydimethylsiloxane (PDMS). These intermediate configurations are shown to be ubiquitous for all charge states and masses present. For each charge state, configurations evolve in two distinctive patterns: an inverse evolution which occurs as an elementary charge attached to the polymer leaves the larger globule and incorporates itself into the appendage, and a forward evolution which reduces the globule without relinquishing a charge while leaving the appendix relatively constant. Forward evolutions are confirmed to form self-similar family shapes that transcend charge states for all polymers. Identical structural changes occur at the same mass over charge regardless of the system, gas or pressure strongly suggesting that conformations are only contingent on number of charges and chain length, and start arranging once the ion is at least partially ejected from the droplet, supporting a charge extrusion mechanism. Configurational changes are smoother for PDMS which is attributed to the larger steric hindrance caused by protruding pendant groups. This study has implications in the study of the configurational space of more complex homopolymers and heteropolymers. Graphical Abstract.

Influence of the amino acid composition on the ionization efficiencies of small peptides

Electrospray ionization is widely used to generate gas phase ions from a variety of molecules ranging from small ions to large proteins, while the ionization mechanism is claimed to depend on the size of the molecule. For small molecules, the ionization efficiency, the amount of gas phase ions produced in the electrospray process, depends on the properties of the compound. Here, we show that the amino acid composition also influences the ionization efficiency of the oligopeptide. Additionally, we show that the ionization efficiencies of oligopeptides consisting of more than five amino acid residues are very similar to one another, and assuming equal ionization efficiencies is feasible. Therefore, accurate ionization efficiency predictions are needed mainly for small oligopeptides. For these oligopeptides, the amino acid composition can be used to estimate the ionization efficiency in an easy to use manner.

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.

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

Molecular dynamics simulations have uncovered mechanistic details of the protein ESI process under various experimental conditions.