Supercharging of Proteins by Salts during Polarity Reversed Nano-Electrospray Ionization

Rapid in situ identification of honey authenticity based on RP-Nano-ESI-MS using online desalting

High-content sugar in honey frequently results in severe matrix effects and requires complex pretreatment prior to analysis, posing significant challenges for the rapid analysis of honey. In this study, the reversal polarity nano-electrospray ionization mass spectrometry (RP-Nano-ESI-MS) analysis was developed for the direct evaluation of honey samples. The results indicated that RP-Nano-ESI-MS significantly mitigated the matrix effects induced by high-content sugar through the implementation of online desalting. Furthermore, RP-Nano-ESI-MS has been proven capable of not only differentiating acacia honey adulterated with 10% rape honey, but also effectively distinguishing six types of honey and exhibiting remarkable proficiency in detecting honey adulteration and botanical traceability. Additionally, RP-Nano-ESI-MS exhibited strong quantitative abilities, effectively characterizing variations in amino acid composition among six types of honey with high stability and reproducibility. Our studies underscore the significant potential of RP-Nano-ESI-MS for its rapid in situ analysis of sugar-rich foods like honey, especially in their authenticity verification.

Recent developments in ionization techniques for single-cell mass spectrometry

The variation among individual cells plays a significant role in many biological functions. Single-cell analysis is advantageous for gaining insight into intricate biochemical mechanisms rarely accessible when studying tissues as a whole. However, measurement on a unicellular scale is still challenging due to unicellular complex composition, minute substance quantities, and considerable differences in compound concentrations. Mass spectrometry has recently gained extensive attention in unicellular analytical fields due to its exceptional sensitivity, throughput, and compound identification abilities. At present, single-cell mass spectrometry primarily concentrates on the enhancement of ionization methods. The principal ionization approaches encompass nanoelectrospray ionization (nano-ESI), laser desorption ionization (LDI), secondary ion mass spectrometry (SIMS), and inductively coupled plasma (ICP). This article summarizes the most recent advancements in ionization techniques and explores their potential directions within the field of single-cell mass spectrometry.

Novel Isobaric Tagging Reagent Enabled Multiplex Quantitative Glycoproteomics via Electron-Transfer/Higher-Energy Collisional Dissociation (EThcD) Mass Spectrometry

Protein glycosylation, covalent attachment of carbohydrates to polypeptide chains, is a highly important post-translational modification involved in many essential physiological processes. Comprehensive site-specific and quantitative analysis is crucial for revealing the diverse functions and dynamics of glycosylation. To characterize intact glycopeptides, mass spectrometry (MS)-based glycoproteomics employs versatile fragmentation methods, among which electron-transfer/higher-energy collision dissociation (EThcD) has gained great popularity. However, the inherent limitation of EThcD in fragmenting low-charge ions has prevented its widespread applications. Furthermore, there is a need to develop a high-throughput strategy for comparative glycoproteomics with a large cohort of samples. Herein, we developed isobaric N,N-dimethyl leucine-derivatized ethylenediamine (DiLeuEN) tags to increase the charge states of glycopeptides, thereby improving the fragmentation efficiency and allowing for in-depth intact glycopeptide analysis, especially for sialoglycopeptides. Moreover, the unique reporter ions of DiLeuEN-labeled glycopeptides generated in tandem MS spectra enable relative quantification of up to four samples in a single analysis, which represents a new high-throughput method for quantitative glycoproteomics.

Investigation on the binary ionization choices for large conjugated amines during electrospray ionization

RATIONALE

Generally, amines form protonated cations ([M + H]⁺ ) in positive polarity during electrospray ionization (ESI). However, it was found that large conjugated amines (LCAs) had binary ionization choices of generating either radical cations (M^•+ ) or [M + H]⁺ during ESI. Investigation on the mechanism would further our understanding of ESI.

METHODS

In this work, the binary ionization behavior of LCAs was reported and studied. Internal factors (functional groups and sizes of conjugated systems) and external factors (solvent type, flow rate, and electrode position) were systematically investigated and discussed.

RESULTS

For the internal factors, electron-donating groups and large conjugated structures of LCAs were conducive to the generation of M^•+ . For the external factors, aprotic solvent, higher flow rate, and shorter distance from the electrode to the spray cone facilitated the formation of M^•+ but hampered the generation of [M + H]⁺ .

CONCLUSION

The present study illustrated that the formations of M^•+ and [M + H]⁺ for LCAs were two independent processes. The M^•+ cations of LCAs were formed on the surface of the electrode through electrochemical oxidation, whereas the [M + H]⁺ cations were generated following the typical ESI evolution process. By regulating the external factors, the ionization results of LCAs could be well modulated.

Direct analysis of hydroxylated polycyclic aromatic hydrocarbons in biological samples with complex matrices using polarity-reversed nanoelectrospray ionization

RATIONALE

Polycyclic aromatic hydrocarbons (PAHs) are a class of environmental contaminants with carcinogenic effect drawing worldwide attention. PAHs can be converted into hydroxylated PAHs (OH-PAHs) through metabolic processes. Thus, they are commonly considered as an important class of biomarkers of PAH exposure. However, direct analysis of related metabolites of these environmental pollutants in biological samples using mass spectrometry is still challenging because of matrix effect and ion suppression during nanoelectrospray ionization (nano-ESI).

METHODS

In our previous work, a polarity-reversed nanoelectrospray ionization (PR-nESI) technique was developed for the analysis of biomolecules in complex matrices. In this work, we further optimized PR-nESI for direct and sensitive analysis of OH-PAHs in different samples under severe salt interference in negative polarity.

RESULTS

Compared with conventional nano-ESI, the optimized PR-nESI method realized sensitive detection of 1-naphthol in samples with a concentration of salt up to millimolar level. The signal-to-noise ratio (S/N) of OH-PAHs was increased by 1-2 orders of magnitude compared with conventional nano-ESI. Six different OH-PAHs were successfully detected with high S/N ratio using PR-nESI. PR-nESI was further successfully applied in the analysis of OH-PAHs in spiked fetal blood serum, human urine, and single-cell samples. For environmentally exposed subjects, the detections of OH-PAHs in single-cell samples and urines from human smokers were successfully conducted.

CONCLUSION

The optimized PR-nESI method was successfully applied for the sensitive analysis of OH-PAHs in complex biological samples with severe salt effects. Based on the present study, PR-nESI can have a promising prospect for the sensitive analysis of other metabolites of environmental pollutants in negative polarity.

Sulfolane-Induced Supercharging of Electrosprayed Salt Clusters: An Experimental/Computational Perspective

It is well-known that supercharging agents (SCAs) such as sulfolane enhance the electrospray ionization (ESI) charge states of proteins, although the mechanistic origins of this effect remain contentious. Only very few studies have explored SCA effects on analytes other than proteins or peptides. This work examines how sulfolane affects electrosprayed NaI salt clusters. Such alkali metal halide clusters have played a key role for earlier ESI mechanistic studies, making them interesting targets for supercharging investigations. ESI of aqueous NaI solutions predominantly generated singly charged [Na_nI_(n-1)]⁺ clusters. The addition of sulfolane resulted in abundant doubly charged [Na_nI_(n-2)Sulfolane_s]²⁺ species. These experimental data for the first time demonstrate that electrosprayed salt clusters can undergo supercharging. Molecular dynamics (MD) simulations of aqueous ESI nanodroplets containing Na⁺/I^- with and without sulfolane were conducted to obtain atomistic insights into the supercharging mechanism. The simulations produced [Na_nI_i]^z+ and [Na_nI_iSulfolane_s]^z+ clusters similar to those observed experimentally. The MD trajectories demonstrated that these clusters were released into the gas phase upon droplet evaporation to dryness, in line with the charged residue model. Sulfolane was found to evaporate much more slowly than water. This slow evaporation, in conjunction with the large dipole moment of sulfolane, resulted in electrostatic stabilization of the shrinking ESI droplets and the final clusters. Hence, charge-dipole stabilization causes the sulfolane-containing droplets and clusters to retain more charge, thereby providing the mechanistic foundation of salt cluster supercharging.

Strong Acid Anions Significantly Increasing the Charge State of Proteins during Electrospray Ionization

Regulation of protein's charge state in electrospray is of great importance to the analysis of proteins. Different methods have been developed so far to increase the charge state of proteins. In this work, we investigated the influence of different anions on the charge state of proteins. Both strong acid anions and weak acid anions were taken into consideration. The results showed that the presence of 5 mM strong acid anions in acidic solutions could significantly increase the charge state of proteins. In comparison, weak acid anions with the same concentration in solution had little impact on the charge state of proteins. The species of the cations in the samples had very limited influence on the charge state. The presence of a certain amount of acid in sample solution was critical to the effect of strong acid anions. Almost no increase of the charge state was observed when no acid was added to the samples. However, remarkable increase of the charge state of myoglobin (Mb) was observed when 0.001% (v/v) acetic acid (HAc) was added to the sample together with 5 mM sodium chloride (NaCl). A higher concentration of acid in samples would further enhance the effect of strong acid anions on the increase of the charge state. Further investigations into the mechanism revealed that the effect of the strong acid anions on the charge state of proteins was based on the unfolding of the protein molecules during electrospray ionization (ESI). The interactions among H⁺, anions, and protein molecules were so strong that it caused the unfolding of protein molecules and resulted in the increasing of proteins' charge states. The key factor that made strong acid anions and weak acid anions different in the results was the hydrolysis of the weak acid anions in acidic solutions. The present work furthers our understanding about electrospray, as well as the regulation of protein charge state. The presence of strong acid anions in acidic solutions can significantly influence the charge state of proteins in electrospray. Attention should be paid to this when regulating the charge state of proteins.

Online desalting and sequential formation of analyte ions for mass spectrometry characterization of untreated biological samples

Current-limited high voltage polarity reversing nanoelectrospray ionization allows online separation of intrinsic metal ions in complex biological samples, resulting in the generation of protonated analytes without interference from salt cations.