High-mass MALDI-MS using ion conversion dynode detectors: influence of the conversion voltage on sensitivity and spectral quality

Inhibiting Protein Aggregation Using Cellulose Nanocrystal in MALDI-TOF MS Analysis: Improving the Sensitivity and Repeatability of Intact Protein in Pueraria

Protein aggregation can induce low sensitivity and poor repeatability of matrix-assisted laser desorption/ionization time-of-fight mass spectrometry (MALDI-TOF MS) analysis for intact protein. Herein, we introduced a strategy to decrease protein aggregation in the sample solution by using cellulose nanocrystal (CNC). The results indicated that protein granule size was effectively reduced by adding CNC to the sample solution. Through MALDI-TOF MS analysis, the signal-to-noise ratio of [M + H]+ peak increased 2-fold, and the detection of limit was <10 μg/mL for intact protein. The CNC also contributed to excellent point-to-point repeatability for MALDI-TOF MS analysis with the coefficient of variation (CV) of 10.0% with CNC vs 48.9% without CNC in Hb solution. Also, the repeatability of Pueraria protein ion signals was improved by using CNC, and the CV with and without CNC was 16.1% and 39.6%, respectively. Moreover, protein ion intensity exhibited great linear relationship (y = 53.04x - 3.474, R2 = 0.9936) with the concentrations (ranging from 0.1 to 10 mg/mL) when using CNC. Further investigation revealed that m/z 19,000 and m/z 21,000 peaks of Pueraria could be used for the adulteration analysis and post-translational modification research, demonstrating our method has the potential for broad applications.

RECENT ADVANCES IN INSTRUMENTAL APPROACHES TO TIME-OF-FLIGHT MASS SPECTROMETRY

Time-of-flight mass spectrometry (TOFMS) is one of the simplest and most powerful approaches for mass spectrometry. Realization of the advantages inherent in TOFMS requires innovation in the theory and practice of the technique. Instrumental developments, in turn, create new capabilities that enable applications in chemical measurement. This review focuses on the recent advances in TOFMS instrumentation. New strategies for ion acceleration, multiplexed detection, miniaturized TOFMS instruments, approaches to extend the length of ion flight, and novel ion detection technologies are reviewed. Techniques that change the basic paradigm of TOFMS by measuring m/z based on ion flight distance are considered, as are applications at the frontiers of instrumental performance. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Development of a focused high-energy macromolecular ion beam

In this work, we report the development of a focused macromolecular ion beam with kinetic energy of up to 110 keV. The system consists of a quadrupole ion trap (QIT), einzel lens and linear accelerator (LINAC). Based on the combination of matrix-assisted laser desorption ionization (MALDI) and quadrupole ion trapping (QIT), ions were desorbed from the surface and trapped with an ion trap to form biomolecular ion packets. Positive- and negative-pulsed voltages were applied on each end-cap electrode of the QIT to extract the ion packets and form an ion beam that was subsequently focused via an einzel lens and accelerated by stepwise pulsed voltages. The tabletop instrument was designed and successfully demonstrated via measurements of molecular ions of insulin, cytochrome c and bovine serum albumin (BSA) with mass-to-charge ratios (m/z) ranging from ∼5.8 to 66.5 k. This is the first report of both a focused and high-kinetic-energy protein ion beam. In addition, both secondary ions and electrons were observed from the surface by hypervelocity ion beam bombardment. This focused macromolecular ion beam has demonstrated its potential in the study of interactions between large molecular ions with other molecules either in the gas phase or upon a surface.

Insight to Functional Conformation and Noncovalent Interactions of Protein-Protein Assembly Using MALDI Mass Spectrometry

Noncovalent interactions are the keys to the structural organization of biomolecule e.g., proteins, glycans, lipids in the process of molecular recognition processes e.g., enzyme-substrate, antigen-antibody. Protein interactions lead to conformational changes, which dictate the functionality of that protein-protein complex. Besides biophysics techniques, noncovalent interaction and conformational dynamics, can be studied via mass spectrometry (MS), which represents a powerful tool, due to its low sample consumption, high sensitivity, and label-free sample. In this review, the focus will be placed on Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI-MS) and its role in the analysis of protein-protein noncovalent assemblies exploring the relationship within noncovalent interaction, conformation, and biological function.

Characterizing the iron loading pattern of ferritin using high-mass matrix-assisted laser desorption ionization mass spectrometry

RATIONALE

Ferritin is an iron storage protein assembly, usually formed by a 24-subunit protein shell and an iron core. The ferritin shell has been well studied using various structural biology tools such as X-ray diffraction and cryo-electron microscopy, whereas the iron status of ferritin is less studied and no well-established method exists for characterizing the distribution of the iron loading of ferritin. Recent advances in mass spectrometry (MS) have expanded the observable m/z range, making the measurement of ferritin possible with MS. In this study, matrix-assisted laser desorption ionization (MALDI)-MS was employed to quantify the iron content of ferritin.

METHODS

The iron content of ferritin was quantified using a MALDI-MS system coupled with a commercially available ion conversions dynode high-mass detector. IgG1 antibody and its aggregates were used as external mass calibrants. The stability of HoloFt and ApoFt was also assessed in this study under different conditions, including various buffer pH, crosslinking agents and MALDI laser intensities.

RESULTS

The differences in peak width of HoloFt, ApoFt and IgG1 indicate the existence of mineral adducts in both HoloFt and ApoFt, and the mineral loading is heterogeneous among the HoloFt and ApoFt population. An average of 2773 ± 1584 iron atoms were determined for a commercial HoloFt sample. The iron core inside the ferritin complex is shown to stabilize and maintain the intact globular complex structure of ferritin.

CONCLUSIONS

This work introduces a MALDI-MS-based workflow for characterizing the ferritin iron loading pattern, which is meaningful for clinical analysis of iron deficiency/overload. In addition, the stability of ferritin is examined under various conditions, providing a guideline for further method development related to ferritin complex.

Single-molecule mass spectrometry

In single-molecule mass spectrometry, the mass of each ion is measured individually; making it suitable for the analysis of very large, heterogeneous objects that cannot be analyzed by conventional means. A range of single-molecule mass spectrometry techniques has been developed, including time-of-flight with cryogenic detectors, a quadrupole ion trap with optical detection, single-molecule Fourier transform ion cyclotron resonance, charge detection mass spectrometry, quadrupole ion traps coupled to charge detector plates, and nanomechanical oscillators. In addition to providing information on mass and heterogeneity, these techniques have been used to study impact craters from cosmic dust, monitor the assembly of viruses, elucidate the fluorescence dynamics of quantum dots, and much more. This review focuses on the merits of each of these technologies, their limitations, and their applications. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 36:715-733, 2017.

Applying mass spectrometry to study non-covalent biomolecule complexes

Non-covalent interactions are essential for the structural organization of biomacromolecules and play an important role in molecular recognition processes, such as the interactions between proteins, glycans, lipids, DNA, and RNA. Mass spectrometry (MS) is a powerful tool for studying of non-covalent interactions, due to the low sample consumption, high sensitivity, and label-free nature. Nowadays, native-ESI MS is heavily used in studies of non-covalent interactions and to understand the architecture of biomolecular complexes. However, MALDI-MS is also becoming increasingly useful. It is challenging to detect the intact complex without fragmentation when analyzing non-covalent interactions with MALDI-MS. There are two methodological approaches to do so. In the first approach, different experimental and instrumental parameters are fine-tuned in order to find conditions under which the complex is stable, such as applying non-acidic matrices and collecting first-shot spectra. In the second approach, the interacting species are "artificially" stabilized by chemical crosslinking. Both approaches are capable of studying non-covalently bound biomolecules even in quite challenging systems, such as membrane protein complexes. Herein, we review and compare native-ESI and MALDI MS for the study of non-covalent interactions.