Ion mobility mass spectrometry (IM-MS) combines the advantages of ion mobility spectrometry (IMS) and MS for effective gas-phase ion analysis. Separation of ions based on their mobilities prior to MS can be performed without a great loss in other analytical figures of merit, and the extra dimension of analysis offered by IM can be beneficial for isomer and complex sample analyses. In this review, basic principles of IMS and IM-MS are described in addition to an introduction to various IMS techniques and commercial IM-MS instruments. The nature of collision cross-section (ΩD), an important parameter determining the transport properties of ions in IMS, is also explained in detail.
Despite its great success in the field of proteomics, mass spectrometry has limited use for determining structural details of peptides, proteins, and their assemblies. Emerging ion mobility spectrometry-mass spectrometry has enabled us to explore the conformational space of protein ions in the gas phase, and further combinations with the gas-phase ion spectroscopy and the colli-sion-induced unfolding have extended its abilities to elucidating the secondary structure and local details of conformational transi-tions. This review will provide a brief introduction to the combined approaches of IMS-MS with gas-phase ion infrared spectroscopy or collision-induced unfolding and their most recent results that successfully revealed higher-order structural details.
Characterization of intact protein structures in the gas phase using electrospray ionization combined with ion mobility mass spectrometry has become an important tool of research. However, the biophysical properties that govern the structures of protein ions in the gas phase remain to be understood. Here, we investigated the impact of host-guest complexation of ubiquitin (Ubq) with macrocyclic host molecules, cucurbit[n]urils (CB[n]s, n = 6, 7), on its structure in the gas phase. We found that CB[n] complexation induces the formation of compact Ubq ions. Both CB and CB exhibited similar effects despite differences in their binding properties in solution. In addition, CB[n] attachment prevented Ubq from unfolding by collisional activation. Based on the experimental results, we suggest that CB[n]s prevent unfolding of Ubq during transfer to the gas phase to promote the formation of compact protein ions. Furthermore, interaction with positively charged residues per se is suggested to be the most important factor for the host-guest complexation effect.
Understanding the mechanisms that control and concentrate the observed electrospray ionisation (ESI) response from peptides is important. Controlling these mechanisms can improve signal-to-noise ratio in the mass spectrum, and enhances the generation of intact ions, and thus, improves the detection of peptides when analysing mixtures. The effects of different mixtures of aqueous: organic solvents (25, 50, 75%; v/v): formic acid solution (at pH 3.26) compositions on the ESI response and chargestate distribution (CSD) during mass spectrometry (MS) were determined in a group of biologically active peptides (molecular wt range 1.3 - 3.3 kDa). The ESI response is dependent on type of organic solvent in the mobile phase mixture and therefore, solvent choice affects optimal ion intensities. As expected, intact peptide ions gave a more intense ESI signal in polar protic solvent mixtures than in the low polarity solvent. However, for four out of the five analysed peptides, neither the ESI response nor the CSD were affected by the volatility of the solvent mixture. Therefore, in solvent mixtures, as the composition changes during the evaporation processes, the pKb of the amino acid composition is a better predictor of multiple charging of the peptides.
In the present study, we report that the charge-directed (assisted) peptide dissociation products, such as b- and y-type peptidebackbone fragments, were the major products in MS/MS and MS3 applications of some o-TEMPO-Bz-C(O)-peptide ions, whileradical-driven dissociation products, such as a/x and c/z-type fragments, were previously shown to be the major products in the freeradical initiated peptide sequencing mass spectrometry (FRIPS MS). Those o-TEMPO-Bz-C(O)-peptides share a common feature intheir sequences, that is, the peptides do not include an arginine residue that has the highest proton affinity among free amino acids. The appearance of b- and y-type fragments as major products in FRIPS MS can be understood in terms of the so-called “mobile-protonmodel”. When the proton is highly mobilized by the absence of arginine, the chare-directed peptide dissociation pathways appearto be more competitive than the radical-driven dissociation pathways, in our FRIPS experiments.
A specific and sensitive liquid chromatography-electrospray ionization tandem mass spectrometry method (LC-ESIMS/MS) was developed and validated for the simultaneous quantification of porphyrins (coproporphyrin, pentacarboxylporphyrin,hexacarboxylporphyrin, heptacarboxylporphyrin, and uroporphyrin) in human plasma and urine. Acidified plasma samplesand urine samples were prepared by using liquid-liquid extraction using ethyl acetate and protein precipitation with acetonitrile,respectively. The separation was achieved onto a Synergi Fusion RP column (150 mm × 2.0 mm, 4 μm) with a gradient elutionof mobile phase A (0.1% formic acid in 2 mmol/L ammonium acetate, v/v) and mobile phase B (20% methanol in acetonitrile, v/v) at a flow rate of 450 μL/min. Porphyrins and the internal standard (IS), coproporphyrin I-15N4, were detected by a tandemmass spectrometer equipped with an electrospray ion source operating in positive ion mode. Multiple reaction monitoring(MRM) transitions of the protonated precursor ions and the related product ions were optimized to increase selectivity and sensitivity. The proposed method was validated by assessing selectivity, linearity, limit of quantification (LOQ), precision, accuracy,recovery, and stability. The calibration curves were obtained in the range of 0.1-100 nmol/L and the LOQs were estimated as0.1 nmol/L for all porphyrins. Results obtained from the validation study of porphyrins showed good accuracy, precision, recovery,and stability. Finally, the proposed method was successfully applied to clinical studies on the autism spectrum disorder(ASD) diagnosis of 203 Korean children.
We developed a bioanalytical method for simultaneous determination of nine NBOMe derivatives (25H-NBOMe, 25B-NBOMe, 25E-NBOMe, 25N-NBOMe, 25C-NBOH, 25I-NBOH, 25B-NBF, 25C-NBF, and 25I-NBF) in human plasma using liquid chromatography tandem mass spectrometry (LC-MS/MS). Human plasma samples were pre-treated using solid-phase extraction. Sepa-ration was achieved on a C18 column under gradient elution using a mobile phase containing 0.1% formic acid in acetonitrile and 0.1% formic acid in water at a flow rate of 0.3 mL/min. Mass detection was performed in the positive ion mode using multiple reaction moni-toring. The calibration range was 1-100ng/mL for all quantitative analytes, with a correlation coefficient greater than 0.99. The intra- and inter-day precision and accuracy varied from 0.85 to 6.92% and from 90.19 to 108.69%, respectively. The recovery ranged from 86.36 to 118.52%, and the matrix effects ranged from 27.09 to 99.72%. The stability was acceptable in various conditions. The LC-MS/MS method was validated for linearity, accuracy, precision, matrix effects, recovery and stability in accordance with the FDA guidance. The proposed method is suitable for reliable and robust routine screening and analysis of nine NBOMe derivatives in forensic field.