Investigation into Small Molecule Isomeric Glucuronide Metabolite Differentiation Using In Silico and Experimental Collision Cross-Section Values

Identifying isomeric metabolites remains a challenging and time-consuming process with both sensitivity and unambiguous structural assignment typically only achieved through the combined use of LC-MS and NMR. Ion mobility mass spectrometry (IMMS) has the potential to produce timely and accurate data using a single technique to identify drug metabolites, including isomers, without the requirement for in-depth interpretation (cf. MS/MS data) using an automated computational pipeline by comparison of experimental collision cross-section (CCS) values with predicted CCS values. An ion mobility enabled Q-Tof mass spectrometer was used to determine the CCS values of 28 (14 isomeric pairs of) small molecule glucuronide metabolites, which were then compared to two different in silico models; a quantum mechanics (QM) and a machine learning (ML) approach to test these approaches. The difference between CCS values within isomer pairs was also assessed to evaluate if the difference was large enough for unambiguous structural identification through in silico prediction. A good correlation was found between both the QM- and ML-based models and experimentally determined CCS values. The predicted CCS values were found to be similar between ML and QM in silico methods, with the QM model more accurately describing the difference in CCS values between isomer pairs. Of the 14 isomeric pairs, only one (naringenin glucuronides) gave a sufficient difference in CCS values for the QM model to distinguish between the isomers with some level of confidence, with the ML model unable to confidently distinguish the studied isomer pairs. An evaluation of analyte structures was also undertaken to explore any trends or anomalies within the data set.

Current status and future prospects for ion-mobility mass spectrometry in the biopharmaceutical industry

Detailed characterization of protein reagents and biopharmaceuticals is key in defining successful drug discovery campaigns, aimed at bringing molecules through different discovery stages up to development and commercialization. There are many challenges in this process, with complex and detailed analyses playing paramount roles in modern industry. Mass spectrometry (MS) has become an essential tool for characterization of proteins ever since the onset of soft ionization techniques and has taken the lead in quality assessment of biopharmaceutical molecules, and protein reagents, used in the drug discovery pipeline. MS use spans from identification of correct sequences, to intact molecule analyses, protein complexes and more recently epitope and paratope identification. MS toolkits could be incredibly diverse and with ever evolving instrumentation, increasingly novel MS-based techniques are becoming indispensable tools in the biopharmaceutical industry. Here we discuss application of Ion Mobility MS (IMMS) in an industrial setting, and what the current applications and outlook are for making IMMS more mainstream.

How useful is molecular modelling in combination with ion mobility mass spectrometry for 'small molecule' ion mobility collision cross-sections?

Ion mobility mass spectrometry is used to measure the drift-time of an ion. The drift-time of an ion can be used to calculate the collision cross-section (CCS) in travelling wave ion mobility (e.g. Waters Synapt and Vion instruments) or directly determine the experimental CCS (e.g. Agilent 6560 instrument and many drift-tube instruments). A comparison of the experimental CCS and theoretical CCS values obtained from trajectory method He(g) parameterised MOBCAL and N2(g) parameterised MOBCAL software, for a range of 20 'small molecules' is presented. This study utilises density functional theory B3LYP methods and the 6-31G+(d,p) basis set to calculate theoretical CCS values. This study seeks to assess the accuracy of a common procedure using CCS calibration with poly-(d/l)-alanine derived from drift-cell measurements and the original release of MOBCAL software and compare it with recent improvements with a drug-like molecule calibration set and a revision of MOBCAL parameterised for N2(g) drift gas. This study represents one of the first quantitative evaluations of the agreement between theoretical CCS and experimental CCS values for a range of small pharmaceutically relevant molecules using travelling wave ion mobility mass spectrometry. Accurate theoretical CCS may allow optimisation of ion mobility separations in silico, provide CCS databases that can confirm structures without the need for alternative analytical tools such as nuclear magnetic resonance spectroscopy (NMR) and assignment of unknowns and positional isomers without requiring reference materials.

Combining density functional theory (DFT) and collision cross-section (CCS) calculations to analyze the gas-phase behaviour of small molecules and their protonation site isomers

Computational methods are employed to study the protomers in ESI-IM-MS.

Can ion mobility mass spectrometry and density functional theory help elucidate protonation sites in 'small' molecules?

RATIONALE

Ion mobility spectrometry-mass spectrometry (IMS-MS) offers an opportunity to combine measurements and/or calculations of the collision cross-sections and subsequent mass spectra with computational modelling in order to derive the three-dimensional structure of ions. IMS-MS has previously been reported to separate two components for the compound norfloxacin, explained by protonation on two different sites, enabling the separation of protonated isomers (protomers) using ion mobility with distinguishable tandem mass spectrometric (MS/MS) data. This study reveals further insights into the specific example of norfloxacin and wider implications for ion mobility mass spectrometry.

METHODS

Using a quadrupole ion mobility time-of-flight mass spectrometer, the IMS and MS/MS spectra of norfloxacin were recorded and compared with theoretical calculations using molecular modelling (density functional theory), and subsequent collision cross-section calculations using projection approximation.

RESULTS

A third significant component in the ion mobilogram of norfloxacin was observed under similar experimental conditions to those previously reported. The presence of the new component is convoluted by co-elution with another previously observed component.

CONCLUSIONS

This case demonstrates the potential of combined IMS-MS/MS with molecular modelling information for increased understanding of 'small-molecule' fragmentation pathways.

Ion mobility spectrometry-mass spectrometry (IMS-MS) of small molecules: separating and assigning structures to ions

The phenomenon of ion mobility (IM), the movement/transport of charged particles under the influence of an electric field, was first observed in the early 20th Century and harnessed later in ion mobility spectrometry (IMS). There have been rapid advances in instrumental design, experimental methods, and theory together with contributions from computational chemistry and gas-phase ion chemistry, which have diversified the range of potential applications of contemporary IMS techniques. Whilst IMS-mass spectrometry (IMS-MS) has recently been recognized for having significant research/applied industrial potential and encompasses multi-/cross-disciplinary areas of science, the applications and impact from decades of research are only now beginning to be utilized for "small molecule" species. This review focuses on the application of IMS-MS to "small molecule" species typically used in drug discovery (100-500 Da) including an assessment of the limitations and possibilities of the technique. Potential future developments in instrumental design, experimental methods, and applications are addressed. The typical application of IMS-MS in relation to small molecules has been to separate species in fairly uniform molecular classes such as mixture analysis, including metabolites. Separation of similar species has historically been challenging using IMS as the resolving power, R, has been low (3-100) and the differences in collision cross-sections that could be measured have been relatively small, so instrument and method development has often focused on increasing resolving power. However, IMS-MS has a range of other potential applications that are examined in this review where it displays unique advantages, including: determination of small molecule structure from drift time, "small molecule" separation in achiral and chiral mixtures, improvement in selectivity, identification of carbohydrate isomers, metabonomics, and for understanding the size and shape of small molecules. This review provides a broad but selective overview of current literature, concentrating on IMS-MS, not solely IMS, and small molecule applications.

"Soft"or "hard" ionisation? Investigation of metastable gas temperature effect on direct analysis in real-time analysis of Voriconazole

The performance of the direct analysis in real-time (DART) technique was evaluated across a range of metastable gas temperatures for a pharmaceutical compound, Voriconazole, in order to investigate the effect of metastable gas temperature on molecular ion intensity and fragmentation. The DART source has been used to analyse a range of analytes and from a range of matrices including drugs in solid tablet form and preparations, active ingredients in ointment, naturally occurring plant alkaloids, flavours and fragrances, from thin layer chromatography (TLC) plates, melting point tubes and biological matrices including hair, urine and blood. The advantages of this technique include rapid analysis time (as little as 5 s), a reduction in sample preparation requirements, elimination of mobile phase requirement and analysis of samples not typically amenable to atmospheric pressure ionisation (API) techniques. This technology has therefore been proposed as an everyday tool for identification of components in crude organic reaction mixtures.