Recognition of Cis-Trans and Chiral Proline and Its Derivatives by Ion Mobility Measurement of Their Complexes with Natamycin and Metal Ion

Single ion mobility monitoring (SIM2) stitching method for high-throughput and high ion mobility resolution chiral analysis

Chiral analysis is of high interest in many fields such as chemistry, pharmaceuticals and metabolomics. Mass spectrometry and ion mobility spectrometry are useful analytical tools, although they cannot be used as stand-alone methods. Here, we propose an efficient strategy for the enantiomer characterization of amino acids (AAs) using non-covalent copper complexes. A single ion mobility monitoring (SIM2) method was applied on a TIMS-ToF mass spectrometer to maximize the detection and mobility separation of isomers. Almost all of the 19 pairs of proteinogenic AA enantiomers could be separated with at least one combination with the chiral references L-Phe and L-Pro. Furthermore, we extended the targeted SIM2 method by stitching several mobility ranges, in order to be able to analyze complex mixtures in a single acquisition while maintaining high mobility resolution. Most of the enantiomeric pairs of AAs separated with the SIM2 method were also detected with this approach. The SIM2 stitching method thus opens the way to a more comprehensive chiral analysis with TIMS-ToF instruments.

A simple strategy for d/l-carnitine analysis in food samples using ion mobility spectrometry and theoretical calculations

This work presents a straightforward approach to the separation d/l-carnitine (d/l-Carn) using ion mobility-mass spectrometry (IM-MS) and theoretical calculations. Natamycin (Nat) was used as separation reagent to interact with the Carn, metal ions (G) were employed as ligand, the resultant ternary complexes [d/l-Carn + Nat + G]+ were observed experimentally. IM-MS results revealed that d/l-Carn could be baseline separated via complex formation using Li+, Na+, K+, Rb+, and Cs+, with a maximum peak separation resolution (Rp-p) of 2.91; Theoretical calculations were performed to determine the optimal conformations of [d/l-Carn + Nat + Li/K]+, and the predicted collisional cross section values were consistent with the experimental values. Conformational analysis was used to elucidate the enantiomeric separation of d/l-Carn at the molecular level via the formation of ternary complexes. Furthermore, quantitative analyses for the determination of the enantiomers were established with effective linearity and acceptable sensitivity. Finally, the proposed method was successfully applied in the determination of d/l-Carn in food samples.

Metal-assisted core-shell plasmonic nanoparticles for small molecule biothiol analysis and enantioselective recognition

Cysteine (Cys) enantiomorphs, important small-molecule biothiols, participate in various antioxidative, flavoring, and poison-removing processes in the food industry. Current cysteine enantiomorph analysis methods require effective strategies for distinguishing them due to their similar structures and reactivity. Herein, we present a metal ion-assisted enantiomorph-selective surface-enhanced Raman scattering (SERS) biosensor based on an amphiphilic polymer matrix (APM), which can promote cysteine enantiomorph (L/D-Cys) identification. The highly selective molecular orientation is perhaps caused by the intermolecular hydrogen bonding with chiral isomers (metal centers). The experimental results show that the SERS biosensor has a sensitivity-distincting factor toward L-Cys and D-Cys. The linear range is from 1 mmol L-1 to 1 nmol L-1, along with a low limit of detection of 0.77 pmol L-1. Moreover, the fabricated Cu-APM biosensor exhibits remarkable stability and high repeatability, with an RSD of 3.7%. Real food cysteine enantiomorph detection was performed with L-Cys-containing samples of onion, cauliflower, garlic, and apple, and D-Cys-containing samples of vinegar, black garlic, cheese, and beer. The results show that the Cu-APM biosensor can be utilized as a powerful tool for real-time determination of Cys enantiomorphs in different food samples. Thus, the metal-ion-assisted enantiomorph-selective SERS biosensor has potential as an adaptable tool for enantiomorph detection and food sample analysis.

A cyclodextrin-based reagent for cis/trans-geometrical isomers separation by mobility measurements and chemical calculations

Identification of cis/trans-carbon-carbon double-bond (CC) isomers remain challenging. Herein, a simple and rapid method for the separation and analysis of cis/trans-maleic acid (MA) and aconitic acid (AA) using Trapped Ion Mobility Spectrometry (TIMS) was developed. α-, β-, γ-cyclodextrin (CD) were served as the separation reagent, slight difference in mobility separation was obtained by [CD-MA/AA-H]^-. Specially, with the addition of divalent metal ion (G²⁺) as coordination metal ion, the separation effect was much increased by [CD-MA/AA + G-H]⁺, and α-CD has better mobility separation effect than β-/γ-CD. Moreover, chemical calculations revealed the binary and ternary complexes are in the inclusion forms, and microscopic interactions between cis/trans-MA/AA, CDs, and G²⁺ are somewhat different that making their mobility separation. Finally, quantifications of cis/trans-isomers were analyzed in food samples, with good linearity (R² > 0.99) and recoveries obtained from 87.25 % to 100.73 %.

Chiral zinc oxide functionalized quartz crystal microbalance sensor for enantioselective recognition of amino acids

Chiral transition metal oxides with tunable structures and multiple physicochemical features have been increasingly applied for chiral sensing and detection. In this work, chiral zinc oxide (ZnO) was first applied as selector to construct quartz crystal microbalance (QCM) sensor for enantioselective recognition of amino acids. The chiral ZnO was prepared by a methionine-induced self-assembly strategy and its high topological chirality was confirmed by several techniques such as circular dichroism spectrum. The chiral discrimination factors were calculated by frequency shifts in response to aspartic acid, phenylalanine, lysine and arginine on L-ZnO surface, achieving 1.89 ± 0.04, 1.76 ± 0.11, 1.66 ± 0.07 and 1.54 ± 0.09, respectively. Notably, L-enantiomers preferred stronger absorptions on L-ZnO surface as compared to D-forms. It was further found that this sensor was appropriate for quantitative analysis and enantiomer excess analysis and adsorption kinetics study. Furthermore, molecular docking revealed the recognition mechanism, where chiral distinction was caused by the different steric interactions between enantiomers and chiral ZnO. This method enjoyed merits of high enantioselectivity, simple preparation and low cost, offering newly chiral sensing method for other molecules.

Rapid discrimination of enantiomers by ion mobility mass spectrometry and chemical theoretical calculation: Chiral mandelic acid and its derivatives

Because R/S-mandelic acids (MA) and their derivatives are critical starting materials or intermediates in the synthesis of chiral drugs, their chirality discrimination is important. In this study, R/S-MA and its derivatives, including R/S-2-phenylpropionic acid (2-PPA), R/S-methoxyphenylaceticacid (MPA), and R/S-2-hydroxy-4-phenylbutyric acid (HPBA), were accurate simultaneous mobility-discriminated by forming diastereomer complexes for the first time, which were obtained by simply mixing with cyclodextrins (α, β, γ-CD) and transition-metal ions (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺). The mass spectra revealed non-covalent diastereomer complexes formed by CD, enantiomers, and metal ions, and ion-mobility spectrometry (IMS) was performed for 109 pairs of complexes. Significant chiral discrimination was observed in the formed diastereomeric complexes, and their separation peak-to-peak resolution (R_p-p) for the enantiomers depended on the transition metal ion type. In most cases, the R_p-p value gradually increases with CD size, with quaternary complexes having the largest R_p-p value. The greatest chiral distinctions of 2-PPA, MA, MPA, and HPBA were obtained by the diastereomeric complex ions of [(2-PPA)(α)₂+Zn²⁺-H]⁺, [(MA)(α)₂+Zn²⁺-H]⁺, [(MPA)₂(β)+Co²⁺-H]⁺, and [(HPBA)(α)₂+Fe²⁺-H]⁺, with R_p-p values of 1.35, 1.57, 1.70, and 0.71, respectively. Furthermore, the favorable conformation and collisional cross section (CCS) value of the different [CD + R/S-MA + Cu-H]⁺ complexes were measured using chemical theoretical calculations to detail their intermolecular interaction, revealing that [α-CD + R/S-MA + Cu-H]⁺ has two favored gas complexes, and the CCS calculated were consistent with the TIMS observed. In addition, R² > 0.99 was obtained for the relative quantification of the chiral enantiomers. Overall, the proposed method provides a promising strategy for distinguishing the enantiomers of MA and their derivatives, with the advantages of simplicity, speed, and accuracy, without the need for complex chemical derivatization or chromatographic separation.

Drug Adulteration Analysis Based on complexation with CD and Metal Ions Using Ion Mobility Spectrometry

Drug adulteration and contamination are serious threats to human health therefore, their accurate monitoring is very important. Allopurinol (Alp) and theophylline (Thp) are commonly used drugs for the treatment of gout and bronchitis, while their isomers hypoxanthine (Hyt) and theobromine (Thm) have no effect and affect the efficacy of the drug. In this work, the drug isomers of Alp/Hyt and Thp/Thm are simply mixed with α-, β-, γ-cyclodextrin (CD) and metal ions and separated using trapped ion mobility spectrometry-mass spectrometry (TIMS-MS). TIMS-MS results showed that Alp/Hyt and Thp/Thm isomers could interact with CD and metal ions and form corresponding binary or ternary complexes to achieve their TIMS separation. Different metal ions and CDs showed different separation effect for the isomers, among which Alp and Hyt could be successfully distinguished from the complexes of [Alp/Hyt+γ-CD + Cu-H]⁺ with separation resolution (R _P-P) of 1.51; whereas Thp and Thm could be baseline separated by [Thp/Thm+γ-CD + Ca-H]⁺ with R _P-P of 1.96. Besides, chemical calculations revealed that the complexes were in the inclusion forms, and microscopic interactions were somewhat different, making their mobility separation. Moreover, relative and absolute quantification was investigated with an internal standard to determine the precise isomers content, and good linearity (R ² > 0.99) was obtained. Finally, the method was applied for the adulteration detection where different drugs and urine were analyzed. In addition, due to the advantages of fast speed, simple operation, high sensitivity, and no chromatographic separation required, the proposed method provides an effective strategy for the drug adulteration detection of isomers.

Simultaneous Differentiation of C═C Position Isomerism in Fatty Acids through Ion Mobility and Theoretical Calculations

Fatty acids play a pivotal role in biological processes and have many isomers, particularly at the C═C position, that influence their biological function. Distinguishing between isomers is crucial to investigating their role in health and disease. However, separating the isomers poses a significant analytical challenge. In this study, we developed a simple and rapid strategy combining ion mobility spectrometry and theoretical chemical calculations to differentiate and quantify the C═C positional isomers in 2-/3-butenoic acid (BA), 2-/3-/4-pentenoic acid (PA), and 2-/3-/5-hexenoic acid (HA). C═C positional isomerism was mobility-differentiated by simple complexation with crown ethers (12C4, 15C5, and 18C6) and divalent metal ions (Mg²⁺, Ca²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, Sr²⁺, and Ba²⁺), that is, converting C═C positional isomers with small structural differences into complexes with large structural differences through the interaction with metal ions and crown ethers. Metallized isomers were formed but could not be differentiated due to their complex and overlapping extracted ion mobiliograms (EIMs). Binary crown ether-isomer complexes were not observed, indicating that C═C positional isomers could not be separated by simple mixing with crown ethers. However, significant EIM differences were obtained for the formed ternary complexes, allowing baseline separation for the isomers. Notably, all crown ethers and metal ions have a separation effect with the isomers, with a calculated separation resolution (R_p-p) of 0.07-2.44. Theoretical chemical calculations were performed to provide in-depth structural information for the complexes and explain the separation principle. Theoretical conformational space showed that the divalent metal ions act as a bridge connecting the crown ether and the isomer. Additionally, the ternary complex becomes more compact as the distance between C═C and -COOH increases. Theoretical results can reflect the features of mobility experiments, with relative errors between the experiment collision cross-section (CCS) and theoretical CCS of no more than ±8.06%. This method was also evaluated in terms of quantification, accuracy, and precision repeatability. Overall, this study establishes that the crown ether-metal ion pair can function as a robust unit for differentiating C═C positional isomerism.