Separation of Enantiomeric Triacylglycerols by Chiral-Phase HPLC

Enantiomeric Separation of Triacylglycerols Consisting of Three Different Fatty Acyls and Their Chiral Chromatographic Elution Behavior

Chromatographic separation of triacylglycerol (TG) enantiomers is a highly challenging task of analytical chemistry because of the similar physicochemical properties. The analysis of chiral TGs is crucial for improving the knowledge of lipid biochemistry and for understanding the nutritional properties of fats and oils. Thus, this study aimed to systematically investigate the chiral resolution of TGs consisting of three different fatty acyls (FAs). Thirty-three asymmetric TG enantiopairs, including 49 synthesized enantiopure TGs and racemic TGs, were analyzed with a recycling chiral HPLC system. Twenty-six enantiopairs were successfully separated. Overall, having both unsaturated and saturated FAs in the outer positions or a difference in carbon chain length between two saturated FAs at the outer positions favored the separation of enantiomers. The retention time at separation correlated negatively with the sn-3 carbon number of the early eluting enantiomer and positively with the carbon number difference between sn-1 and sn-3. When the samples were studied in separate groups based on unsaturation and regioisomers, both the acyl carbon number and the degree of unsaturation of FAs in all three positions influenced the separation and elution behavior of chiral TGs, indicating an active role of both intermolecular interactions and steric hindrances. This is the first systematic study of the chiral separation of TGs consisting of three different FAs using a large number of enantiopairs. The novel findings on the behavior of TG enantiomers in a chiral environment provide important guidance and reference for a stereospecific study of the chemistry and biochemistry of natural lipids.

Three-dimensional liquid chromatography with parallel second dimensions and quadruple parallel mass spectrometry for adult/infant formula analysis

Three dimensions of chromatographic separation, using split-flow two-dimensional liquid chromatography (SF-2D-LC) with two parallel second dimensions, LC × 2LC, combined with quadruple parallel mass spectrometry (LC3MS4) is demonstrated for analysis of NIST SRM 1849a adult/infant formula. The first dimension, ¹D, was a conventional non-aqueous reversed-phase (NARP) HPLC separation using two C18 columns in series, followed by detection using an ultraviolet (UV) detector, a fluorescence detector (FLD), with flow then split to a corona charged aerosol detector (CAD), and then dual parallel mass spectrometry (MS), conducted in atmospheric pressure photoionization (APPI) and electrospray ionization (ESI) modes. The first second dimension, ²D(1), UHPLC was conducted on a 50.0 mm C30 column using a NARP-UHPLC parallel gradient for separation of short-chain triacylglycerols (TAGs) from long-chain TAGs, with detection by UV and ESI-MS. The second dimension, ²D(2), UHPLC was conducted using a 100.0 mm C30 column with a NARP-UHPLC parallel gradient for improved separation of TAG isomers, with detection by UV, an evaporative light scattering detector, and high-resolution, accurate-mass (HRAM) ESI-MS. Transferred eluent dilution was used to refocus peaks and keep them sharp during elution in both ²Ds. The separation space in the ²D(2) was optimized using multi-cycle (aka, "constructive wraparound") elution, which employed flow rate programming. In the ¹D, calibration lines for quantification of fat-soluble vitamins were constructed. A lipidomics approach to TAG identification and quantification by HRAM-ESI-MS was applied to the ²D(2). These experiments can be represented: LC1MS2 × (LC1MS1 + LC1MS1) = LC3MS4, or three-dimensional liquid chromatography with quadruple parallel mass spectrometry.

Deep Structural Annotation of Glycerolipids by the Charge-Tagging Paterno-Büchi Reaction and Supercritical Fluid Chromatography-Ion Mobility Mass Spectrometry

Glycerolipids (GLs) are essential for cellular lipid homeostasis, while dysregulation in GL metabolism is often associated with the onset or progression of human-related metabolic diseases. The profile of GLs is thus frequently used as a molecular readout for disease phenotyping. Although mass spectrometry (MS) is the method of choice for GL profiling, the current MS methods are unable to differentiate two major types of structural isomers due to the fact that fatty acyls can be linked to different positions on the glycerol backbone (sn-positions) and the site(s) of unsaturation in acyl chains. Herein, by utilizing charge-tagging Paterno-Büchi (PB) derivatization of carbon-carbon double bond (C═C), supercritical fluid chromatography (SFC), and mobility aligned tandem mass spectrometry (MS/MS), a workflow has been developed for the sensitive and structurally informative analysis of GLs. SFC allows fast separation (within 25 min) of sn-isomers of diacylglycerols (DGs) and separation of triacylglycerols (TGs) of different chain lengths and degrees of unsaturation. Time-aligned parallel fragmentation enables multiple-stage MS/MS of the PB-derivatized lipids in a high-throughput fashion and allows pinpointing C═C location to a specific fatty acyl chain. This workflow reveals the presence of more than 500 molecular structures of neutral lipids from pooled human plasma. A comparison of human plasma samples between type 2 diabetes (N = 7) and control (N = 7) shows significant changes in isomer compositions (C18:1 Δ9 vs Δ11) from nine groups of TG and DG. These findings suggest that the developed workflow can be potentially applied to lipid marker discovery for disease monitoring or diagnosis.

Regioisomeric and enantiomeric analysis of primary triglycerides in human milk by silver ion and chiral HPLC atmospheric pressure chemical ionization-MS

Triglycerides (TG) not only provide energy for infants but have important physiological functions. Understanding the composition and structure of TG in human milk is conducive to the development of infant formulas. In this study, TG species in human milk from 3 provincial capitals (Zhengzhou, Wuhan, and Harbin) in different regions of China were determined through C18 HPLC electrospray ionization tandem mass spectrometry (MS). The results showed that in human milk from these 3 regions, oleoyl-palmitoyl-linoleoylglycerol (OPL; 16.55, 19.20, and 18.67%, respectively) was more abundant than oleoyl-palmitoyl-oleoylglycerol (OPO; 10.08, 10.22, and 12.03%, respectively). Subsequently, regioisomeric and enantiomeric analysis of main TG in the human milk were performed on silver ion and chiral HPLC atmospheric pressure chemical ionization mass spectrometry (APCI)-MS, respectively. The results showed that rac-OPL (above 85%), rac-OPO (above 85%), rac-palmitoyl-oleoyl-oleoylglycerol (PPO; above 90%), and rac-OLaO (above 70%) were the main regioisomers of OPL, OPO, PPO, and lauroyl-oleoyl-oleoylglycerol (LaOO), respectively. The relative ratios of enantiomer pairs of rac-OPL (rac-OPL1 and rac-OPL2) were about 37 and 63%, respectively.

Comparison of the Fatty Acid and Triglyceride Profiles of Big Eye Tuna (Thunnus obesus), Atlantic salmon (Salmo salar) and Bighead Carp (Aristichthysnobilis) Heads

Big eye tuna (Thunnus obesus), Atlantic salmon (Salmo salar) and bighead carp (Aristichthys nobilis) are three representative marine and fresh water fishes. In this study, the content of total lipids (TL), triglyceride (TG) fraction, and the fatty acid profiles in the corresponding fish heads were analyzed. Meanwhile, their complicated TG molecular species were further characterized. The results showed that TG was the major lipid in these three fish heads (60.58–86.69%). Compared with other two fish heads, big eye tuna head was the most abundant in polyunsaturated fatty acids, among which eicosapentaenoic acid (EPA) + docosahexaenoic acid (DHA) accounted for 64.29% and 32.77% in the TL and TG fraction, respectively. It is also worth noting that EPA+DHA/total fatty acid (TFA) value of TL and TG fraction from bighead carp head showed no significant difference with Atlantic salmon head, a typical marine fish. There were 146 TG molecules detected in big eye tuna head, 90 in Atlantic salmon and 87 in bighead carp heads. DHA or EPA accounted for 56.12%, 22.88%, and 5.46% of the total TG molecules in these three fish heads, respectively. According to principal component analysis, orthogonal projection to latent structures-discriminant analysis and the constructed heat map, the three samples could be completely differentiated based on their TG molecule fingerprints. This study is the first to compare marine and fresh water fish from the perspective of their heads’ fatty acid and TG molecule profiles.

Arsenolipids in the green alga Coccomyxa (Trebouxiophyceae, Chlorophyta)

Lipid-like compounds containing a dimethylarsinoyl group, i.e. Me₂As(O)-, have been identified by liquid chromatography/inductively coupled plasma mass spectrometry (LC/ICP-MS) and non-aqueous reversed-phase high-performance liquid chromatography (positive and/or negative high-resolution tandem electrospray ionization mass spectrometry (NARP-HPLC/HR-ESI^+(-)-MS/MS) from three strains of green algae of the genus Coccomyxa (Trebouxiophyceae, Chlorophyta). The algae were cultivated in a medium containing 10 g arsenic/L, i.e. 133.5 mmol/L of Na₂HAsO₄.7H₂O. After extraction by methyl-tert-butyl ether (MTBE), total lipids were analyzed by ICP-MS or ESI-MS without any further separation or fractionation. A total of 39 molecular species of arsenic triacylglycerols (AsTAG), 15 arsenic phosphatidylcholines (AsPC), 8 arsenic phosphatidylethanolamines (AsPE), 6 arsenic phosphatidylinositols (AsPI), 2 arsenic phosphatidylglycerols (AsPG) and 5 unknown lipids (probably ceramides) were identified. The structures of all molecular species were confirmed by tandem MS. Dry matter of the individual strains contained different amounts of total arsenolipids, i.e. C. elongata CCALA 427 (0.32 mg/g), C. onubensis (1.48 mg/g), C. elongata S3 (2.13 mg/g). On the other hand, there were only slight differences between strains in the relative abundances of individual molecular species. Possible biosynthesis of long-chain lipids with the end group Me₂As(O) has also been suggested.

An untargeted lipidomic approach for qualitative determination of latent fingermark glycerides using UPLC-IMS-QToF-MSE

More detailed fundamental information is required about latent fingermark composition in order to better understand fingermark properties and their impact on detection efficiency, and the physical and chemical changes that occur with time following deposition. The composition of the glyceride fraction of latent fingermark lipids in particular is relatively under-investigated due in part to their high structural variability and the limitations of the analytical methods most frequently utilised to investigate fingermark composition. Here, we present an ultra performance liquid chromatography-ion mobility spectroscopy-quadrupole time-of-flight mass spectrometry (UPLC-IMS-QToF-MS^E) method to characterise glycerides in charged latent fingermarks using data-independent acquisition. Di- and triglycerides were identified in fingermark samples from a population of 10 donors, through a combination of in silico fragmentation and monitoring for fatty acid neutral losses. 23 diglycerides and 85 families of triglycerides were identified, with significant diversity in chain length and unsaturation. 21 of the most abundant triglyceride families were found to be common to most or all donors, presenting potential targets for further studies to monitor chemical and physical changes in latent fingermarks over time. Differences in relative peak intensities may be indicative of inter- and intra-donor variability. While this study represents a promising step to obtaining more in-depth information about fingermark composition, it also highlights the complex nature of these traces.

Regioisomeric and enantiomeric analysis of triacylglycerols

A survey of useful methods for separation and identification of regioisomers and enantiomers of triacylglycerols. Gas chromatography, gas chromatography-mass spectrometry, ¹³C NMR determination of regioisomers by enzymatic methods, and supercritical fluid chromatography are briefly surveyed, whereas a detailed description is given of the analysis of triacylglycerols by liquid chromatography, especially with silver ion (Ag⁺; argentation), and nonaqueous reversed phase liquid chromatography. Special attention is paid to chiral chromatography. Details of mass spectrometry of triacylglycerols are also described, especially the identification of important triacylglycerol ions such as [M + H-RCOOH]⁺ in atmospheric pressure chemical ionization mass spectra.

Retention behavior of lipids in reversed-phase ultrahigh-performance liquid chromatography-electrospray ionization mass spectrometry

Reversed-phase ultrahigh-performance liquid chromatography (RP-UHPLC) method using two 15cm sub-2μm particles octadecylsilica gel columns is developed with the goal to separate and unambiguously identify a large number of lipid species in biological samples. The identification is performed by the coupling with high-resolution tandem mass spectrometry (MS/MS) using quadrupole - time-of-flight (QTOF) instrument. Electrospray ionization (ESI) full scan and tandem mass spectra are measured in both polarity modes with the mass accuracy better than 5ppm, which provides a high confidence of lipid identification. Over 400 lipid species covering 14 polar and nonpolar lipid classes from 5 lipid categories are identified in total lipid extracts of human plasma, human urine and porcine brain. The general dependences of relative retention times on relative carbon number or relative double bond number are constructed and fit with the second degree polynomial regression. The regular retention patterns in homologous lipid series provide additional identification point for UHPLC/MS lipidomic analysis, which increases the confidence of lipid identification. The reprocessing of previously published data by our and other groups measured in the RP mode and ultrahigh-performance supercritical fluid chromatography on the silica column shows more generic applicability of the polynomial regression for the description of retention behavior and the prediction of retention times. The novelty of this work is the characterization of general trends in the retention behavior of lipids within logical series with constant fatty acyl length or double bond number, which may be used as an additional criterion to increase the confidence of lipid identification.

Bioprospecting of microalgae: Proper extraction followed by high performance liquid chromatographic-high resolution mass spectrometric fingerprinting as key tools for successful metabolom characterization

Currently, the interest in microalgae as a source of biologically active components exploitable as supplementary ingredients to food/feed or in cosmetics continues to increase. Existing research mainly aims to focus on revealing and recovering the rare, cost competitive components of the algae metabolom. Because these components could be of very different physicochemical character, a universal approach for their isolation and characterization should be developed. This study demonstrates the systematic development of the extraction strategy that represents one of the key challenges in effective algae bioprospecting, which predefines their further industrial application. By using of Trachydiscus minutus as a model microalgae biomass, following procedures were tested and critically evaluated in order to develop the generic procedure for microalgae bioprospecting: (i) various ways of mechanical disintegration of algae cells enabling maximum extraction efficiency, (ii) the use of a wide range of extraction solvents/solvent mixtures suitable for optimal extraction yields of polar, medium-polar, and non-polar compounds, (iii) the use of consecutive extractions as a fractionation approach. Within the study, targeted screening of selected compounds representing broad range of polarities was realized by ultra-high performance liquid chromatography coupled with high resolution tandem mass spectrometric detection (UHPLC-HRMS/MS), to assess the effectiveness of undertaken isolation steps. As a result, simple and high-throughput extraction-fractionation strategy based on consecutive extraction with water-aqueous methanol-hexane/isopropanol was developed. Moreover, to demonstrate the potential of the UHPLC-HRMS/MS for the retrospective non-target screening and compounds identification, the collected mass spectra have been evaluated to characterize the pattern of extracted metabolites. Attention was focused on medium-/non-polar extracts and characterization of lipid species present in the T. minutus algae. Such detailed information on the composition of native (non-hydrolyzed) lipids of this microalga has not been published yet.

Temperature dependence of production of structured triacylglycerols in the alga Trachydiscus minutus

This study describes the identification of regioisomers and enantiomers of triacylglycerols of C20 polyunsaturated fatty acids (PUFAs) in the alga Trachydiscus minutus cultivated at different temperatures using reversed- and chiral-phase liquid chromatography-mass spectrometry. The use of the two different phases contributes to ready identification, both qualitative and semiquantitative, of regioisomers and enantiomers of triacylglycerols containing eicosapentaenoic and arachidonic in the molecule. The ratio of regioisomers and enantiomers of triacylglycerols (TAG) depends on the temperature of cultivation; with lowering temperature the proportion of the achiral TAG increases and the enantiomer ratio diverges from 1:1.