Analysis of triacylglycerols— approaching the molecular composition of natural mixtures

Lipase and Its Unique Selectivity: A Mini-Review

Contrary to other solid catalysts, enzymes facilitate more sophisticated chemical reactions because most enzymes specifically interact with substrates and release selective products. Lipases (triacylglycerol hydrolase, EC 3.1.1.3), which can catalyze the cleavage and formation of various acyl compounds, are one of the best examples of enzymes with a unique substrate selectivity. There are already several commercialized lipases that have become important tools for various lipid-related studies, although there is still a need to discover novel lipases with unique substrate selectivity to facilitate more innovative reactions in human applications such as household care, cosmetics, foods, and pharmaceuticals. In this mini-review, we focus on concisely demonstrating not only the general information of lipases but also their substate selectivities: typoselectivity, regioselectivity, and stereoselectivity. We highlight the essential studies on selective lipases in terms of enzymology. Furthermore, we introduce several examples of analysis methodology and experimental requirements to determine each selectivity of lipases. This work would stress the importance of integrating our understanding of lipase chemistry to make further advances in the relevant fields.

Regio- and Stereospecific Analysis of Triacylglycerols—A Brief Overview of the Challenges and the Achievements

The efforts to reveal, in detail, the molecular and intramolecular structures of one of the main lipid classes, namely, triacyl-sn-glycerols, which are now known to affect their specific and important role in all living organisms, are briefly overviewed. Some milestones of significance in the gradual but continuous development and improvement of the analytical methodology to identify the triacylglycerol regio- and stereoisomers in complex lipid samples are traced throughout the years: the use of chromatography based on different separation principles; the improvements in the chromatographic technique; the development and use of different detection techniques; the attempts to simplify and automatize the analysis without losing the accuracy of identification. The spectacular recent achievements of two- and multidimensional methods used as tools in lipidomics are presented.

Solvent-based strategy improves the direct determination of key parameters in edible fats and oils by 1 H NMR

BACKGROUND

Edible fats and oils are very important in nutrition and as a main source of energy and are also essential nutrients. There are several methods for the analysis of edible fats and oils, but nowadays nuclear magnetic resonance (NMR) is emerging as a powerful tool (albeit complex and high-tech demanding) to identify, quantify, and differentiate many types of food, including fats and oils. In this sense, the challenges of this technique are the simplification of methodology and taking advantage of a 400 MHz NMR instrument.

RESULTS

Through an adequate mixture of solvents, we have developed a methodology to quantify essential parameters in edible fats and oils, including 1,2-diacylglycerol, 1,3-diacylglycerol, and 1-monoacylglycerol, by using a single experiment and without the need for matrix derivatization.

CONCLUSION

This methodology has been successfully applied to the analysis of olive, sunflower, corn, sesame, and peanut oils, as well as butter, walnut, salmon, and spicy pork sausage. Moreover, the evolution of thermal oxidation and lipolysis of virgin olive oil and sunflower has been analyzed. © 2020 Society of Chemical Industry.

Characterisation of triacylglycerols from bovine milk fat fractions with MALDI-TOF-MS fragmentation

In this study, we used MALDI-TOF-MS to profile and characterise the triacylglycerol (TAG) species of anhydrous bovine milk fat (AMF) and its low melting (olein) and high melting (stearin) fractions obtained by dry fractionation. Silver-ion solid phase extraction (Ag⁺-SPE) cartridges were utilised to separate the TAGs according to saturation degree. Then, several TAG species were selected and fragmented via post-source decay (PSD) fragmentation. MALDI-TOF-MS TAG profiles and fragmentation patterns were compared to the TAG and fatty acid (FA) compositions obtained by gas chromatography-flame ionization detector (GC-FID). We found that, olein was rich in medium length chain TAG species like CN38:0 and CN40:1, whereas stearin was rich in saturated long chain TAG species from CN42:0 to CN52:0. Separation of the TAGs based on saturation degree allowed successful selection of the TAG parent-ion for fragmentation by eliminating the interferences of TAG species that have the same carbon number but vary in number of double bonds. The TAG fragmentation patterns indicated significant differences between AMF, olein and stearin as a result of the dry fractionation process. Compared to AMF, olein yielded in higher fragments of short-chain saturated and middle-chain unsaturated FAs. Whereas, stearin yielded in saturated and monounsaturated long chain FA fragments. Fragmentation of unsaturated long chain TAGs showed that the TAGs in olein contained more C18:1 and C18:2 than that of AMF and stearin. Stearin on the other hand, contained higher amount of TAG species containing C16:0. These results were in line with the FA compositions obtained from GC-FID and suggest that Ag⁺-SPE cartridges coupled with MALDI-TOF-MS offer an informative and practical approach to characterise fats and oils with complex TAG composition.

Supercritical fluid chromatography for lipid analysis in foodstuffs

The task of lipid analysis has always challenged separation scientists, and new techniques in chromatography were often developed for the separation of lipids; however, no single technique or methodology is yet capable of affording a comprehensive screening of all lipid species and classes. This review acquaints the role of supercritical fluid chromatography within the field of lipid analysis, from the early developed capillary separations based on pure CO₂ , to the most recent techniques employing packed columns under subcritical conditions, including the niche multidimensional techniques using supercritical fluids in at least one of the separation dimensions. A short history of supercritical fluid chromatography will be introduced first, from its early popularity in the late 1980s, to the sudden fall and oblivion until the last decade, experiencing a regain of interest within the chromatographic community. Afterwards, the subject of lipid nomenclature and classification will be briefly dealt with, before discussing the main applications of supercritical fluid chromatography for food analysis, according to the specific class of lipids.

Determination and comparison of seed oil triacylglycerol composition of various soybeans (Glycine max (L.)) using ¹H-NMR spectroscopy

Seed oil triacylglycerol (TAG) composition of 32 soybean varieties were determined and compared using ¹H-NMR. The contents of linolenic (Ln), linoleic (L), and oleic (O) ranged from 10.7% to 19.3%, 37.4%-50.1%, and 15.7%-34.1%, respectively. As is evident, linoleic acid was the major fatty acid of soybean oil. Compositional differences among the varieties were observed. Natural oils containing unsaturated groups have been regarded as important nutrient and cosmetic ingredients because of their various biological activities. The TAG profiles of the soy bean oils could be useful for distinguishing the origin of seeds and controlling the quality of soybean oils. To the best of our knowledge, this is the first study in which the TAG composition of various soybean oils has been analyzed using the ¹H-NMR method.

Cyclic acyloxonium ions as diagnostic aids in the characterization of chloropropanol esters under electron impact (EI), electrospray ionization (ESI), and atmospheric pressure chemical ionization (APCI) conditions

During mass spectrometric analysis of various lipids and lipid derivatives such as the chlorinated counterparts of triacylglycerols, the detailed structure of the characteristic and common ions formed under electron impact (EI), electrospray ionization (ESI), and atmospheric pressure chemical ionization (APCI) conditions by the loss of a single fatty acid remains ambiguous. These ions are designated in the literature as "diacylglyceride ions" and are frequently depicted with a molecular formula without showing any structural features and sometimes represented as cyclic acyloxonium ions. Characterization of these ions is of considerable importance due to their utility in structural identification of lipid derivatives. This study provides complementary evidence on the cyclic nature of "diacylglyceride ions" through the use of the simplest 3-monochloropropanediol diester as a model and the use of isotope labeling technique. Tandem MS/MS studies have indicated that the ion at m/z 135.6 generated from 1,2-bis(acetoyl)-3-chloropropane through the loss of an acetyl group was identical to the ion at m/z 135.6 generated from 4-chloromethyl-2,2-dimethyl-1,3-dioxolane, the latter being generated from a cyclic precursor through the loss of a methyl radical, keeping the dioxolane ring structure intact, thus confirming the cyclic nature of these ions. The corresponding cyclic oxonium ions generated from longer chain chloropropanol diesters, such as the ion at m/z 331.2 originating from 3-monochloropropanediol (3-MCPD) diesters containing palmitic acid(s), could serve as chemical markers for the presence chloropropanol esters.

¹H-nuclear magnetic resonance analysis of the triacylglyceride composition of cold-pressed oil from Camellia japonica

Camellia japonica (CJ) has oil-rich seeds, but the study of these oils has received little attention and has mainly focused only on their health properties. In the present work the relative composition of the fatty acid (FA) components of the triglycerides in cold-pressed oil from CJ is studied by ¹H-NMR. The results obtained were: 75.75%, 6.0%, 0.17% and 18.67%, for oleic, linoleic, linolenic and saturated FA respectively. Levels of C₁₈ unsaturated FA found in CJ oil were similar to those reported for olive oils. We also checked the possibility of using ¹³C-NMR spectroscopy; however, the results confirmed the drawback of ¹³C over ¹H-NMR for the study of FA components of CJ triglycerides due to its low gyromagnetic ratio and its very low natural abundance.

Analytical strategies for discriminating archeological fatty substances from animal origin

Mass spectrometry (MS) is an essential tool in the field of biomolecular archeology to characterize amorphous organic residues preserved in ancient ceramic vessels. Animal fats of various nature and origin, namely subcutaneous fats of cattle, sheep, goats, pigs, horses, and also of dairy products, are those most commonly identified in organic residues in archeological pottery. Fats and oils of marine origin have also been revealed. Since the first applications of MS coupled with gas chromatography (GC) in archeology at the end of 1980s, several developments have occurred, including isotopic determinations by GC coupled to isotope ratio MS and identification of triacylglycerols (TAGs) structure by soft ionization techniques (ESI and APCI). The combination of these methods provides invaluable insights into the strategies of exploitation of animal products in prehistory. In this review, I focus on the analytical strategies based upon MS that allow elucidation of the structure of biomolecular constituents and determination of their isotopic values to identify the nature of animal fat components preserved in highly complex and degraded archeological matrices.

Contribution of mass spectrometry to assess quality of milk-based products

The vast knowledge of milk chemistry has been extensively used by the dairy manufacturing industry to develop and optimize the modern technology required to produce high-quality milk products to which we are accustomed. A thorough understanding of the chemistry of milk and its numerous components is essential for designing processing equipment and conditions needed for the manufacture and distribution of high-quality dairy products. Knowledge and application of milk chemistry is also indispensable for fractionating milk into its principal components for use as functional and nutritional ingredients by the food industry. For all these reasons, powerful analytical methods are required. Because of the complexity of the milk matrix, mass spectrometry, coupled or not to separation techniques, constitutes a key tool in this area. In the present manuscript, we review the contribution and potentialities of mass spectrometry-based techniques to assess quality of milk-based products.

The bottom-up solution to the triacylglycerol lipidome using atmospheric pressure chemical ionization mass spectrometry

Presented here is an approach to representing the data from atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) of triacylglycerols (TAG) using a set of one, two, or three Critical Ratios. These Critical Ratios may be used directly to provide structural information concerning the regioisomeric composition of the triacylglycerols (TAG), and about the degree of unsaturation in the TAG. An AAA-type, or Type I, TAG has only one Critical Ratio, the ratio of the protonated molecule, [M + H]+, to the DAG fragment ion, [AA]+. The Critical Ratio for a Type I TAG is [MH]+/Z[DAG]+, and the mass spectrum of a Type I TAG can be reproduced from only this one ratio. An ABA/AAB/BAA, or Type II, TAG has two Critical Ratios, the [MH]+/sigma[DAG]+ ratio and the [AA]+/[AB]+ ratio. The [AA]+/[AB]+ ratio for a single TAG or TAG mixture can be compared with the [AA]+/[AB]+ ratios of pure regioisomeric standards, and the percentage of each regioisomer can be estimated. The abundance of the protonated molecule and the abundances of the two [DAG]+ fragment ions can be calculated from the two Critical Ratios for a Type II TAG. To calculate the abundances, the Critical Ratios are processed through the Bottom-Up Solution to the TAG lipidome. First, Critical Limits are calculated from the Critical Ratios, and then the Critical Ratios are classified into Cases by comparison with the Critical Limits. Once the Case classification is known, the equation for the abundance of each ion in the mass spectrum is given by the Bottom-Up Solution. A Type III TAG has three different FA and three Critical Ratios. The [MH]+/Z[DAG]+ ratio is the first Critical Ratio, the [AC]+/([AB]+ + [BC]+) ratio is the second Critical Ratio, and the [BC]+/[AB]+ ratio is the third Critical Ratio. The second critical ratio for a Type III TAG can be compared with regioisomeric standards to provide an estimate of the percentage composition of the regioisomers. The three Critical Ratios for a Type III TAG can be processed through the Bottom-Up Solution to calculate the four ion abundances that make up the APCI-MS mass spectrum. The Critical Ratios constitute a reduced data set that provides more information in fewer values than the raw abundances.

Mass spectrometric methods prove the use of beeswax and ruminant fat in late Roman cooking pots

Lipid extracts of sherds of archaeological late Roman cooking pots were analysed using high temperature-gas chromatography coupled to a mass spectrometer and liquid chromatography with atmospheric pressure chemical ionization mass spectrometer detection (LC-APCI-MS). With these advanced techniques the use of beeswax was shown through identification of the constituting alkanes, mono and diesters. The detection of high amounts of saturated triacylglycerols (TAGs) further indicated that animal fat was processed in these pots. Part of the animal fat was characterised as originating from ruminants due to the presence of trans-fatty acids. The distribution of saturated TAGs and the higher concentration of stearic acid compared to palmitic acid in the transesterified lipid extract indicated that this was sheep fat. The results illustrate how complex mixtures can be unravelled and original contents of ancient ceramic vessels can be determined using specialised analytical equipment.

Elucidation of the composition of bovine milk fat triacylglycerols using high-performance liquid chromatography-atmospheric pressure chemical ionisation mass spectrometry

Bovine milk fat triacylglycerols (TAGs) have been characterised using high-performance liquid chromatography-atmospheric pressure chemical ionisation mass spectrometry (HPLC-APCI-MS) and high-temperature gas chromatography-mass spectrometry (GC-MS). The complex nature of the fat meant that prefractionation was necessary to provide simpler fractions for more detailed molecular analyses. Silica thin-layer chromatography gave rise to two fractions, one of which contained predominantly butyric acid containing TAGs. Gel permeation chromatography (GPC) gave rise to 16 fractions, which were subsequently analysed using HPLC-APCI-MS. Twelve of the GPC fractions were also analysed by high-temperature GC-MS using a capillary column coated with a polarisable stationary phase. TAGs present in the fractions were correlated with those in the chromatogram of the whole milk fat through retention time comparison and the use of mass chromatograms. In total, 120 TAGs were identified.

Analysis of triacylglycerols with non-aqueous reversed-phase liquid chromatography and positive ion electrospray tandem mass spectrometry

A non-aqueous reversed-phase liquid chromatographic method coupled to electrospray ionisation (ESI) tandem mass spectrometry was developed for the analysis of triacylglycerols (TGs). The synthetic TGs studied were separated according to their equivalent carbon number with a gradient of methanol (containing 0.01% (v/v) formate adjusted to pH 5.3 with ammonia) and chloroform. ESI mass spectra of TGs yielded positive ion current signals for [M + NH(4)](+) and [M + NH(4)-RCOONH(4)](+). The mass spectra also showed signals believed to arise from [M + K](+). Collision-induced dissociation (CID) of the [M + NH(4)](+) precursor ion yielded [M + NH(4) - RCOONH(4)](+), [RCO + 74](+) and [RCO](+) product ions as aids for the structural elucidation of the TGs. In addition, [RCO - 18](+) and small amounts of [RCO - 2](+) product ions were also found. The latter ions were observed only for TGs containing unsaturated fatty acids. CID of ammoniated 1-stearoyl-2-oleoyl-3-linoleoyl-glycerol (18:0/18:1/18:2) indicated that neutral loss of the sn-2 fatty acid was energetically less favourable than loss of the fatty acid from the sn-1 or sn-3 position.

Determination of the conjugated linoleic acid-containing triacylglycerols in New Zealand bovine milk fat

Reversed-phase high-performance liquid chromatography (HPLC) with ultraviolet (UV) detection at 233 nm was used to separate, quantify, and identify the triacylglycerols (TAG) of milk fat that contain conjugated linoleic acid (CLA). The absorbance at 233 nm was substantially due to CLA-TAG (chromatography of some representative TAG devoid of CLA, such as tripalmitin and triolein, showed poor responses at 233 nm, 1/800th that of CLA-TAG). A CLA molar extinction coefficient at 233 nm of 23,360 L mol(-1) cm(-1) and an HPLC UV response factor were obtained from a commercially available cis-9,trans-11-CLA standard. This molar extinction coefficient was only 86% of reported literature values. Summation of all chromatographic peaks absorbing at 233 nm using the corrected response factor gave good agreement with independent determinations of total CLA by gas chromatography and UV spectrophotometry. This agreement allowed quantification of individual CLA-TAG peaks in the HPLC separation of a typical New Zealand bovine milk fat. Three CLA-containing TAG, CLA-dipalmitin, CLA-oleoyl-palmitin and CLA-diolein, were prepared by interesterification of tripalmitin with the respective fatty acid methyl esters and used to assign individual peaks in the reversed-phase chromatography of total milk fat, of which CLA-oleoyl-palmitin was coincident with the largest UV peak. Band fractions from argentation thin-layer chromatography of total milk fat were similarly employed to identify five predominant CLA-TAG groups in total milk fat: CLA-disaturates, CLA-oleoyl-saturates, CLA-vaccenyl-saturates, CLA-vaccenyl-olein, and CLA-diolein.

Lipids as carriers of anthropogenic signals from prehistory

Studies performed during the last two decades have shown that lipids are preserved in association with a wide range of artefacts and ecofacts recovered from archaeological sites, e.g. pottery vessels and skeletal remains. The majority of work in this area has focused on the use of molecular structures (‘biomarkers’) and distributions (‘fingerprints’) to assess the nature and origin of commodities associated with past cultural, economic and agricultural practices. However, since lipids, like all other classes of biomolecule, are affected by degradation (both pre– and post–burial), emphasis is now being placed on the complementary use of diagenetically robust, compound–specific stable isotope measurements to enhance the scope and reliability of archaeological interpretations. A feature of the δ 13 C values of individual lipids, rather than bulk measurements of biochemically more heterogeneous materials, lies in their capacity to reflect differences in both the isotopic composition of the carbon sources used in their biosynthesis and the routing of dietary lipids and their metabolites in consumer organisms. This isotopic information, accessible by gas chromatography–combustion–isotope ratio mass spectrometry, has opened up new avenues of investigation concerning human activity in prehistory.