Software (MSPECTRA) for automatic interpretation of triacylglycerol molecular mass distribution spectra and collision induced dissociation product ion spectra obtained by ammonia negative ion chemical ionization mass spectrometry

Advanced Tandem Mass Spectrometric Analysis of Complex Mixtures of Triacylglycerol Regioisomers: A Case Study of Bovine Milk Fat

Comprehensive analysis of triacylglycerol (TAG) regioisomers is extremely challenging, with many variables that can influence the results. Previously, we reported a novel algorithmic method for resolving regioisomers of complex mixtures of TAGs. In the current study, the TAG Analyzer software and its mass spectrometric fragmentation model were further developed and validated for a much wider range of TAGs. To demonstrate the method, we performed for the first time a comprehensive analysis of TAG regioisomers of bovine milk fat, a very important and one of the most complex TAG mixtures in nature containing FAs ranging from short to long carbon chains. This analysis method forms a solid basis for further investigation of TAG regioisomer profiles in various natural fats and oils, potentially aiding in the development of new and healthier foods and nutraceuticals with targeted lipid structures.

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.

Strategy for stereospecific characterization of natural triacylglycerols using multidimensional chromatography and mass spectrometry

Stereoisomeric determination of individual triacylglycerols (TAGs) in natural oils and fats is a challenge due to similar physicochemical properties of TAGs with different fatty acid combinations. In this study, we present a strategy to resolve the enantiomeric composition of nutritionally important TAGs in sea buckthorn (Hippophaë rhamnoides) as an example food matrix. The targeted strategy combines 1) fatty acid profiling with GC, 2) separation of TAGs with RP-HPLC, 3) stereospecific separation with chiral-phase HPLC and 4) structural characterization with MS. Three major asymmetric diacid- and triacid-TAG species were analyzed in sea buckthorn pulp oil. Off-line coupling of RP-HPLC and chiral-phase HPLC allowed separation of several TAG regioisomers and enantiomers, which could not be resolved using one-dimensional techniques. Enantiomeric ratios were determined and specific structural analysis of separated TAGs was performed using direct inlet ammonia negative ion chemical ionization method. Of the TAG 16:0/16:1/16:1 palmitic acid (C16:0) was located predominantly in a primary position and the enantiomeric ratio of TAG sn-16:1-16:1-16:0 to sn-16:0-16:1-16:1 was 70.5/29.5. Among the TAGs 16:0/16:0/18:2 and 16:0/16:0/16:1, only ca 5% had C16:0 in the sn-2 position, thus, ca 95% were symmetric sn-16:0-18:2-16:0 and sn-16:0-16:1-16:0. The enantiomeric ratio of triacid-TAGs containing C16:0 and two unsaturated fatty acids (palmitoleic C16:1, oleic C18:1 or linoleic acids C18:2) could not be resolved due to lack of commercial enantiopure reference compounds. However, it became clear that the targeted strategy presented offer unique and convenient method to study the enantiomeric structure of individual TAGs.

Direct inlet negative ion chemical ionization tandem mass spectrometric analysis of triacylglycerol regioisomers in human milk and infant formulas

A previously developed direct inlet tandem mass spectrometric method for analysis of triacylglycerol (TAG) regioisomers was updated and validated for operation with current instrumentation with improved sensitivity and throughput. TAG regioisomers in pooled Chinese and Finnish human milk samples, two bovine milk samples and 11 infant formulas were identified and quantified. A total of 241 TAG regioisomers were identified and quantified, consisting of over 60 mol% of all TAGs in the human milk samples. The infant formulas deviated largely from human milk in regioisomeric composition of TAGs. In the Finnish and Chinese human milks, the most abundant ones were 1,3-dioleoyl-2-palmitoylglycerol (OPO; 7.4 and 8.8 mol% of all TAGs) and 1(3)-linoleoyl-2-palmitoyl-3(1)-oleoylglycerol (LPO; 4.7 and 8.3 mol% of all TAGs). In the infant formulas 1,2(2,3)-dioleoyl-3(1)-palmitoylglycerol (OOP) and 1(3)-linoleoyl-2-oleoyl-3(1)-palmitoylglycerol/1(3)-palmitoyl-2-linoleoyl-3(1)-oleoylglycerol (LOP/PLO) were more abundant than OPO and LPO. The differences between human milk and infant formula prompt for further development of current formulas.

A simple and economical strategy for obtaining calibration plots for relative quantification of positional isomers of YYX/YXY triglycerides using high-performance liquid chromatography/tandem mass spectrometry

RATIONALE

Positional analysis of intact triglycerides could provide greater insights into the link between fatty acid position and lipotoxic diseases. However, this methodology has been impeded by lack of commercial availability of positionally pure triglycerides. This work reports on a strategy for defining calibration plots for YXY/YYX triglyceride systems based on the product ion intensities in the collision-induced dissociation spectra of ammoniated precursor ions.

METHODS

A set of triglycerides were synthesized and analyzed by electrospray ionization tandem mass spectrometry using an ion trap mass spectrometer. The product ion spectra of the ammoniated precursor ions were collected for 42 triglyceride systems of the form YXY/YYX, where Y represents C_16:0 , C_18:1(c-9) and C_{20:4(cccc-5,8,11,14)} . Three-point calibration plots were prepared by plotting the relative abundance of the YY⁺ product ion vs. the relative abundance of the YYX positional isomer.

RESULTS

The calibration plots were shown to give relative abundances of positional isomers accurate to within ±0.02 for most systems. Using an ion trap, under a controlled set of collision parameters, the slopes of the calibration plots can be used to compare the sensitivities of the product ion intensities to fatty acid position for various triglyceride systems. The average slopes of the calibration plots for the C_16:0 , C_18:1(c-9) and C_{20:4(cccc-5,8,11,14)} systems were 0.29 ± 0.05, 0.21 ± 0.05 and 0.045 ± 0.005, respectively.

CONCLUSIONS

While the presence of multiple unsaturated fatty acids tends to slightly decrease the slopes of the calibration plots, the data suggest that the sensitivities are sufficient for performing positional analysis of most triglyceride systems. However, the presence of unsaturated fatty acids that contain double bonds close to the carbonyl group, such as arachidonic acid, tends to dramatically decrease positional sensitivity.

Triacylglycerol regioisomers in human milk resolved with an algorithmic novel electrospray ionization tandem mass spectrometry method

A highly sensitive mass spectrometric (MS) method was developed and validated to analyze ratios of regioisomeric triacylglycerols (TAGs) in fats and oils. UPLC resolution of lithiated TAGs followed by daughter scan MS/MS of positive ions revealed several indicative ions for quantitative analysis. Reference TAGs containing C14-C20 fatty acids (FAs) showed good linear response. Analysis of Finnish and Chinese pooled human milk samples revealed hundreds of regioisomeric TAGs. At least 64mol% of the TAGs were quantified with relative standard deviation <17%. When present in the same TAG molecule together with C18 FAs, palmitic acid was typically in the sn-2 position. When together with FAs 10:0, 12:0, 14:0, 20:1 and 20:2, the sn-2 preference of 16:0 was less clear. Oleic acid occupied typically the sn-1/sn-3 positions but when together with FAs 20:1, 20:2, 18:2, 14:1, 12:0 or 10:0 the positioning of 18:1 did not follow these rules.

Tandem mass spectrometric analysis of human milk triacylglycerols from normal weight and overweight mothers on different diets

The composition and structures of TAGs in the human milk from mothers with different food choices and prepregnancy body mass index were determined with two tandem mass spectrometric methods (negative APCI-MS/MS and positive UHPLC/ESI-MS/MS) at the infant's age of three months. The normal weight mothers with recommended food choices had more 18:3n-3 and less 18:0 in their milk than normal weight mothers with non-recommended food choices. A significant difference between the normal weight mothers on the non-recommended food choices and the other groups was seen in acyl carbon number: number of double bond (ACN:DB)-groups 54:6, 54:5, 54:3 and 54:2. In ACN:DB 52:7 and 52:6 the two recommended food choices-groups differed significantly from the two non-recommended food choices-groups. The regioisomerism of TAGs varied little despite differences in mother's weight and diet with sn-18:1-16:0-18:1 as the most prevalent regioisomer in the milk (13.8±2.7%). The results of this study highlight the importance of structure specific human milk substitutes and the careful selection of the MS/MS methods for analysis of mixtures of several isobaric TAGs.

Glycerolipid and cholesterol ester analyses in biological samples by mass spectrometry

Neutral lipids are a diverse family of hydrophobic biomolecules that have important roles in cellular biochemistry of all living species but have in common the property of charge neutrality. A large component of neutral lipids is the glycerolipids composed of triacylglycerols, diacylglycerols, and monoacylglycerols that can serve as cellular energy stores as well as signaling molecules. Another abundant lipid class in many cells is the cholesterol esters that are on one hand sterols and the other fatty acyl lipids, but in either case are neutral lipids involved in cholesterol homeostasis and transport in the blood. The analysis of these molecules in the context of lipidomics remains challenging because of their charge neutrality and the complex mixtures of molecular species present in cells. Various techniques have been used to ionize these neutral lipids prior to mass spectrometric analysis including electron ionization, atmospheric chemical ionization, electrospray ionization and matrix assisted laser desorption/ionization. Various approaches to deal with the complex mixture of molecular species have been developed including shotgun lipidomics and chromatographic-based separations such as gas chromatography, reversed phase liquid chromatography, and normal phase liquid chromatography. Several applications of these approaches are discussed. .

Quantification of triacylglycerol regioisomers by ultra-high-performance liquid chromatography and ammonia negative ion atmospheric pressure chemical ionization tandem mass spectrometry

The regioisomer composition of triacylglycerols (TAGs) in various vegetable oils was determined with a new liquid chromatography/tandem mass spectrometry (LC/MS/MS method). A direct inlet ammonia negative ion chemical ionization (NICI) MS/MS method was improved by adapting it to LC negative ion (NI) atmospheric pressure chemical ionization (APCI) MS/MS system using ammonia as nebulizer gas. The method is based on the preferential formation of [M-H-RCOOH-100](-) ions during collision-induced dissociation by loss of sn-1/3 fatty acids from [M-H](-) ions. Calibration curves were created from nine reference TAGs: Ala/L/L, Gla/L/L, L/L/O, L/O/O, P/O/O, P/P/O, Po/Po/V, Po/Po/O, and C/O/O. The calibration curves were used to quantify the regioisomer compositions of selected TAGs in rapeseed oil, sunflower seed oil, palm oil, black currant seed oil, and sea buckthorn pulp oil. The method discriminates the different regioisomers and the results obtained by this method were in good agreement with previous results. This proves that this new method can be used for the determination of regiospecific distribution of fatty acids in TAGs.

Computer-assisted interpretation of triacylglycerols mass spectra

Triacylglycerols (TGs) are principal components of vegetable oils and animal fats. Natural TGs form extremely complex mixtures composed of tens or hundreds of molecular species. HPLC/MS suits well for their analyses, but manual data processing is laborious and time-consuming. Specialized software algorithms are needed to accelerate the interpretation process. Here we present software named TriglyAPCI for interpreting APCI, APPI, or ESI MS/MS spectra of TGs. The chapter shows how to build and use the software, what are its advantages and limitations. The algorithm uses diacylglycerol fragments and molecular adducts for determining TG structure. Each ion in a spectrum is tested whether it might be a fragment or a molecular adduct. If so, the number of carbons and double bonds is assigned to it. The relations among the ions are searched and possible structures are suggested. TriglyAPCI allows interpreting spectra of single compounds, mixtures, or incomplete spectra lacking one of the diagnostic ions. The fragment intensities are used to distinguish regioisomers. An efficient chromatographic separation of TGs is shown to be crucial for correct spectra interpretation and avoiding false positive results. TriglyAPCI performance is demonstrated for HPLC/APCI-MS data of TGs isolated from bumblebee Bombus rupestris.

Software tools for analysis of mass spectrometric lipidome data

New software tools for quantitative analysis of mass spectrometric lipidome data have been developed. The LIMSA tool finds and integrates peaks in a mass spectrum, matches the peaks with a user-supplied list of expected lipids, corrects for overlap in their isotopic patterns, and quantifies the identified lipid species according to internal standards. Three different algorithms for isotopic correction (deconvolution) were implemented and compared. LIMSA has a convenient user interface and can be applied on any type of MS spectrum. Typically, analysis of one spectrum takes only a few seconds. The SECD tool, designed for analysis of LC-MS data sets, provides an intuitive and informative display of MS chromatograms as two-dimensional "maps" for visual inspection of the data and allows the user to extract mass spectra, to be further analyzed with LIMSA, from arbitrary regions of these maps. More reliable analysis of complex lipidome data with improved signal-to-noise ratio is obtained when compared to standard time-range averaged spectra. The functionality of these tools is demonstrated by analysis of standard mixtures as well as complex biological samples. The tools described here make accurate, high-throughput analysis of extensive sample sets feasible and are made available to the scientific community free of charge.

Quantification of triacylglycerol regioisomers in oils and fat using different mass spectrometric and liquid chromatographic methods

The regioisomers (sn-ABA/sn-AAB) of four triacylglycerols (TAGs), 18:2/18:2/18:1 (LLO), 18:2/18:1/18:1 (LOO), 16:0/18:1/18:1 (POO), and 16:0/16:0/18:1 (PPO), were quantified in lard, rapeseed oil, and sunflower seed oil by three different mass spectrometric methods using liquid chromatography (LC) and two different mass spectrometers. The ionization methods used were positive ion atmospheric pressure chemical ionization (APCI), positive ion electrospray ionization (ESI), and negative ion chemical ionization (NICI) with ammonia as the reagent gas. The LC/APCI-MS results with two different instrumentation types, LC/ESI-MS/MS and direct inlet ammonia NICI-MS/MS, were compared. The LC/APCI-MS method is based on the preferential formation of diacylglycerol (DAG) fragment ions during ionization by loss of sn-1/3 fatty acids from [M+H]+ ions. Similar formation of the DAG ions from [M+NH4]+ ions by collision-induced dissociation (CID) in the LC/ESI-MS/MS method and the [M-H--RCOOH-100]- ions from [M-H]- ions by CID in the direct inlet ammonia NICI-MS/MS method is observed. These methods were found to be useful and reliable in determining the regioisomeric structure of TAGs. No statistically significant differences were found between the results obtained with these methods. For LLO, LOO, and POO the proportions of sn-ABA isomer calculated from the results from all four methods were in rapeseed oil 7.7 +/- 6.5, 57.9 +/- 3.3, and 4.5 +/- 6.1%, respectively, and in sunflower seed oil 12.2 +/- 6.9, 34.0 +/- 5.2, and 1.4 +/- 2.8%, respectively. The proportions of ABA of POO and PPO in lard were 95.3 +/- 3.2 and 4.9 +/- 5.6%, respectively. This study also proved that the LC/APCI-MS/MS method examined is not applicable in the quantification of TAG regioisomers because the formation of DAG ions is not clearly dependent on the positional distribution of the fatty acids.

Quantification of triacylglycerols in butterfat by gas chromatography-electron impact mass spectrometry using molar correction factors for [M-RCOO]+ ions

A quantitative electron impact GC-MS method using molar correction factors (MCFs) for [M-RCOO]+ ions has been developed for determination of molecular species of triacylglycerols (TAGs). MCFs were determined by linear calibration for 226 ions of 104 TAG species with good reproducibility: on the average, coefficient of determination was 0.975 +/- 0.043 and 0.963 +/- 0.115 for saturated and unsaturated TAGs, respectively. The MCFs of the sn-1(3) regioisomers of short-chain TAGs were lower than those of sn-2 isomers indicating ca. 2-3-fold higher cleavage of butyroyl and caproyl groups from the primary positions than from the secondary position. The method enabled quantification of 139 and 135 individual TAG species of butterfat (BF) and interesterified butterfat, respectively, including several regioisomers of short-chain TAGs. The most abundant molecular species of the even-numbered TAGs in BF were butyroylpalmitoyloleoylglycerol (5.05 mol%), butyroyldipalmitoylglycerol (4.75 mol%), and palmitoyldioleoylglycerol (3.32 mol%). The method provides an alternative for elucidation of nutritional and technological properties of relatively saturated TAG mixtures.

Analysis of triacylglycerols of seeds and berries of sea buckthorn (Hippophaë rhamnoides) of different origins by mass spectrometry and tandem mass spectrometry

TAG of seeds, berries, and fruit pulp/peel of different subspecies of sea buckthorn (Hippophaë rhamnoides) were analyzed by MS and tandem mass spectrometry (MS/MS). The seeds contained mainly TAG with acyl carbon number (ACN) of 52 with 2-6 double bonds (DB) (20-30%), and TAG of ACN 54 with 3-9 DB (70-80%). In the pulp/peel fraction, the major TAG were species with ACN:DB of 48:1 to 48:3 (19-49%), 50:1 to 50:4 (31-41%), and 52:1 to 52:6 (9-19%). The molecular weight species of whole berries largely resembled those of fruit pulp/peel with additional species of ACN 54 from the seeds (5-24%). Subspecies (ssp.) sinensis differed from ssp. mongolica and rhamnoides by having a higher proportion of TAG of ACN 52 (27% vs. 21% and 22%, P< 0.05) and a lower proportion of ACN 54 (71% vs. 79% and 78%, P < 0.01) in seed TAG. Seed TAG of ssp. mongolica contained a higher proportion of more unsaturated species compared with those of the two other subspecies. Berry TAG of ssp. mongolica had the highest proportion of molecular species of ACN 48 due to the higher proportion of palmitic and palmitoleic acids and the lower seed content of the berries. Overall, palmitic acid favored the sn-1 and sn-3 positions. The order of preference of unsaturated FA for the sn-2 position depended at least partially on the FA combination of TAG. Seed TAG of ssp. mongolica contained a higher proportion of alpha-linolenic acid in the sn-2 position than those of ssp. sinensis. In berry TAG, ssp. mongolica had the highest proportions of palmitoleic and linoleic acids in the sn-2 position, and the lowest proportion of oleic/cis-vaccenic acid in the sn-2 position, among the three subspecies.

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.

Computer-assisted interpretation of atmospheric pressure chemical ionization mass spectra of triacylglycerols

Current lipidomics approaches require simple and rapid algorithms enabling the interpretation of mass spectra of lipids. Most lipids are complex mixtures of related components in which the composition of the aliphatic fatty acid chains varies from one molecule to the next. Triacylglycerols (TAGs) are an example of such a lipid class. Fatty acid chains are the only parts of the molecule to change from one species to another. Fatty acids, and consequently also TAGs, can be characterized by two parameters; the number of carbon atoms and the number of double bonds. All calculations reflecting relations among ions in the spectra can be easily made using these parameters. An algorithm for the automated interpretation of TAGs from atmospheric pressure chemical ionization mass spectra (TriglyAPCI) is presented in this paper. The algorithm first identifies diacylglycerol fragments and molecular adducts. In the next step, relations among the ions are searched and possible TAG structures are suggested. Individual features of the algorithm are described in detail and the software performance is demonstrated for the liquid chromatography/mass spectrometric (LC/MS) analysis of TAGs isolated from the termite Prorhinotermes canalifrons.

Regiospecific determination of short-chain triacylglycerols in butterfat by normal-phase HPLC with on-line electrospray-tandem mass spectrometry

This study uses normal-phase HPLC with on-line positive ion electrospray mass spectrometry (ESI-MS) to obtain quantitative compositional data on both synthetic and butterfat short-chain TAG. The product ion tandem MS of standards averaged 11.1 times lower in abundance of the ion formed by cleavage of FA from the sn-2-position for the pairs of regioisomers in the TAG classes: L/L/S-L/S/L and L/S/S-S/L/S, where L denotes long and S short acyl chain (C2-C6). The molar correction factors, determined for 42 regioisomeric pairs of short-chain TAG of 20 randomized mixture of standards, differed by 1.4-80% as the ratios varied between 0.217 and 5.847. Butterfat TAG were resolved into four fractions on short flash chromatography grade silica gel columns. Pairs of regioisomers in the TAG classes L/S/S-S/L/S with predominance of L/S/S isomers and the sole regioisomers in the TAG classes L/L(M)/S were identified by tandem MS, where M denotes either 8:0 or 10:0 acyl chain. The total proportion of L/L(M)/S isomers was estimated at 34.7 and that of L/S/S-S/L/S at 1.0 mol%, including a small proportion of S/S/S. In contrast to previous work, the present data indicate the presence of a small proportion of butyric and caproic acids in the sn-1-position. The overall distribution of the FA in the short-chain TAG of butterfat, calculated from direct MS measurements, was consistent with the results of indirect determinations based on stereospecific analyses of total butterfat.

Positional distribution of decanoic acid: effect on chylomicron and VLDL TAG structures and postprandial lipemia

Although medium-chain FA (MCFA) are mainly absorbed via the portal venous system, they are also incorporated into chylomicron TAG; therefore, the positional distribution of MCFA in TAG is likely to affect their metabolic fate. We studied chylomicron and VLDL TAG structures, as well as the magnitude of postprandial lipemia, after two oral fat loads containing decanoic acid (10:0) predominantly at the sn-1(3),2 (MML) or at the sn-1,3 positions (MLM) of TAG in a randomized, double-blind, crossover clinical trial with 10 healthy, normal-weight volunteers. An MS-MS method was used to analyze TAG regioisomers. The position of decanoic acid in chylomicron TAG reflected its position in the TAG ingested, and TAG with none, one, two, or three decanoic acid residues were detected after ingestion of both fats. More (P < 0.05) 30:0 and 38:1 TAG (acyl carbons:double bonds) and fewer 46:5, 54:5, and 54:4 TAG were found in chylomicrons after ingestion of MML than after MLM. The VLDL TAG composition did not differ between the fat loads but did change (P < 0.05) 2 to 6 h after ingestion of both fats. No statistical differences were seen between the fat loads in areas under the plasma, chylomicron, or VLDL TAG response curves or in FFA concentrations. Thus, the positional distribution of MCFA in TAG affects their metabolic fate, but the magnitude of postprandial lipemia does not seem to be dependent on the positional distribution of MCFA in the ingested fat.

Regio- and stereospecific analysis of glycerolipids

In recent years researchers have recognized the potential value of comprehensive lipid profiling (lipidomics), which was invented and promoted by lipidologists who recognized the many valuable applications that grew out of the fields of DNA profiling (genomics) and protein profiling (proteonomics). Through lipid class-selective intrasource ionization and subsequent analysis of two-dimensional cross-peak intensities, the chemical identity and mass composition of individual molecular species of most lipid classes can now be determined in a chloroform extract. There remains, however, the necessity to distinguish the enantiomers and isobaric regioisomers resulting from enzymatic and chemical reactions, which conventional high performance liquid chromatography/mass spectrometry (HPLC/MS) has been slow to accommodate, and tandem MS unable to provide. While reversed-phase HPLC can separate regioisomers, normal-phase HPLC can resolve diastereomers, and chiral-phase HPLC can effect dramatic resolution of enantiomers, the full potential of the combined systems has seldom been exploited. The present chapter calls attention to both recent and earlier combinations of these methodologies with mass spectrometry, which allows the HPLC/ESI (electrospray ionization)-MS/MS separation and identification of enantiomeric diacylglycerols, triacylglycerols, and glycerophospholipids as well as their isobaric regioisomers. These developments permit further expansion of lipid profiling (lipidomics) and better understanding of lipid metabolism.

Examining the collision-induced decomposition spectra of ammoniated triglycerides as a function of fatty acid chain length and degree of unsaturation. I. The OXO/YOY series

A series of positionally pure triglycerides (TAGs) of the form OXO and YOY, where O is the oleate moiety and X and Y are large arrays of different fatty acid moieties, was synthesized and analyzed by reversed-phase high-performance liquid chromatography/tandem mass spectrometry. The intensities of the collision-induced decomposition (CID) products of ammoniated TAGs (ammonium ion adducts) were examined as a function of chain length, degree of unsaturation, double-bond position, and cis/trans configuration of X and Y. The major CID products, the diglyceride fragment ions and the MH+ ion, were plotted as functions of chain length for the saturated and mono-unsaturated series of X and Y. Different trends for each of these series were observed. Trends in the abundances of these fragment ions were also characterized as a function of degree of unsaturation in the TAGs. In general, the fractional abundances of the MH+ ions vary linearly with degree of unsaturation. However, the presence of double bonds positioned close to the carbonyl carbon of the fatty acid chain promotes the formation of the diglyceride fragment ion corresponding to loss of that fatty acid. Mechanisms of the formation and decomposition of ammoniated TAGs are proposed that fit the trends observed in the data. Extensions of this work are described, and a vision of a derived library of CID spectra is discussed as a platform for comprehensive analysis of complex TAG mixtures.

Chylomicron and VLDL TAG structures and postprandial lipid response induced by lard and modified lard

Alterations in chylomicron and VLDL TAG and the magnitude of postprandial lipemia were studied in healthy volunteers after two meals of equal FA composition but different TAG FA positional distribution. Molecular level information of individual lipoprotein TAG regioisomers was obtained with a tandem MS method. The incremental area under the response curve of VLDL TAG was larger (P = 0.021) after modified lard than after lard. In plasma TAG, the difference did not quite reach statistical significance (P = 0.086). In general, there were less TAG with palmitic acid in the sn-2 position and more TAG with oleic acid in the sn-2 position in chylomicrons than in fat ingested. From 1.5 to 8 h postprandially, the proportion of individual chylomicron TAG was constant or influenced by TAG M.W. VLDL TAG regioisomerism was similar regardless of the positional distribution of fat ingested. Significant alterations were seen in VLDL TAG FA, in M.W. fractions, and in individual regioisomers with respect to time. The TAG sn-14:0-18:1-18:1 + sn-18:1-18:1-14:0, sn-16:0-16:1-18:1 + sn-18:1-16:1-16:0, and sn-16:1-18:1-18:1 + sn-18:1-18:1-16:1 decreased (P < 0.05); and sn-16:0-16:0-18:2 + sn-18:2-16:0-16:0, sn-16:0-16:0-18:1 + sn-18:1-16:0-16:0, sn-16:0-18:1-16:0, and sn-16:0-18:1-18:2 + sn-18:2-18:1-16:0 increased (P < 0.05) after both meals. In conclusion, positional distribution of TAG FA was found to affect postprandial lipid metabolism in healthy normolipidemic subjects.

Quantitative analysis of triacylglycerol regioisomers in fats and oils using reversed-phase high-performance liquid chromatography and atmospheric pressure chemical ionization mass spectrometry

Positional distribution of fatty acyl chains of triacylglycerols (TGs) in vegetable oils and fats (palm oil, cocoa butter) and animal fats (beef, pork and chicken fats) was examined by reversed-phase high-performance liquid chromatography (RP-HPLC) coupled to atmospheric pressure chemical ionization using a quadrupole mass spectrometer. Quantification of regioisomers was achieved for TGs containing two different fatty acyl chains (palmitic (P), stearic (S), oleic (O), and/or linoleic (L)). For seven pairs of 'AAB/ABA'-type TGs, namely PPS/PSP, PPO/POP, SSO/SOS, POO/OPO, SOO/OSO, PPL/PLP and LLS/LSL, calibration curves were established on the basis of the difference in relative abundances of the fragment ions produced by preferred losses of the fatty acid from the 1/3-position compared to the 2-position. In practice the positional isomers AAB and ABA yield mass spectra showing a significant difference in relative abundance ratios of the ions AA(+) to AB(+). Statistical analysis of the validation data obtained from analysis of TG standards and spiked oils showed that, under repeatability conditions, least-squares regression can be used to establish calibration curves for all pairs. The regression models show linear behavior that allow the determination of the proportion of each regioisomer in an AAB/ABA pair, within a working range from 10 to 1000 microg/mL and a 95% confidence interval of +/-3% for three replicates.

Triacylglycerol molecular weight and to a lesser extent, fatty acid positional distribution, affect chylomicron triacylglycerol composition in women

Postprandial composition of chylomicron triacylglycerols (TAG) and their clearance may be affected by the molecular weight of TAG, their fatty acid (FA) combinations and the positional distribution of FA in TAG. Delayed postprandial TAG clearance is a risk factor for cardiovascular disease. However, due to the complexity of traditional analysis methods, the composition of individual TAG molecules is frequently overlooked. In this study, chylomicron TAG molecular weight distribution and regioisomerism were followed in 10 healthy female volunteers after two fat loads with identical FA composition but different positional distributions (palm oil and transesterified palm oil). An efficient tandem mass spectrometric method of analysis was applied. During the 6-h observation period, the relative concentrations of TAG with 48:2 [48 acyl carbons and 2 double bonds (ACN:DB)], 50:3 and 50:2 decreased, whereas the proportions of 48:0 (tripalmitin), 52:3 and 54:4 remained constant and the proportion of 54:3 (triolein) increased (P < 0.05). The existence of seven regioisomers containing palmitic, oleic and linoleic acids in different sn-positions was studied. The amount of 1,3-dipalmitoyl-2-oleoyl-sn-glycerol was less (P < 0.05) 1.5 h postprandially than at 2-5 h after palm oil, and less (P < 0.05) at 1.5 h than at 2-6 h after transesterified palm oil. This may be an indication of a loss of palmitic acid in the gut. Taken together, TAG molecular weight composition and to a lesser extent, positional distribution, seem to affect the rates of chylomicron TAG clearance in humans.

Software algorithm for automatic interpretation of mass spectra of glycerolipids

A new software algorithm for automatic interpretation of mass spectra of glycerolipids has been developed. The algorithm utilizes a user-specified list of parameters needed to process the spectra. The compounds in mass spectra are identified according to range of measured m/z values, after which the spectra are automatically corrected by the content of naturally occurring isotopes and ion intensities of identified compounds by response correction factors. Automatic processing of the spectra was shown to be accurate and reliable by testing with numerous spectra of glycerophospholipids obtained by liquid chromatography/electrospray ionization mass spectrometry and by comparing the results with manual interpretation of the spectra. If quantitative analysis using internal standards is performed, all the identified compounds in the sample are quantified automatically. A dilution factor may be defined for each sample and is applied to correct the alterations in sample concentration during sample preparation. Processing of several replicate spectra simultaneously produces mean results with standard deviations. The software may also be used to subtract the results of two analyses and to calculate the mean result of replicate subtractions. The algorithm was shown to save time and labor in repetitive processing of mass spectra of similar type. It may be applied to processing of spectra obtained by various mass spectrometric methods.

Automated molecular weight assignment of electrospray ionization mass spectra

Process improvements in the synthesis of therapeutic agents and their intermediates are often facilitated by identification of reaction by-products. Analysis by liquid chromatography/mass spectrometry (LC/MS) with electrospray ionization is a powerful approach for obtaining molecular weight information for these compounds. Such analyses are well suited for 'open-access' mass spectrometry using generic chromatographic conditions, provided spectral interpretation for unknown compounds is facile. We have developed a software application (MassAssign) that facilitates automated data processing and molecular weight assignment for chromatographic peaks detected by any standard ultraviolet-visible wavelength detector. The program assigns [M + H](+) ions (and thus molecular weight) in the mass spectra using predetermined criteria. This evaluation process differentiates [M + H](+) ions from other signals in a complex mass spectrum such as those resulting from chromatographic coelution or the presence of multiple species (i.e., fragment ions, singly charged ions, doubly charged ions, adduct ions, proton-bound dimers, etc.). Once the program has evaluated all ions in a mass spectrum that exceed a preset abundance threshold, MassAssign reports either a numeric value-indicating the chromatographic peak consists of a single component having the displayed molecular weight, 'MC'-indicating the peak consisted of multiple components, or 'ND'-that a molecular weight could not be determined unequivocally. The performance of the program was evaluated by comparing mass assignments made by MassAssign against manual interpretation for 55 samples analyzed by positive electrospray ionization using a generic HPLC method. Correct molecular weight assignments were obtained in 90% of the cases.