Metabolic Profiling of the Oil of Sesame of the Egyptian Cultivar ‘Giza 32’ Employing LC-MS and Tandem MS-Based Untargeted Method

Sesame (Sesamum indicum L.) is a global oil crop. Sesame oil has been regarded as functional oil with antioxidant properties in several in vivo studies but little is known about its minor fraction. In this line, this study figures out the profile of the polar fraction of Egyptian cultivar Giza 32 sesame oil (SG32 oil) employing reversed-phase high-performance liquid chromatography coupled with diode array detection and electrospray ionization-quadrupole-time-of-flight-mass spectrometry and tandem MS. The characterization of the sesame oil metabolites depended on the observation of their retention time values, accurate MS, and MS/MS data, with UV spectra, and compared with relevant literature and available standards. Remarkably, 86 metabolites were characterized and sub-grouped into phenolic acids, lignans, flavonoids, nitrogenous compounds, and organic acids. From the characterized metabolites, 72 compounds were previously characterized in SG32 cake, which presented antioxidant properties, and hence it could contribute to SG32 oil antioxidant properties. Further studies are required to state the presence of such phenolics in commercial sesame oils and what of these compounds resist oil refining.


Introduction
Family Pedaliaceae belongs to the order Lamiales and consists of 14 genera and 70 species, including the famously known as sesame (Sesamum indicum L.) [1]. It is a cropproducing oil whose cultivation is distributed globally. On the base of total production, sesame seeds production is nearly 7 million tons with a production of 2 million tons of sesame oil [2]. Anciently, sesame originated from India. It was known in the Ancient Egyptian Civilization for the treatment of asthma since the third century BC [3][4][5]. Sesame seeds contain fats, proteins, carbohydrates, vitamins, and dietary fibers [5,6].
Besides their nutritional properties, several scientists explored possible biological activities and phytoconstituents of sesame seeds. In this sense, Dravie et al. [7] examined the antioxidant properties, total phenols, and flavonoids contents of Ghanaian sesame seeds via a multiple solvent extractions model. It was clear that the acetone extract was by 5 mL methanol:water (80:20, v/v) based on Ishtiaque et al. [20] to get the polar fraction of SG32 oil. The extraction mixture was agitated, centrifuged, and the supernatant collected. The previous step was repeated twice (methanol:water (80:20, v/v) × 2.5 mL). The methanolic-aqueous extracts were combined and defatted with n-hexane (2 mL) to eliminate any residual fat and concentrated using a speed-vacuum Concentrator plus (Eppendorf AG, Hamburg, Germany) at 30 • C for above 3 h. The polar oil fraction was appropriately dissolved in 2 mL of aqueous methanol (80:20, v/v) previously to subjection to RP-HPLC-DAD-ESI-QTOF-MS and -tandem MS analysis.

Analysis by RP-HPLC-DAD-ESI-QTOF-MS and -Tandem MS
The HPLC was an Agilent 1200 series equipped with a binary pump, an autosampler, and a diode array detector (DAD), (Agilent Technologies, Santa Clara, CA, USA) [21,22]. The separating column was a core-shell Halo C18 (150 mm × 4.6 mm, 2.7 µm particle size, Advanced Materials Technologies, Wilmington, DE, USA). The system was hyphenated to a 6540 Agilent Ultra-High-Definition (UHD) Accurate-Mass Q-TOF LC/MS equipped with an Agilent Dual Jet Stream electrospray ionization (Dual AJS ESI) interface. MassHunter Workstation software (Agilent Technologies) was used for data acquisition (2.5 Hz) in profile mode. The spectra were acquired over a mass-to-charge (m/z) range from 70 to 1500 in negative-ion mode. The detection window was set to 100 ppm.

RP-HPLC-DAD-ESI-QTOF-MS and Tandem-MS of SG32 Oil
The phenolic fraction of SG32 oil was analyzed in negative ionization mode via core-shell RP-HPLC-DAD-ESI-QTOF-MS and tandem MS. Tables 1 and 2 classify the characterized metabolites into phenolics and non-phenolics. Besides, they demonstrate for each candidate the time (RT), experimental m/z, generated molecular formulas, mass errors, scores, double bond equivalents (DBE), UV maxima (if present), tandem mass fragments, and relative abundance (area of chromatographic profiles of all characterized metabolites), respectively. This information was used for the characterization work, which was based on the strategy followed in our previous studies. Basically, the RT, molecular formula, and the fragmentation patterns were compared to those found in literature, databases, and standards, when possible. Moreover, fragmentation patterns enabled us to obtain clues about the functional groups, basic constituents, and/or polyphenol nucleus [10,23,35]. Nonetheless, further confirmation is required by NMR to also establish the stereochemistry.
Also, Tables S1 and S2 (supplementary material) mention metabolites classifications, and cite their previous description in the literature. A total of 86 metabolites were characterized with 11 metabolites reported for the first time in sesame, 59 metabolites observed for the first time in sesame oil, and 3 new proposed structures.
Moreover, Figure 1a represents the base peak chromatogram obtained by RP-HPLC-DAD-ESI-QTOF-MS showing the complexity of the minor constituents of sesame oil as that for the seed cake ( Figure 1c).

Phenolic Acids
The presence of phenolic acids was noticed with 32 phenolic compounds, being the major class of the annotated metabolites in qualitative and quantitative terms (Figure 1b, see Section 3.2). They belonged to three subclasses viz., hydroxybenzoic acids (12), hydroxycinnamic acids (19), and a hexahydroxydiphenic acid dilactone (Table 1 and Table S1).
Concerning hydroxybenzoic acids, compounds at m/z 121.03 and 135.05 showed the loss of CO (28 Da) and CO 2 (44 Da), and λ max 278 and 273 nm, respectively. They were described as benzoic acid and a methyl derivative [10]. Methylated and methoxy derivatives were also tentatively identified. In this sense, compounds at m/z 151.40 (C 8 H 8 O 3 ) exerted neutral losses of methyl (CH 3 , 15 Da) followed by decarboxylation (CO 2 , 44 Da), which are typical of the presence of methoxy groups and phenolic acids, respectively. They were annotated as methoxybenzoic acid isomers I-II according to the Reaxys database. Figure 2a describes the main fragments of methoxybenzoic acid isomer II. Similarly, a methylated derivative was observed at RT 14.61 min and was characterized as hydroxybenzoic acid methyl ester. In this case, a loss of CH 2 was observed instead of CH 3 as in the aforementioned cases. Moreover, a mono-hydroxylated benzoic acid was also characterized, hydroxybenzoic acid (m/z 137.02, C 7 H 6 O 3 ), with the sequential loss of water and CO 2 [21].

Phenolic Acids
The presence of phenolic acids was noticed with 32 phenolic compounds, being the major class of the annotated metabolites in qualitative and quantitative terms (Figure 1b, see Section 3.2). They belonged to three subclasses viz., hydroxybenzoic acids (12), hydroxycinnamic acids (19), and a hexahydroxydiphenic acid dilactone (Tables 1 and S1).
Concerning hydroxybenzoic acids, compounds at m/z 121.03 and 135.05 showed the loss of CO (28 Da) and CO2 (44 Da), and λmax 278 and 273 nm, respectively. They were described as benzoic acid and a methyl derivative [10]. Methylated and methoxy derivatives were also tentatively identified. In this sense, compounds at m/z 151.40 (C8H8O3) exerted neutral losses of methyl (CH3, 15 Da) followed by decarboxylation (CO2, 44 Da), which are typical of the presence of methoxy groups and phenolic acids, respectively. They were annotated as methoxybenzoic acid isomers I-II according to the Reaxys database. Figure 2a describes the main fragments of methoxybenzoic acid isomer II. Similarly, a methylated derivative was observed at RT 14.61 min and was characterized as hydroxybenzoic acid methyl ester. In this case, a loss of CH2 was observed instead of CH3 as in the aforementioned cases. Moreover, a mono-hydroxylated benzoic acid was also characterized, hydroxybenzoic acid (m/z 137.02, C7H6O3), with the sequential loss of water and CO2 [21].  Dihydroxybenzoic acids were detected, and illustrated as, protocatechuic and vanillic acids (O-methylated derivative), at RT 11.26 and 16.79 min, respectively. Both revealed the neutral loss of water (18 Da) and CO 2 with an additional loss of a methyl group (CH 3 ) in the case of vanillic acid [36] (Figure 2b). A glycosylated derivative of this compound (vanillic acid hexoside) was also characterized (m/z of 329.09, C 14 H 18 O 9 ), which showed a loss of a hexose (162 Da) as being linked through the hydroxyl moiety of the vanillic acid, as well as CH 3 and CO 2 , as for the aforementioned phenolic acids. It bears noting that this is the first report of it in sesame oil [10].
Concerning hydroxycinnamic acids, 19 derivatives were observed. They could be divided into cinnamic acid (non-hydroxylated), p-coumaric acid, and m-coumaric acid derivatives (mono-hydroxylated), caffeic acid, and ferulic acid derivatives (di-hydroxylated), and sinapic acid derivatives (tri-hydroxylated). It is noteworthy that the presence of m-coumaric, p-coumaric acid, chlorogenic (caffeoylquinic I), and ferulic acids was unambiguously confirmed with standards. Another isomer of caffeoylquinic acid was observed at RT 17.23 min with the presence of the fragment ions of quinic acid with its dehydrated ion (m/z 191.0556 and 173.0450) and caffeic acid with its dehydrated and decarboxylated ions (m/z 179.0327, 161.0233, and 135.0455) [24]. Besides, the occurrence of m-coumaric acid and caffeoylquinic acids I-II is observed for the first time in sesame oil. In this line, a caffeoyl phenylethanoid derivative (m/z 623.20, C 29 H 36 O 15 ) was observed at RT 21.12 min. The main detected fragments unraveled the neutral loss of a caffeoyl and a deoxyhexosyl moieties (m/z 461.17 and 315.11) with the detection of caffeic acid ion and its dehydrated form (m/z 179.03 and 161.02). The hydroxytyrosol ion (phenylethanoid) was observed (m/z 153.05) after the neutral loss of hexosyl moiety (Figure 3a) and so it is verbascoside according to previous studies [10,24].
Additionally, peak 21 (m/z of 147.05, C 9 H 8 O 2 ) showed the characteristic neutral loss of 44 Da of phenolic acids and thus it was annotated as cinnamic acid [10] (Table 1,  Table S1). Four isomers of p-coumaric acid hexosides were characterized at m/z values of 325.09 (C 15 H 18 O 8 ) and with MS/MS revealing neutral losses of hexose, which release the aglycone nuclei (m/z 163.04), followed by decarboxylation. Similar fragmentation patterns were obtained for four isomers of ferulic acid hexoside (peaks 30, 31, 39, and 45) [10]. Concerning tri-hydroxylated cinnamic acids, four sinapic acid derivatives were firstly detected in sesame oil. Briefly, sinapic acid hexoside was observed (m/z 385.11, C 17 H 22 O 10 ) with neutral loss of the hexosyl moiety (162 Da) followed by the common fragments of sinapic acid [10]. In the same manner, sinapic acid deoxyhexoside hexoside was annotated at RT 17.83 min with the aforementioned fragmentation pattern and the additional loss of deoxyhexose. Finally, two undescribed sinapoyl-dehydroshikimic acid hexosides were detected, showing the neutral loss of a hexose (m/z 377.12) with subsequent sinapic acid ion (m/z 223.06) and the sequential loss of a methyl group (m/z 209.0455) and water (m/z 191.03). Moreover, dehydroshikmic acid ion was detected with m/z 171.03 followed by its dehydrated (m/z 153.0537) and decarboxylated (m/z 127.04) forms (Table 1  and Table S1), Figure 3b shows the detailed fragmentation pattern.
In line with hexahydroxydiphenic acid dilactone, ellagic acid was observed upon comparison with a standard at RT 20.93 min.

Flavonoids
The presence of flavonoids in SG32 oil was widely observed with 19 derivatives. They are sub-grouped into flavonols (5), flavones (10), a flavan-3-ol, a proanthocyanidin, and flavanones (2) ( Table 1 glucopyranoside, (-)-epicatechin, procyanidin A2, and naringenin was based on standards comparison. They were all mentioned for the first time in sesame oil, while procyanidin A2 was also described for the first time in Pedaliaceae. This compound is found in other families like Ericaceae [37]. Figure 4a,b shows examples of the fragmentation patterns, highlighting the typical losses of hexose of O-glycosylated compounds.

Flavonoids
The presence of flavonoids in SG32 oil was widely observed with 19 derivatives. They are sub-grouped into flavonols (5), flavones (10), a flavan-3-ol, a proanthocyanidin, and flavanones (2) ( Table 1). It is noteworthy that the detection of kaempferol luteolin, luteolin 7-O-β-D-glucopyranoside, (-)-epicatechin, procyanidin A2, and naringenin was based on standards comparison. They were all mentioned for the first time in sesame oil, while procyanidin A2 was also described for the first time in Pedaliaceae. This compound is found in other families like Ericaceae [37]. Figure 4a,b shows examples of the fragmentation patterns, highlighting the typical losses of hexose of O-glycosylated compounds.   [38]. Furthermore, UV absorbance λ max 281 nm indicated a flavanone structure. As far as we know, it is the first description of it in Pedaliaceae [39].
Mostly, flavones were represented in SG32 oil as C-glycosides conjugates of either apigenin or luteolin with the typical fragmentation of C-glycosides. This is characterized by the loss of 18 Da (H 2 O), 44 Da (CO 2 ), 60 Da (2 × (CH 2 O)), 90 Da (3 × (CH 2 O)), and/or 120 Da (4 × (CH 2 O)), according to previously reported studies [10,40]. In this line, apigenin C-pentoside C-hexoside (I-III) isomers were noticed exerting fragments at m/z values of 541.13, 503.12, 473.11, 443.10, 383.08, and 353.07, and with the common ion at m/z 117.03 ( 1,3 B − ), Figure 4c [21,38,39,41]. As for luteolin derivatives, two isomers of luteolin C-hexoside I-II and luteolin C-deoxyhexoside-C-hexoside I-II were annotated being characterized by a similar fragmentation pattern to the aforementioned apigenin derivatives and comparison with reported studies [10,42]. To our knowledge, this is the first report of apigenin and luteolin derivatives in sesame oil. Another luteolin derivative, but O-glycosylated, was observed at m/z 593. 15 (C 27 H 30 O 15 ) and thus it showed the neutral loss of a deoxyhexosyl (m/z 447.09) and a hexosyl (m/z 285.04) moieties. The aglycone also revealed fragment 133.03 suggesting the ion ( 1,3 B − ) and hence was described as luteolin deoxyhexoside hexoside, which was not reported before in genus Sesamum, as far as we know.

Lignans
Lignans are dimeric β-β'-linked phenylpropanoid compounds that are widely distributed in Kingdom Plantae and possess several biological activities [43]. Ten lignan derivatives were observed in SG32 oil. All of them are classified as furofuran lignans and they occurred as sugars conjugates where the loss of sugars was observed and the aglycones analogs were compared with previously reported studies [10,43,44]. Concisely, two isomers of pinoresinol dihexoside were annotated exerting the neutral loss of two hexosyl moieties (m/z 357.1302, 2 × 162 Da) with an aglycone fragmentation showing an m/z of 151 resulted from the cleavage of the tetrahydrofuran ring complying with earlier reports [10,[43][44][45] ( Table 1 and Table S1). Similarly, the ion m/z of 841.28 (RT 22.52 min, C 38 H 50 O 21 .) exhibited a neutral loss of three hexosyl moieties consecutively, leading the fragment ions m/z 679. 22, 458.15, and 355.12). Besides, the fragments m/z 161 and m/z 149 were noticed standing for 1,3-dioxymethylenephenyl-CHCHCH 2 and 1,3-dioxymethylenephenyl-CO, respectively [44]. Consequently, it was tentatively characterized as xanthoxylol trihexoside. Correspondingly, the unreported analog xanthoxylol dihexoside was observed at RT 24.46 min with a similar fragmentation pattern (Table 1 and Table S1, Figure 5a).

Coumarins, Phenol Aldehydes, and Derivatives
Besides the aforementioned classes, the coumarin 7-hydroxycoumarin (umbelliferone) presence was unambiguously confirmed upon comparison with a standard, while sesamol (m/z 137.02) [10] and vanillin (phenol aldehyde) were tentatively characterized [47] 3.1.2. Non-Phenolic Compounds About sesaminol, the presence of sesaminol trihexoside (I-III) and sesaminol tetrahexoside (I-II) isomers was noticed. As before, they were characterized by the corresponding losses of hexosyl moieties with the appearance of sesaminol aglycone (m/z 369.10). Besides, the latter aglycone exhibited the fragments of m/z 161 and m/z 149 complying with furofurano lignans [10,46]. Moreover, hydroxysesaminol trihexoside was detected as being characterized by the ion of the aglycone m/z 385 with the latter characteristic ion at m/z 161 (Figure 5b). All these lignans glycosides have been observed for the first time in sesame oil.

Non-Phenolic Compounds Nitrogenous Compounds
Concerning nitrogenous compounds, the occurrence of amino acids was observed by eight derivatives namely pyroglutamic acid (I-II) leucine/isoleucine (I-III), tyrosine, phenylalanine, and tryptophan. They exhibited the neutral loss of ammonia (17 Da) and/or CO 2 (44 Da) complying with several reports [10,21,24,36,45]. Besides, the aromatic amino acids phenylalanine, tyrosine, and tryptophan were confirmed with standards. It bears noting that this is the first report of pyroglutamic acid in Pedaliaceae (Table 2 and Table S2). In this line, oxidized glutathione (GSSG) was detected (RT 6.32 min, m/z 611.14) showing both glutathione (m/z 306.08) and glutamyl (m/z 128.04) moieties [10] and hence giving a clue of the occurrence of reduced glutathione (GSH) in SG32 oil being susceptible to conversion to GSSG during sampling and analysis [48]. As a matter of fact, GSH is a natural cellular antioxidant that prevents the onset and progression of many serious diseases [48]. This is the first report of GSSG in sesame oil and its presence in sesame oil provides a new aspect of its functionality (Table 2 and Table S2).

Semi-Quantitative Analysis
Semi-quantitative analysis was performed via estimation of the total peak area obtained by MS denoting the relative amount of each characterized metabolite. In the perspective of subclasses of phenolic metabolites, phenolic acids subclass was the most abundant with (38.4%) followed by flavonoids (33.7%) then lignans (26.9%) ( Figure 6). propylmalic, and azelaic acids. Their characterization are according to earlier reports [10,24,36,47,49,50]. All of them, as far as we know, were not reported before in sesame oil (Tables 2 and S2).

Semi-Quantitative Analysis
Semi-quantitative analysis was performed via estimation of the total peak area obtained by MS denoting the relative amount of each characterized metabolite. In the perspective of subclasses of phenolic metabolites, phenolic acids subclass was the most abundant with (38.4%) followed by flavonoids (33.7%) then lignans (26.9%) ( Figure 6).

Comparison between Sesame Seed Oil and Cake
Our previous study on SG32 cake counterpart exhibited the presence of 112 metabolites [10]. They belonged to the same classes of phytoconstituents in SG32 oil. Remarkably, 72 metabolites were detected in both SG32 cake and oil (Tables S1 and S2). The remaining metabolites in SG32 oil consist of generally minor phenolic compounds, and no sugars were observed as in the cake [10]. All the characterized lignans were of furofurano type, as in the seed cake. GSSG was detected in both samples.
Basically, sesame oil is considered a functional oil [12,13]. Since SG32 cake showed antioxidant activities, the common phenolic compounds between SG32 cake and oil could contribute to the biological potential of the oil. The phenolic profile in vegetable oils depends on the source and the processing. For example, olive oil is rich in hydroxytyrosol derivatives, among other phenolic compounds. Some of these compounds come from the olive fruit and pass to the oil, which is obtained by milling, malaxation, and centrifugation [51]. Nonetheless, the processing conditions affect the profile and the content of phenolic compounds in olive oil and thus its oxidative stability and shelf life [52,53]. Another example is tea seed oil obtained by screw pressing extraction, which also present tea phenolic compounds [54] and they contribute to the antioxidant stability [55]. In this context, it is important to establish the phenolic profile in vegetable oils due to the contribution of phenolic compounds to the functional and antioxidative properties of the vegetable oils. Therefore, other studies should be performed to address the phenolic composition integrity of virgin cold-pressed sesame oil and refined oil as industrial processes could lead to the loss or modification of phenolics compounds [56,57]. The employment of the state of art hyphenated techniques as U/HPLC, like in the current work, and/or GC coupled to high-resolution MS could help in this purpose as other authors have shown for other vegetable oils [51,53,54,58]. Even more, the data obtained here can be the basis for those characterization studies on sesame oils when using U/HPLC-MS. In fact, lignan aglycones have not been found in this work. It seems that lignans can be transformed during the oil production process, and some of them, like sesamolin and sesamin, are unstable [59].

Conclusions
In the present study, core-shell RP-HPLC-DAD-ESI-QTOF-MS and -MS/MS were utilized for the analysis of the oil of the Egyptian cultivar of sesame Giza 32'. Collectively, 86 metabolites were characterized in sesame SG32 oil, with 64 phenolic compounds with 3 unreported metabolites. The observed phenolic compounds were classified into phenolic acids, flavonoids, lignans, and others. All the characterized lignans were of furofurano type, as in the seed cake. Moreover, this is the first report showing oxidized glutathione in sesame oil. Mostly, the phenolic metabolites and other phytoconstituents in SG32 oil were present in SG32 cake counterpart, suggesting that they can pass from the seed cake to the oil, while in some cases the oil counterpart seems to be enriched. Consequently, further studies are required to detect the presence of important bioactive metabolites in commercial samples.