a thin-layer chromatography method for the identification of three different olibanum resins...

Research Article

Received: 9 March 2011; Revised: 6 May 2011; Accepted: 7 May 2011 Published online in Wiley Online Library: 20 August 2011

(wileyonlinelibrary.com) DOI 10.1002/pca.1341

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A Thin‐layer Chromatography Method for theIdentification of Three Different OlibanumResins (Boswellia serrata, Boswellia papyriferaand Boswellia carterii, respectively,Boswellia sacra)Michael Paul, Gerit Brüning, Jochen Bergmann and Johann Jauch*

ABSTRACT:Introduction – Resins of the genus Boswellia are currently an interesting topic for pharmaceutical research since several phar-macological activities (e.g. anti‐inflammatory, anti‐microbial, anti‐tumour) are reported for extracts and compounds isolatedfrom them. Unambiguous identification of these resins, by simple and convenient analytical methods, has so far not clearlybeen verified.Objective – For differentiation and identification of three important Boswellia species (Boswellia serrata Roxb., Boswelliapapyrifera Hochst. and Boswellia carterii Birdw., respectively Boswellia sacra Flueck.), possible even for minimally equippedlaboratories, a thin‐layer chromatography (TLC) method was developed, allowing unambiguous identification of the threespecies.Methodology – Crude resin samples (commercial samples and a voucher specimen) were extracted with methanol or diethylether and subjected to TLC analysis (normal phase). A pentane and diethyl ether (2:1) with 1% acetic acid eluent was used.Chromatograms were analysed by UV detection (254nm) and dyeing with anisaldehyde dyeing reagent. Significant spotswere isolated and structures were assigned (mass spectrometry; nuclear magnetic resonance spectroscopy).Results – Incensole and incensole acetate are specific biomarkers for Boswellia papyrifera. Boswellia carterii/Boswellia sacrareveal ß‐caryophyllene oxide as a significant marker compound. Boswellia serrata shows neither incensole acetate norß‐caryophyllene oxide spots, but can be identified by a strong serratol and a sharp 3‐oxo‐8,24‐dien‐tirucallic acid spot.Conclusion – The TLC method developed allows unambiguous identification of three different olibanum samples (Boswelliapapyrifera, Boswellia serrata, Boswellia carterii/Boswellia sacra). Evidence on the specific biosynthesis routes of these Boswelliaspecies is reported. Copyright © 2011 John Wiley & Sons, Ltd.

Supporting information can be found in the online version of this article.

Keywords: Thin‐layer chromatography; Frankincense resins; Boswellia papyrifera; Boswellia serrata; Boswellia carterii/Boswellia sacra

* Correspondence to: J. Jauch, Organic Chemistry II, Universität desSaarlandes, Am Campus C4.2, D‐66123 Saarbrücken, Germany. E‐mail:[email protected]‐saarland.de

Organic Chemistry II, Universität des Saarlandes, Saarbrücken, Germany

IntroductionBoswellia resins are described in numerous ancient texts bymany civilizations (Martinetz et al., 1988) and are nowadays pre-dominantly known as incense in catholic churches and other re-ligious traditions. In addition to their application for spiritualpurposes, these resins have become more and more interestingfor pharmaceutical research since several papers report of phar-macological activities in isolated extracts and compounds fromthem (Moussaieff and Mechoulam, 2009; Ammon, 2010). Treesof the genus Boswellia are mainly distributed over northeast Africa(Ethopia, Eritrea, Sudan and Somalia), the Arabian Peninsula (Yemen,Oman) and India. Economically important species often reported inthe literature are Boswellia sacra (Arabian frankincense; Thulinand Warfa, 1987; Coppi et al., 2010), Boswellia carterii (Somalia;Thulin and Warfa, 1987), Boswellia papyrifera (Ethopia, Eritrea,Sudan; Ogbazghi et al., 2006) and Boswellia serrata (IndianFrankincense; Sunnichan et al., 2005). However, the identity ofthe resin analysed is not always clearly verified in the literature.

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For example (chosen randomly from the literature), Moussaieffet al. (2007) claim that incensole (1) and incensole acetate (2)were isolated from B. carterii (the structures of all compoundsdiscussed here are shown in Fig. 1). Banno et al. (2006) andAkihisa et al. (2006) also report the isolation of 1 and 2 fromB. carterii. These claims, however, were never really provenand the authors instead relied on the information providedby the distributors of the products. Reliance on distributors(e.g. Scents of Earths, USA; Caesar & Loretz GmbH, Germany)under these circumstances is obviously not an ideal situation.Fortunately Hamm et al. (2005), Camarda et al. (2007) and

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Figure 1. Chemical structures of incensole (1), incensole acetate (2), 3‐oxo‐8,24‐dien‐tirucallic acid (3), serratol (4), ß‐caryophyllen‐oxide (5), boswellicacids (α+ß‐BAs), acetylated boswellic acids (α+ß‐ABAs) and 11‐keto boswellic acids (KBA, AKBA). NMR data of compounds 1, 2, 4 and 5 are presentedin the supplementary material.

Thin‐Layer Chromatography Identification Of Olibanum

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Obermann (1977) have already discussed the issue of identifica-tion (e.g. 1 and 2 in conveniently detectable quantities refer to B.papyrifera, ‘Eritrean type). Furthermore, Hamm et al. (2005),Mathe et al. (2004) and Thulin and Warfa (1987) provide convinc-ing evidence that B. carterii can be considered the same speciesas B. sacra, but to the best of our knowledge there are no reportsof the isolation or detection of 1 and 2 from B. sacra.

Unfortunately there is no convenient method so far reportedfor the unambiguous identification of olibanum species. Toimprove this situation we report here a simple thin‐layerchromatography (TLC) method that allows the unambiguousidentification of B. papyrifera (Bpap), B. serrata (Bser) andB. carterii/B. sacra (Bcar/Bsac). In addition our results arecompared with reliable data obtained from the literature.

Quantitation of boswellic acids in olibanum by high‐performance(HP) TLC has been reported recently by Krohn et al. (2001),Pozharitskaya et al. (2006) and Shah et al. (2007). Qualitativeidentification of terpenes from olibanum by TLC has beenattempted in the past. Obermann (1977) and Klein andObermann (1978) basically reported similar results to those weobtained for the identification of resins sold as type ‘Aden’and ‘Eritrea’. The ‘Aden type’ can be considered as Bcar fromSomalia (B. carterii from Somalia is shipped to Aden, Yemen,and therefore traded as ‘Aden‐type’; personal communicationfrom Siegfried Tietz, a gum and resin expert, Gerhard Eggebrecht,Süderau, Germany). The ‘Eritrea‐type’ is synonymous with


B. papyrifera (Bpap; Hairfield et al., 1984). Consistent with Ober-mann’s TLC analysis we could identify 1 and 2 only in the type‘Eritrea’ and thus Bpap. This result is consistent with the GC‐MSresults of Hamm et al. (2005) and Camarda et al. (2007) andwith our RP‐DAD‐HPLC results (HPLC data will be reportedelsewhere).Basar (2005) compared several olibanum samples using

both GC‐MS and TLC. Even though Basar considered theGC‐MS and TLC results characteristic for Bcar, the data addi-tionally imply that the resin purchased as Bcar was actuallyBpap. Despite this uncertainty, the work is critical for the un-ambiguous identification of Bcar/Bsac by TLC and providessupport for the work published here (see supporting informa-tion). Two more publications on TLC analysis (Hairfield et al.,1984, 1989) conclude, by qualitative comparison of TLC chroma-tograms, that the ‘Eritrean type’ is similar to Bpap and that‘Somalian olibanum’ is similar to Bcar.

Experimental

Material and methods

TLC Silica gel 60 F254 Multiformat prescored to 5 × 10 cm glass plateswere purchased from Merck (Darmstadt, Germany). Pentane (> 99%),diethyl ether (Et2O, > 99%), HPLC‐quality methanol (MeOH) and aceticacid (> 99%) were obtained from Sigma Aldrich (Steinheim, Germany).Pentane and diethyl ether were distilled prior to application. Materials

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and methods for the extraction and isolation of marker compounds arereported in the supporting information.

Plant material

The Burseraceae resins B. papyrifera Hochst. (‘Eritrea 1st choice’), B.carterii Birdw. (‘Somalia 1st choice ’), B. sacra Flueck. (‘Oman white No.1’) and B. serrata Roxb. (‘Indian siftings’) were purchased from GerhardEggebrecht Vegetabilien & Harze (Süderau, Germany). Identity of resinswas evaluated by Siegfried Tietz, a gum and resin expert (GerhardEggebrecht, Süderau), and Johannes Ertelt (Pharmacist, HeidelbergPharmacy, Bisingen, Germany). For comparison of the commercialfrankincense samples we obtained a voucher specimen (Bpap and Bsac)from the Economic Botany Collections in Kew Gardens, UK, donated byMrs J. Steele. Certified samples of Bsac and Bcar were also received byJochen Bergmann from Dr M. Al‐Amri (Ministry of Agriculture, RumaisResearch Station, Muscat, Oman) during his visit to Oman in 2004–2005and from Mr Giama, Bremerhaven, Germany. Original H15 Gufic (Bser,Sallaki Tablets®) from India were obtained from AureliaSan GmbH,Tübingen, Germany. Each reference was identical to each commercial resin(see supporting information) obtained from Gerhard Eggebrecht GmbH(TLC and HPLC analysis). Furthermore, the identity of resins analysed isdiscussed by comparison with reliable literature results.

Sample preparation and TLC analysis

The different olibanum resins were cooled in a freezer to −30°C (1–2 h)and powdered in an electrical mixer. The powder (1 g) was dissolved in80mL methanol or diethyl ether (MeOH and Et2O as solvent for extrac-tion of the powdered resins give identical TLC chromatograms) in a100mL volumetric flask and sonicated for 20min (the resin will not bedissolved completely). Afterwards, 20mL MeOH (or Et2O) were added(final volume= 100mL). Aliquots (1mL) were taken from the filteredsolution (0.2 µm; Filtropur S; Sarstedt, Nürnbrecht, Germany) for furtherTLC analysis. A mixture of two parts pentane and one part diethylether plus 1% (v/v) of acetic acid was used as the mobile phase. TLCplate start lines were drawn (by pencil) at 1.5 cm, giving an elutiondistance of 8.5 cm. With a capillary, each olibanum aliquot (ca. 10 μL)was carefully put on in line. Chromatograms were developed in a classicvertical TLC chamber, saturated with the eluent. UV detection (254nm)was carried out by a conventional UV light source (Duo‐UV‐Source forthin‐layer and column chromatography, Desaga, Heidelberg, Germany).As a dyeing reagent, 0.5mL anisaldehyde in 50mL glacial acetic acidplus 1mL H2SO4 (concentrated) was used. After development of thechromatograms, the TLC plate was completely immersed into asolution of dyeing reagent and heated with a heat gun until thecoloured spots appeared. Finally, after colour development of thechromatograms at room temperature (ca. 15–30min), significant spotscan be detected visually.

Results and Discussion

Solubility properties

Bpap can be easily distinguished from the other resins (Bcar/Bsac and Bser) by sensorial evaluation, since it shows a distinctiveorange kind of fragrance (note that Bpap contains relatively highquantites of n‐octyl acetate (Hamm et al., 2005; Camarda et al.,2007), which has an orange fragrance; see also basic text bookson organic chemistry). Furthermore, a medium polar (Et2O) ex-tract of the resin can be easily dissolved in MeOH, whereasthe medium polar (Et2O) extracts of Bcar/Bsac and Bser givea dim white colloidal solution when dissolved in MeOH. Todifferentiate between Bcar/Bsac and Bser is more difficult.

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These darker resins have a more frankincense‐like aromaticfragrance (‘coniferous forest’) and as described above givethe same colloidal patterns when the Et2O extract is dissolvedin MeOH.

TLC chromatograms

In Fig. 2, a drawn model chromatogram is compared with realsample TLC chromatograms (Et2O and MeOH extract). All follow-ing discussions refer to Fig. 2. For a better understanding of thefollowing discussion, Table 1 explains the main differences andcharacteristics of the marker molecules and resin species ana-lysed. The model distinguishes the significant differences be-tween the ‘three’ olibanum species (Bpap, Bser, Bcar/Bsac) afterdyeing with anisaldehyde solution, heating and colour develop-ment for a few minutes at room temperature. Bpap shows twostrong dark brownish spots at Rf = 0.27 (1) and 0.68 (2), whichare very specific for this species. These two spots are the samespots described by Obermann (1977), Hairfield et al. (1984) andBasar (2005). A strong blue spot at Rf = 0.22 refers to 3‐oxo‐8,24‐dien‐tirucallic acid (3; isolation and structural assignment of3 and all other terpenic acids from Boswellic species mentionedhere can be viewed in the literature (Seitz, 2008) and is not partof the topic discussed here). Bser also exhibits the 3 spot andhas a strong green spot at Rf = 0.46, which refers to serratol (4).Bsac and Bcar show also the green spot of 4 at Rf = 0.46 anda very significant pink spot at Rf = 0.50, which is very specificonly for Bcar and Bsac and provides further evidence oftheir common identity, i.e. both gums have the same chromato-gram in common. The pink spot was identified after isolationas ß‐caryophyllene oxide (5). Additionally, Bcar/Bsac can be dis-tinguished by the lack of a strong blue spot referring to 3, whichis consistent with HPLC analysis, where Bcar and Bsac showsmaller peak areas for 3 compared with Bpap and Bser (HPLCdata not shown here). Compound 1 was also isolated from Bcarand the TLC chromatograms imply that 1 is also present in Bcar/Bsac and even Bser, but in much lower quantities compared withBpap (compare the strong brown spot at Rf = 0.27 with theslightly brownish spots of Bser and Bcar/Bsac). Compound 2has not been detected in Bser and Bcar/Bsac (view respectiveRf at 0.68). Thus, there seems to be a remarkable difference inthe biogenesis of these terpenes. Compound 4 is reported as abiosynthetic precursor for 1 and thus 2 by Klein and Obermann(1978), which is plausible since the spot of compound 4diminishes when compound 1 increases (compare the Bsaccolumn at Rf = 0.46 and 0.27).

All species have the classic boswellic acids in common. At Rf =0.25 (violet spots) α‐acetyl‐boswellic acid and ß‐acetyl‐boswellicacid (α+ß‐ABAs) are present and at Rf = 0.19 (violet spots) α+ß‐BAs are visible. Both spots surround the spot of compound3, giving a specific elution pattern. When these spot areas areslowly heated after dyeing, 3 becomes first a greenish spotand later turns to blue (after ca. 24 h 3 turns into brown), whichis significant for compound 3 and thus useful for identification.The Rf = 0.26 (blue spot) eluting between compound 1 andα+ß‐ABAs is not verified yet. This spot may belong totriterpenes with a functional group (e.g. α‐amyrine, ß‐tirucallol,etc.), as reported by Obermann (1977). The spots (dark blue,violet) at Rf = 0.80, almost eluting at the front, probably belongto all kinds of non‐polar terpenes without any functionalgroup. We tested several non‐polar terpenes, such as ß‐pinene,limonene, α‐humulene and ß‐caryophyllene, and all eluted at

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Table 1. Properties of marker compounds referring to the resin species. Compounds and referring abbreviation (Abbr.) or number(Nr.). Colour after dyeing with anisaldehyde spray reagent, UV‐activity at 254 nm (only KBA and AKBA) and the species (Bpap, Bser,Bcar/Bsac) for which the corresponding marker compound is characteristic (Expressions in parentheses declare, if the spot is strongor weak and thus significant for a certain species). For further discussion details see text and chromatograms in Figs. 2 and 3

Compound Compoundreference

Rfvalue

Colour(after dyeing)

UVdetection(254 nm)

Species

Incensole 1 0.27 Brown — Bpap (strong), Bser, Bcar/BsacIncensole acetate 2 0.68 Brown — Bpap3‐Oxo‐8,24‐dien‐tirucallic acid 3 0.22 Blue — Bpap, Bser(both strong) Bcar/BsacSerratol 4 0.46 Green — Bser, Bcar/Bsac (both strong)ß‐Caryophyllene oxide 5 0.50 Pink — Bcar/Bsacα‐Acetyl and ß‐Acetyl boswellic acid α+ß‐ABAs 0.25 Violet — Bpap, Bser, Bcar/Bsacα‐ and ß‐Boswellic acid α+ß‐BAs 0.19 Violet — Bpap, Bser, Bcar/Bsac11‐Keto‐ß‐boswellic acid KBA 0.10 Yellow golden

(if pure spot)Visible Bpap, Bser, Bcar/Bsac

3‐O‐Acetyl‐11‐keto‐ß‐boswellic acid

AKBA 0.16 Yellow golden(if pure spot)

Visible Bpap (strong), Bser, Bcar/Bsac

R = 0.80

R = 0.68

R = 0.50

R = 0.46

R = 0.27R = 0.26R = 0.25R = 0.22R = 0.19R = 0.16

R = 0.10

Figure 2. Model chromatogram (right) and actual chromatograms (middle: diethyl ether extract; left: MeOH extract; after dyeing and ca. 15min de-velopment at room temperature). The model shows only the significant spots that can be used for resin identification. Conditions as described inthe text. Rf = ca. 0.10 (KBA, UV active, see also Fig. 3); Rf = ca. 0.16 (AKBA, UV active, see also Fig. 3); Rf = ca. 0.19 (α+ß‐BAs, violet); Rf = ca. 0.22 (3, blue);Rf = ca. 0.25 (α+ß‐ABAs, violett); Rf = ca. 0.26 (not verified, blue); Rf = ca. 0.27 (1, brown); Rf = ca. 0.46 (4, green); Rf = ca. 0.50 (5, pink); Rf = ca. 0.68(2, brown); Rf = ca. 0.80 (terpenes without functional group, e.g. limonene, ß‐pinene, α‐humulene, ß‐caryophyllene, etc.; dark blue/violet). Definitionof spots in the model chromatogram: solid spots (clearly visible), hollow spots (slightly visible), hollow and dashed spots (barely visible). For furtherdiscussion see text.


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this front, showing almost identical dyes after reaction with ani-saldehyde reagent. 3‐O‐acetyl‐11‐keto‐ß‐boswellic acid (AKBA)at Rf = 0.16 and 11‐keto‐ß‐boswellic acid (KBA) at Rf = 0.10, whichare typical boswellic species markers, can be monitored at254 nm UV detection (see Fig. 3). It is noticeable that Bpap shows


the strongest AKBA spot. For Bser and Bcar/Bsac the AKBA spothas approximately the same absorbance (consistent with HPLCdata, not shown here). KBA is clearly detectable only for Bpapand Bser, whereas for Bcar/Bsac the spot can only be guessedby UV detection at this concentration level.


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Figure 3. UV detection of AKBA (Rf = 0.16) and KBA (Rf = 0.10) at 254 nm(column 1: Bpap; column 2: Bser; column 3: Bcar, column 4: Bsac). Thetwo spots in each resin column refer to the extraction method, Et2Oand MeOH. For further discussion see text, Table 1 and Fig. 2.

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Validation of results with the literature

Good support of the data here is given in the dissertation ofBasar (2005), where 34 different TLC chromatograms of essentialoils from different olibanum samples are published (see supportinginformation or original dissertation).

Additionally, our conclusion about chemical markers is veri-fied since Camarda et al. (2007) and Hamm et al. (2005) detected5 only in Bcar/Bsac. ß‐Caryophyllene as a precursor of 5 isreported by Hamm et al. (2005) only in Bcar/Bsac, and byCamarda et al. (2007) in Bcar/Bsac and in Bser (but in lowerquantities). Co‐injection of 5 and ß‐caryophyllene (HPLC analy-sis) produced positive results only for Bcar/Bsac. For Bser andBpap no co‐elution could be detected (data not shown here).Since it is known that ß‐caryophyllene is easily oxidised (Skoldet al., 2006), we conclude that Bcar/Bsac biogenetically producemore ß‐caryophyllene, at least in detectable quantities, thanBpap and Bser. Hence, detection of 5may refer to the harvestingprocess (or even is a side‐product of isolation), where the gum isharvested by incision in the bark of the trees and dried for a fewweeks before it is finally collected (Martinetz et al., 1988).

In summary, the TLC method described here allows unambig-uous identification of B. papyrifera, B. serrata and B. carterii/B.sacra. Furthermore, our results are consistent with datapublished in the literature (Thulin and Warfa, 1987; Basar, 2005;Hamm et al., 2005; Camarda et al., 2007). Additionally, it is shownthat B. carterii shows the same chemical component pattern asB. sacra. Thus, these two resins must be classified the same, asreported by Thulin and Warfa (1987). For B. papyrifera, highquantities of incensole and its acetate are very specific. Boswellia

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carterii and B. sacra show a significant pink ß‐caryophylleneoxide spot above the green serratol spot. Boswellia serratareveals only the serratol spot.

Finally, we would suggest that these differences in the chem-ical patterns may be significant for the pharmacologicalactivities of these olibanum species (incensole and its acetatefor Bpap (Moussaieff et al., 2007, 2008); ß‐caryophyllenefor Bcar/Bsac (Gertsch et al., 2008)), and thus to distinguishbetween B. papyrifera (‘African frankincense’), B. serrata (‘IndianFrankincense’) and B. carterii/B. sacra (‘Arabian frankincense’).As a plausible proposal to clarify nomenclature, and due to thefact that B. serrata is identified in the European Pharmacopoeia6.0 as Olibanum Indicum, this classification would make mostsense. Furthermore, the term ‘Arabian frankincense’ wouldrecognise the great cultural and historical importance of the frank-incense trade of the Arabian Peninsula (Martinetz et al., 1988).

Supporting information

Supporting information can be found in the online version ofthis article.

Acknowledgements

We acknowledge the generous support of our work from SaarlandUniversity, Bundesministerium für Wirtschaft und Technologie(ProInno‐Project II) and AureliaSan GmbH. Thanks are due to Prof.Wittko Francke, University of Hamburg, for the permission ofpublication of data (supporting information) from the dissertationof Simla Basar.

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a thin-layer chromatography method for the identification of three different olibanum resins...

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