A standardized method for the identification of lichen products

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A. STANDARDKZED

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IDENTIFICATION

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,,., ,..” I,) A: pro&ure .for. the ,routine identification .of ,the .products of -lichen-forming fungi. by thin-layer. chromatography is described. Microextracts of plant ..fragments are chromatographed. in three solvent systems.: The, spots, of. unknowns are assigned to RF iclasses. defined :by the RF, values of marker, controls ,of two lichen substances (atranorin and norstictic .acid) chromatographed qnevery ,plate., The unknowns are tentatively identified. by sorting (by. Rp,,classes) .punched cards .summarizing microchemical .data. for: all. compounds: previously studied. .The preliminary identification is then confirmed by .additional : microchemical tests. The open&ded, system, can incorporate new and unknown compounds,as well as information from other chromatographic systems: Data obtained rby the standardized procedure are ,given ,for 104 products. ,’ ,._I. : ,.

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; INTRODUCTION

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The natural products of the lichen-forming fungi have become, .intimately involved in’ systematics and classification l. Some 300 specific. substances, have been reported from about 2,000 of the 18,000 described, species of lichen@. Approximately 100 of these compounds are aromatic products that have been frequently mentioned in the taxonomic literature. ,Today the lichen systematist ,faces the problem of learning to identify. these natural products without a set procedure to guide him. An important paper,,by .SANTESSON~records RR values for spots of some 80 compounds on thin-layer chromatograms ruu in a:variety of solvent systems. The compounds are grouped
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C. E'..CULBERsoN,I.

KRISTINSSON,.

the worker surveying previously ,unstudied lichens, requires REV data for-all lichen compounds in the same solvent .systems. For, example, if the lichen thallus produces coloratiiih; With_,+-pheriylenediamine i (PD):, one ‘:may choose:,a, solvent system recom-, mended’.for PD-positive. substances, only to find that there are also PD-negative constituents present in the same plant and for which there are no Rr;, data for that solvent. It is then necessary to,select other solvent systems; hoping,by trial and,error to find a suitable one. In spite of the,intensive interest in lichen chemistry+0 stand,ardized system has been presented. The method ‘described in, this report- attempts.,& overcome (I), the hit-and-miss aspects of microchemical identifications, ,,(z),..the:djffi:, culties due to varying Rp values, and (3) the problem of incorporating and retrieving microchemical information. The system described was developed for use by: the lichenologists at .Duke University. While. the specific solvent systems and TLC plates used h&e been most successful in our laboratory, .the general plan of the method can be modified and applied to any solvents used with any media. In essence the system involves the following. Chromatographic data from three stan,dard solvent systems for all compounds previously studied,are stored on punched cards? ‘The?ca~d~~,a;re’ke~~d~,not~to,specificR~~‘vdues in the’three~solutions~but:‘rather td’~R~,classes the limits ‘of tihich &e,‘determined.‘by the’ R$ values of a control mixture. cousisteritly~used (as ~&marker :on:every :chromatogram. UnknoGns. are, identified, by, deterrW&ig their, I?$ classes in the. three solvent systems’ and :theu ‘by sorting! the ! punched- cards for the jkhown compounds:belonging to all the ‘same ‘R&lasses. ; :“,I.:.:.;,/;,, .These possibilities are’ tlieni narrowed do&i .by ctireful comparison of (a) ‘actual R~$‘&hi”s%vith respectto’the!R~ values’of the control~marker spots; (b)‘color’reactions, (c) :“appearance ,?oof, tlie’spot ,.in shorti 8and- longwave’ UYI light; (d) solubility, 1and any other’pertinent’data available::The’final identification’of the compoundis achieved by appropriate Iconfirmatory’ tests: ,:Punched ‘cards ‘made ‘for compouiids’ that cannot. be identified are added to the deck to permit the recognition of the same substances in sp.ecies ,, ; ,. ., (,studied ,. in, the future! . .‘. I,, ,. (,

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‘,,I.: /Ii’, ,’ Ghvomato&ag5hic materials asd solvent systems :; ,.f>$i .:Cliromatograms,:were :developed in Brinkman tanks to’ a ,height of IO cm on MerckSilica! Gel:Faii. thin-layer plates. that ,had!been stored .in a desiccator ‘over CaClsNaOH ; ,but not ‘activated; .: ‘The solvent systems are ; solvent: A : .benzene_dioxaneacetic, acid: 1(go .:,25.: it;’ 238 ml) s;’1solvent B : ‘hexane-ethyl ether&formic acid (5 :4:1, ‘. , ., 1 ‘., , .. 200 ,ml) ;.solvent CI toluen+acetic acid (85: 15, 240 ml) .,;: .I: :”” /. : i rThe tbenzene;.hexane,. and toluene .uiere,dried (CaCla); redistilled; and stored over sodiu,ni, 2The;‘ dioxiine and. anhydrous ethyl ,ether iwere 8passed through, columns : of alumina: tb removeperoxides; The,,ether was stored,.'over sodium; The same, s&e& systems prepared without !these
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IbENTIIW%~~ON

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BY TLC

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were replaced when the Rp values of the controls deviated significantly from their usual values. Atranorin and norstictic acid were selected for marker controls because of their ready availability and. because of their behavior in the particular solvent systems used. A mixture of these two compounds is available in a single extract of many common species, for example Parmelia perforata (Jacq.) Ach. in North, America and P. acetabulzcm (Neck.) Duby in Europe. : (Eighteen spots tiere applied to the plates z,cm from the bottom and g mm apart. On every ‘plate the first, ninth, and eighteenth positions were used for controls. The controls on a plate tiere spotted ‘from the same solution to ehminate RF variation due to concentration. ” ‘. ., .’ ‘. 1. .’ ,,.

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Determistatioh of Ap classes The developed plates were air-dried, examined under UV light (254 nm and 366 nm), sprayed with 10% HPSOI, and heated at about IIO' until colors developed. The colors of,the spots were recorded as soon as the plates were taken from the oven. ‘In order to identify RF classes, first the’centers of the atranorin spots at positions ‘. ii and,:g,and ,at positions: g and: 18,are connected by a rule4 line in pencil. The same is ‘done for the norstictid acid. spots. (Usually ‘spots’are slightly lo&& towards the center of the plate, the effect being more pronounced at lower RF values, but sometimes the direction of variation is reversed.) The distance between the’origin and the solvent front tias divided arbitrarily into:eight regions measured’ with respect to the RP values’of the controls. ?he iimits ,of ‘the. eight .Rp classes are shotin in Fig. I. All RF ‘values for spots were recbrded and

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Fig. I. A copy of a plate run in solvent C showing the controls of atranorin and norstictic acid, at positions I, g, and 18, the horizontal penciled lines connecting the control spots, and the RF clssses of the spots of the principal compounds chromatographed. The number (or numbers) associated .with each spot indicate its Rp class and the diagram to the right of, the plate summarizes the Rp classes, The identities of the principal spots at each position on the plate are (I) atranorin (A), norstictic acid (N), and an unknown substance, an acetone extract of Pavmelia perforata; (2) acetylportentol; (3) alectorialic acid (class 4) and barbatolic acid (class 3); (4) alectoronic acid;, (5) anziaic, acid (classes 4-5) ; (6) atranorin; (7) baeomycesic acid (class 5) and squa’matik acid (class 3’) ; (8) barbatic acid (classes 5-6) ; (9) atranorin (A) and norstictic acid, (N), the control; (IO) fumarprotocetraric acid and protocetraric acid (running together as a smglc spot) ; (I I) chloroatranorin ; ( 12 ) confluentic acid ; (13) atranorin (class 7) and a-collatolic acid acid ; (class ,5) ; (14) constictic acid; (I 5) cryptochlorophaeic acid ; (16) q-0-demethylbarbatic (I 7) ,djdy&ic acid ; aad (I 8). atranorin (A) and norstictic acid (N), the control: .. J. Claromaiog., 46 (rg70)‘85-g3

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IDEXUTIIVCAT~ON

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Chemotaxonomic studies’ ontlichens are usually based upon chemical ,analysis of fragments of herbarium specimens. In our 1aboratory;the following procedure is used : cleaned plant material (I cm2 or less) is placed in a shell vial (Mercer Glass Works, Inc., N.Y. ; shell vials, short style, 112 dr, 35 x 12 mm) numbered in India ink. The fragment is extracted three times by g-ro-min soakings in benzene at roomtemper; ature. The extracts are evaporated at about 50” on a microscope slide. The processes are’ repe,ated ‘using ,acetone to extract the sample at elevated temperature &do), and the %cetone extracts’ are’ evaporated on a’second slide. ,Usually ,many Specimens’are analyzed simultaneously,. and ‘the .bennene ‘and ( acetone ‘extracts ‘of ‘each sample iare chromatographed’\ side-by-side on: the thin-layer. plate’.’ *As usual,: ,at positions * I, :9; ; ‘.’ and, 18; the controls of atranorin and norstictic acid are’ included.’ ; .: .’ ; “,, “’ I To spot the chromatograms, a drop or two of acetone,is flowed, over’ the residue on ,the slide,,and. .collected:in’a micropipette:.,Three ! plates may, -be, spotted isimultaneously.: The’ Rp values of the ,spots are recorded and the .Rp classes :determined; The punched cards make possible the identification of, all previously studied &d pounds with the same Rp classes. Of the 104 compounds listed in Table I the maximum number of compounds with the same RF classes is six. The additional information on the cards (Fig. 2) reduces the number of possibilities still further and suggests .the best solution or solutions in which to rechromatograph the unknown with appropriate J; Chvomatog.;

46 (.zg7o) 85+3

C; F.!CuLBERSON, H..,KRISTINSSON

92

standards; If the compound is.riot identifiable, a punched card for. it is prepared and l ., ., ,. , ‘, . .:I ,, ; ’ ,.,I ::;: added to the deck. ‘.

RESULTS AND DISCUSSION :

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%, ” Table.eJ .summari&s, the.:.data:,obtained for the ,mosi -common. aromatic. and triterpenoid!products~df lichens;.The compounds. are. arranged aticprdingito’ increasing J?p’classes so $hat,.those:with,.the,.most~simil~ RF-val.ueS,in,the ,thti+!e .solvent systems ‘are grouped,,tog&her,. -It is urilikely: that, these RF valGes could. be reproduced exactly inanother labor&ory,‘since oier:a period of some mbnths the Rp values of the controls ofatranorin and &stictic~acid~chan&e!slightly, But it is expected that the RIPclasses ” which depend only on,the relative order of spot& Gill be much’ti&e con+ant. Iq,t:,$heidentification;,,pf.,upk~iiq,wnsY-from.cr!uc+‘]@ctracts df ,litihqs containing sever@,comp,ound&t-inay be diffic&+b distinguish whi&sp&s bn,tl&chromatograms run in$ffereqt sol&fqq~ ,qq,‘diie to. the ‘game compounds; Sometimes, one or more spotsa:’ ; .; ( ,>i,,:1 .‘;j ,, : , i ; : :,i / .;] 1.: i, :‘ac ,.Y>.;: ,:. :.l.jc;:i; ‘, :, ,,, ,. :

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ACli4NOWLEDG~MENTS;.!~,..; f:;;,' ,:;,i I+

carpiea@id; lD&;.S.;:ti~;~~nhE~~forl’a~&le.of-h&em&h&molic;acid:; .Dr; :S. ~-~UNECK. for .samples::of: acetylporfentol and planaic. :acid.;; Dr., .W:, .S..6, :MMSS! for' +&ples .of several pultiinic acid, derivatives ; and, Dr.. C ., A.: WACNTMEISTER for samplds: of p&o,::, !,, ;,. .,; lichenic,arid:thiqphanic acids.):, ,.. I ..,. :.;..:‘-:,. ., II. +,, .:.;:,‘_, : ;,:I; ,,..I ,-i c;.tr;.!:rl’r;lis investigation was supported hy..a.grant.’ (G&&45) ‘from, the I&i&ion of Gen&al:.Medical. Sciences,; Naii0na.l Institutes .of
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IDENTIFICATION

OF LICHEN:

PRODUCTS

BY

TLC

93

REFERENCES

I W. L! CULBERSON, Taxon; 18 (1969) 152. C. I?.: CVLBERSQN, ‘Univerpjty’ of ,Notih : Czkolina I?ros’s, Cha&?l Hill; 3 J. SANTESSON, Acta Cl&em. SC&&, ?I (x.967) 1162.

2

4 G. BENDZ,

J. SANTESSON'AND Ir:T~r3tiii, Acta iShbm,~~S&d.~ Acta Chcm. Stand.. 19: (1963) 2254.

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