A CHEMOTAXONOMIC STUDY OF DITERPENES FROM MARINE

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The Science of the Total Environment, 75 (1988) 271-283 271Elsev ier Science Publishers B.V., Am sterdam - - Prin ted in The Netherlands

A C H E M O T A X O N O M I C S T U D Y O F D I T E R P E N E S F R O M M A R I N EB R O W N A L G A E O F T H E G E N U S D I C T Y O T A

VALE RIA L. TEIXEIRALaboratorio de Botanica Marinha, Faculdade de Biologia e Psicologia maria Thereza, RuaViscone do Rio Branco 869, 24, 020 Niteroi, RJ (Brazil)ALPHONSE KELECOMLaboratory of Marine Natural Products, Departamento de Biologia Geral, Universidade FederalFluminense (UFF), C.P. 100.183, 24.000 Niteroi, RJ (Brazil)

ABSTRACTDiterpenes from Dictyota species appear to be valuable taxonomic m arkers and showed, forvarious species, s t rong correlat ion with botanic data. Our resul ts corroborate the Theory ofMicromolecular Evo lut ion that s tates tha t h igher oxidation levels, in a given taxon, are charac-ter is t ic of evolut iona ri ly more advanced components.

INTRODUCTIONP h a e o p h y t a a r e b r o w n a l g a e , a l m o s t e x c l u s i v e ly m a r i n e , l i to p h y l ic , o cc a -

s i o n a l l y e p i p h y t i c , a n d f o u n d p r i n c i p a l l y i n c o l d a n d t e m p e r a t e w a t e r s [1]. T h ed i v i s i o n P h a e o p h y t a p o s s e s s e s o n e s i n g l e c l a s s , P h a e o p h y c e a e , c o m p o s e d o fa b o u t 2 5 0 g e n e r a a n d m o r e t h a n 1 5 0 0 s p e c i e s [ 2 ] . A c c o r d i n g t o W y n n e a n dL o i s e a u x [3], P h a e o p h y c e a e a r e d i s t r i b u t e d i n 13 o r d e r s , t w o o f w h i c h , F u c a l e sa n d D i c t y o t a l e s , a r e p r e d o m i n a n t i n t r o p i c a l a n d s u b - t r o p i c a l s e a s [ 2 ] .

T h e o r d e r D i c t y o t a l e s i n c l u d e s 16 f u l ly d e f i n e d g e n e r a [4]. A m o n g t h e s e , t h eg e n e r a Dictyota L a m o u r o u x , Dilophus J . A g a r d h , Glassophora J . A g a r d h ,Pachydictyon J . A g a r d h , Spatoglossum K u t z i n g a n d Stoechospermum K u t z i n gp r o d u c e a s e r i e s o f d i t e r p e n e s t h a t a r e p r o b a b l y i n v o l v e d in h e r b i v o r y c o n t r o l[5, 6].

T h e g e n u s Dictyota, w i t h s o m e 3 0 s p e c i e s , h a s b e e n c h e m i c a l l y t h e m o s te x t e n s i v e l y i n v e s t i g a t e d : m o r e t h a n 9 0 d i t e r p e n e s o f 17 s k e l e t a l c l a s s e s h a v eb e e n i s o l a t e d f r o m 1 8 s p e c i e s d i s t r i b u t e d i n a l l t h e o c e a n s [ 7 ] . S i n c e p r o b l e m ss t i ll e x i s t i n e s t a b l i s h i n g t h e s e p a r a t i o n l im i t s b e t w e e n s p e c i e s a n d v a r i e t i e s o fv a r i o u s Dictyota r e p r e s e n t a t i v e s [ 8 , 9 ] , i t h a s b e e n s u g g e s t e d t h a t t h e a l g a ed i t e r p e n e s m i g h t b e o f c h e m o t a x o n o m i c i n t e r e s t [1 0-1 2] . H o w e v e r , t h e r e h a v eb e e n n o t a x o n o m i c n o r p h y l o g e n e t i c i n f e r e n c e s f r o m t h e c h e m i c a l d a t ar e s u l t i n g f r o m t h e i s o l a t i o n a n d i d e n t i f i c a t i o n o f d i t e r p e n e s f r o m Dictyotas p e c ie s . I t is th e a i m o f t h i s w o r k t o i l lu s t r a t e t h e g r e a t p o t e n t i a l o f Dictyotad i t e r p e n e s a s c h e m o t a x o n o m i c a n d p h y l o g e n e t i c m a rk e r s .


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272METHODOLOGY

Usually, chemo systemati c studies are based on a presence/absence record ofmetabolites [13]. It cannot be over emphasized, however, that such a criterionmay not be considered as fully reliable for chemotaxonomic purposes for atleast two reasons. First, because the presence of metabolites in an organismmay have been overlooked, and second because the isolation of knowncompounds, even from new sources, is generally not reported (and even notaccepted for publication) in the mai n journals. Consequently, compounds th atare in fact present will be considered absent, and this m ay obviously alter thechemotaxonomic conclusions. Hence, a methodology that is not based on apresence/absence criterion is necessary for the use of natural products intaxonomy. Such a tool has been devised by O.R. Gottlieb [14] and has beenextensively used for this study.

Accordingly, all diterpenes isolated from Dictyota species up to June 1986have been characterized by two indexes. First, the skeleton specializationindex (SI), calcul ated from the th eoreti cal number of steps, involving C-C bondformat ion or cleavage, tha t are nec essary to derive a part icul ar skeleton fromthe common g eran yl-g eran iol precursor (see Figs 1-3). Second, the oxid ationindex (OI), which considers identical all compounds hav ing the same molecularformulae and functionality irrespective of the isomer. Each Dictyota specieswas then characterized by the skeletal evo lutionary advancement index (EAs)and by the mean oxidation index (OI), both obtained as the arithmetic meanrespectively of the SI and OI values of each diterpene iso lated from consideredspecies. For a detailed description of the methodology, see Gottlieb [14].RESULTS

The 17 diterpene skeletons obt ained from Dictyota species are shown in Figs1-3, and the number of diterpenes of each skeletal class is reported in Table 1.The unidenti fied species for which chemical dat a are av ailable were arbit rarilynumbered, in order to allow further discussion.

Based on a revised biogenetic scheme proposed by the present auth ors in anearlier study [7], the diterpenes have been distri buted int o thr ee groups (I-III),depending on the first formal cyclization of the geranyl-geraniol precursor.Diterpenes of Group I are mainly prenylated derivatives of known sesquiter-pene skeletons th at result from a first cyclization of gera nyl- gera nio l betweenposit ions 1 and 10 (Fig. 1). Group II con tai ns dit erpenes derived by a cycli zatio nof the precursor between positions 1 and 11 (Fig. 2). With the exception of thedolabellane skeleton, the diterpene skeletons of this group are restricted,among Dictyotales, to Groups I and III. Whet her or not D. spinulosa, for whichonly one xeniane-re lated diterpene had been reported [12], belongs to the la ttergroup will be discussed later. Finally, the cosmopolite D. dichotoma [2], whichis considered to be the type species of the genus [4], is the only species thatproduced diterpenes from all three groups [13-55].Our results, obtained by numerical tre atm ent using the mean oxidati on (OI)


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pyrophosphate

~ I,iiall-trans-geranyl-geraniol

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I d i c t y m a n e\/ dolabellane08} 8

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dictyoxetane I

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secodolastane 4

seco 10,14

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dolastane 1 7

Fig. 2. Hypothetical biosynthetic pathway for Dictyota diterpenes of Group II.

and th e mean skeletal evolutive adv ancem ent (EAs) indexes for all the Dictyotaspecies considered in this work, are reported in Fig. 4.The A area includes species that yield exclusively diterpenes from Group I.

These are cha rac ter ized by low OI (~ - 1.43) and EAs ( ~ 0.10) values. Dictyotamasonii was te ntat ivel y included (dotted line) in sub-group A for havin g onlyone diterpene reported [19] whose OI and EAs indexes (OI = -1 . 5 0 ;EAs = 0.05) were found to be in disc repancy with the values observed for theother species of sub-group A.Sub-group B corresponds to species that produce diterpenes belonging ex-

clusively to Group II. Their main characterist ics are high OI (~ 1.26) andEAs (~ 0.15) values. Among these species, Dictyota sp 1 is peculiar in being the


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al i- trans-geranyl-geraniolpyrophosphate

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crenu l idane ]

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tricyc lodictyotane 3 dichotomane 2

Fig. 3. Hypothetical biosynthetic pathway for Dictyota diterpenes of Group III.only one that afforded dolabellanes, when all the other species yieldeddolastanes.Sub-group C includes the algae that furnish diterpenes from Groups I and III.This sub-group is character ized by the lowest EAs value (~ 0 .09) and by an OIvalue of - 1.32. In this sub-group, Dictyo ta pro l i ficans i s d i sc r epan t fo r hav ing


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A CHEMOTAXONOMIC STUDY OF DITERPENES FROM MARINE