n-alkanes from Paepalanthus Mart . species ( Eriocaulaceae )

(n-alkanes from Paepalanthus Mart. species (Eriocaulaceae)). This work presents the study of nonpolar compounds from plants belonging to the genus Paepalanthus Mart. (Eriocaulaceae). Long-chain linear aliphatic hydrocarbons were identified by GCFID and GC-MS. The results indicate that Paepalanthus subg. Platycaulon species present a very homogenous profile, with carbon chains of n-alkanes ranging from C 25 to C 31 , most samples presenting higher frequencies of C27 and C29 homologues. Paepalanthus subg. Paepalocephalus species may be distinguished from one another by the distribution of main n-alkanes. P. macrocephalus, subsect. Aphorocaulon species, presents alkanes with odd-carbon numbers and P. denudatus and P. polyanthus, Actinocephalus species, present alkanes with quite distinctive profiles, with many shorter chains and a high frequency of even-carbon number, especially P. polyanthus. The results obtained indicate that the distribution of alkanes can be a useful taxonomic character, as do polar compounds like flavonoid glycosides.


Introduction
Eriocaulaceae comprises around 1,200 species in 10 genera (Giulietti et al. 2000).It is a natural group of herbaceous monocotyledons, characterized by small flowers densely arranged in capitula.Despite the low number of genera when compared to other plant families, the Eriocaulaceae encompasses many infrageneric levels.Paepalanthus is the largest genus of this family, comprising about 500 species (Giulietti & Hensold 1990) and it is subdivided in many subgenera, sections and subsections (Sano 2004).Although the huge amount of botanical work on this group, it is still difficult to clearly define these levels (Giulietti et al. 1995;Sano 2004).On the other hand, little is known about the chemical composition of the plants from this family.Previous reports (Andrade et al. 1999;Vilegas et al. 1999a;Vilegas et al. 1999b;Dokkedal et al. 2004) have shown that polar compounds like flavonoids glycosides can be usefull as a taxonomic character.
Among plant secondary metabolites, alkanes from epicuticular waxes have acquired a very consolidated condition as indicators of taxonomic relations between plant groups.Considerable interest has been shown in the systematic distribution of such compounds throughout the plant kingdom.Attention has been directed towards the possibility of using their distribution as a means of establishing a taxonomic system based on chemical characteristics (Eglinton et al. 1962a;Eglinton et al. 1962b;Eglinton & Hamilton 1963;Herbin & Robins 1968;Manners & Davis 1984;Gneco et al. 1988;Salatino et al. 1988;Salatino et al. 1989).However, inconsistencies of the alkanes of plant waxes as taxonomic markers have been pointed out; some authors have observed that the alkane distribution could be strongly affected by several factors, among them the age of the plant organ (Wilkinson & Kasperbauer 1972;Stocker & Wanner 1975;Nordby & Nagy 1977).Others (e.g.Smith & Martin-Smith 1978) concluded that no chemotaxonomic relationship could be derived from the composition of n-alkanes as the intraspecific variation was greater than the interspecific variations.Gradually, it seems that alkanes have regained the chemotaxonomists confidence (Sorensen et al. 1978;Faboya et al. 1980;Cowlishaw et al. 1983;Broschat & Bogan 1986).Salatino et al. (1991) have analysed a variable number of individuals of 12 species of Velloziaceae.Three degrees of plasticity of alkane profiles were recognized, depending on the species considered.They found that for most taxa, alkanes may be taxonomically useful at the species level if some precautions are taken.Skorupa et al. (1998) found that foliar epicuticular hydrocarbon patterns of 11 species of Pilocarpus represent useful evidences for its taxonomy at the interspecific, specific e infraspecific hierarchic levels.Merino et al. (1999) observed that alkane patterns are taxonomically valuable to explain Lupinus species relationships.
In the present study we have undertaken the investigation of nonpolar extracts by GC-FID and GC-MS from capitula of 11 additional species of Paepalanthus, distributed in two subgenera as follows: P. subg Platycaulon (P.bromelioides, P. latipes, P. planifolius, P. speciosus, P. vellozioides) and P. subg.Paepalocephalus (P.macrocephalus, P. denudatus, P. hilairei, P. polyanthus, P. ramosus and P. robustus).Capitula of each plant (1g) were extracted with 10 mL of hexane by maceration for one week.The extracts were concentrated at 40 °C in a rotary evaporator and the final solutions were evaporated under a gentle nitrogen flow until almost dry, and diluted to 200 µL with hexane.The extracts were transferred onto a silica gel Sep-Pak cartridge (690 mg 8 µm), which was previously conditioned with 5 mL of hexane and sequentially eluted with 1.5 mL of hexane.Fractions from each plant were evaporated under a stream of nitrogen until almost dry.The residues were dissolved in 100 µL of hexane and then analyzed by GC-FID with standards of n-alkanes (C 20 , C 26 and C 32 -Aldrich Chemie) and GC-MS.Gas chromatography (GC) analyses were performed using a Varian 3380 gas chromatograph equipped with a fused silica CBP-5 capillary column (25m×0.33mm i.d.; film thickness 0.5 m) and a flame ionization detector (FID).Hydrogen was used as the carrier gas (60 Kpa), and the injection split ratio was 1:30.The injection temperature was 250 °C; the column temperature was held at 100 °C for 2 min, then increased to 280 °C at 10 °C/min, and this temperature was held for 15 min; the detector temperature was 280 °C.Samples of 1 µL were injected using a 10 µL Hamilton syringe.High resolution (HR) chromatography-mass spectrometry (GC-MS) analyses were performed using a Hewlett Packard (HP) 5970 MSD, with electron impact ionization (70eV), coupled to an HP 5890 GC.The column was a 25 m× 0.25 mm i.d.HP-1 (cross-linked methyl silicon; 0.3 µm film thickness).Samples of 1 µL were injected using the split mode (split ratio 1:30), with the injector and the interface both maintained at 280°C.The temperature program used was the same as described above.Hydrogen was used as carrier gas (100 Kpa).The MS scan range was 50-500 a.m.u.Data were processed on an HP 7946/HP 9000-300 CPU.

Material and methods
The calibration curve was constructed injecting standard hydrocarbons C 20 , C 26 , C 32 .The calibration curve graph was constructed using log Retention Time vs Carbon atom number.The first fractions of each hexane extract from each Paepalanthus species were then analyzed by HRGC-FID under the same condition as that of the hydrocarbons standard aforementioned to obtain the chromatograms.
The first fractions of each hydrocarbon standard were injected into the GC-MS equipment using the same conditions as described above.The identification of the compounds was based on interpretation of the fragmentograms and retention index calculations.

Results and discussion
The chromatograms of the fractions analyzed by GC-FID and GC-MS show a typical profile of wellresolved peaks separated by 14 a.m.u (CH 2 ) corresponding to long-chain aliphatic hydrocarbons.
The molecular ion is weak in every case.Major peaks were identified as being C 25 H 52 , C 27 H 56 , C 29 H 60 and C 31 H 64 , respectively.Figures 1 and 2 show the n-alkanes profile of each plant based on the intensity of the peaks obtained from GC-FID analyses.
The results indicate that Paepalanthus subg.Platycaulon (Fig. 1) species present a very homogenous profile, with carbon chains of the n-alkanes ranging from C 25 to C 31 , most samples presenting higher frequencies of C 27 and C 29 homologues.In all species C 27 is the main alkane, excluding P. latipes, where C 29 is the main one.These results agree with data of previous reports using polar compounds like flavonoid glycosides (Vilegas et al. 1999a;Vilegas et al. 1999b) and reinforce the homogeneity of these taxa, since subg.Platycaulon is characterized by naphthopyranone derivatives and 7-methoxy flavonol derivatives.On the other hand, in the subg.Paepalocephalus species (Fig. 2), although C 27 -C 29 homologues present higher frequencies in all samples, they may be distinguished from one another by the distribution of n-alkanes, taking into account the main alkanes quoting the main one outside and the second one inside parentheses: P. macrocephalus -C 21 , C 27 and C 29 (no real predominance of either alkane); P. hilairei -C 27 (C 29 ); We also can see that P. macrocephalus (subsect.Aphorocaulon species) presents just alkanes with oddcarbon numbers and P. denudatus and P. polyanthus (Actinocephalus species) present a quite distinctive profile, with many shorter chains and a high frequency of alkanes with even-carbon numbers, especially P. polyanthus.These results are different from those described by Salatino et al. (1988), who detected C 27 as the main homologue in all species of P. subg.Paepalocephalus.These data are in agreement with the cladistic analysis (Giulietti et al. 2000) where P. subg.Platycaulon form a clade, which is sister to P. subsect.Aphorocaulon and P. sect.Actinocephalus (both belonging to P. subg.Paepalocephalus).The results obtained indicate that the distribution of alkanes can be useful as a taxonomic character, although a more detailed inventory of alkanes profiles of species of Paepalanthus are needed, based on wide sampling for each species.

Aknowledgments
We thank the financial support of FAPESP, CNPq and FUNDUNESP.

Figure 1 .
Figure 1.n-Alkanes profile of plants of Paepalanthus subg.Platycaulon based on the intensity of the peaks obtained from GC-FID analyses.