Taxonomic significance of alkaloids and iridoid glucosides in the tribe Psychotrieae (Rubiaceae)

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Biochemical Systematics and Ecology 32 (2004) 1187–1195

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Taxonomic significance of alkaloids and iridoidglucosides in the tribe Psychotrieae (Rubiaceae)

Sılvia Lopes, Gilsane L. von Poser, Vitor A. Kerber,Fabiane M. Farias, Eduardo L. Konrath, Paulo Moreno,

Marcos E. Sobral, Jose A.S. Zuanazzi, Amelia T. Henriques �

Programa de Pos-Graduacao em Ciencias Farmaceuticas, Universidade Federal do Rio Grande do Sul

(UFRGS), Av. Ipiranga, CEP 90.610-000, 2752 Porto Alegre, RS, Brazil

Received 30 August 2002; accepted 30 April 2004

Abstract

Leaves of 15 Brazilian species of Psychotria, three of Rudgea and Palicourea rigida, wereanalyzed for their alkaloid and iridoid content. Alkaloids were found in three of Rudgeaand 14 species of Psychotria, and iridoids were found in Psychotria leiocarpa, which pro-duces asperuloside and deacetylasperuloside. Palicourea rigida yielded no alkaloids but loga-nin was isolated. The results illustrate the significance of the alkaloids in thechemotaxonomy of some taxa of Psychotrieae. The phytochemical data indicate that theAmerican species of Psychotria with Palicourea could be joined to form the genus Hetero-psychotria.# 2004 Elsevier Ltd. All rights reserved.

Keywords: Psychotria; Palicourea; Rudgea; Rubioideae; Rubiaceae; Alkaloids; Iridoid glucosides;

Taxonomy

1. Introduction

The tribe Psychotrieae (subfamily Rubioideae) comprises about 50 genera; some

of them with unclear generic limits due to the lack of adequate morphologicalcharacters to define their boundaries. The generic delimitation of this tribe has

been the object of investigations by several authors (Taylor, 1989, 1996; Nepok-roeff et al., 1999) initially using purely morphological characters, but more recent

S. Lopes et al. / Biochemical Systematics and Ecology 32 (2004) 1187–11951188

molecular data suggest that there should be a revision of generic limits, mainly inthe Psychotria alliance.

Psychotria L., distributed in tropical regions is one of the largest genera offlowering plants with 1000–1650 species worldwide and is taxonomically complex(Nepokroeff et al., 1999). Petit (1964, 1966) and Steyermark (1972) recognizedthree subgenera: Psychotria (pantropical), Tetramerae (includes some species fromAfrica and Madagascar) and Heteropsychotria (includes the remainder of the spe-cies of Psychotria in the neotropics). The division into subgenera is based on mor-phological characters and geographical distribution. Species of Palicourea Aubl.and Rudgea Salisb. are most closely related to members of subg. Heteropsychotriabased on their shared persistent and continuously connate stipules, versus thedeciduous stipules found in other Psychotria (Taylor, 1989, 1996). Molecularphylogenetic analysis proposed the fusion of Palicourea and the subgenus Hetero-psychotria in a genus of its own (Nepokroeff et al., 1999). If this viewpoint isaccepted, this new inclusive genus would be named Psychotrophum P. Browne,based on Taylor’s (1996) arguments of priority rules.

The present paper is part of a search for alkaloids and iridoid glucosides innative Rubiaceae species of Brazil. It deals with the chemical screening of 15 spe-cies of Psychotria, (subg. Heteropsychotria), Palicourea rigida H.B.K., three speciesof Rudgea and contributes to the systematic analysis of these taxa.

2. Materials and methods

2.1. Plant material

The species of Psychotria surveyed are shrubs to treelets of tropical and sub-tropical forest formations of southern and southeastern Brazil; those of Rudgea aretreelets from southern Brazilian coastal forests and Palicourea rigida is a shrubfrom rocky areas of central Brazil. Voucher specimens were deposited in the her-barium of the Universidade Federal do Rio Grande do Sul (ICN). Collected spe-cies, voucher number and sites of collection are presented in Table 1.

2.2. Chemical methods

2.2.1. Iridoid detectionThe dried leaves were powdered, extracted with EtOH and the concentrated

extracts were partitioned in Et2O–H2O. The aqueous phases were concentrated giv-ing the crude extracts. These crude extracts were analyzed by TLC eluting withCH2Cl2/MeOH (4:1) and submitted to 1H NMR analysis which can detect concen-tration in iridoids lower than 0.01% (Jensen et al., 1988).

A larger batch of Psychotria leiocarpa (180 g) and Palicourea rigida (50 g) (whichshowed the characteristic 1H NMR signals of iridoid glycosides) was extracted asdescribed above. The concentrated aqueous extracts (8.2 and 3.4 g, respectively) weresubmitted to column chromatography on silica gel using a CHCl3/MeOH gradientsystem followed by preparative TLC eluting with CH2Cl2/MeOH (4:1, V/V)

1189S. Lopes et al. / Biochemical Systematics and Ecology 32 (2004) 1187–1195

providing (1) asperuloside (13 mg) and (2) deacetylasperuloside (9 mg) from Psycho-

tria leiocarpa and (3) loganin (37 mg) from Palicourea rigida. The products were

identified by 1H NMR and 13C NMR (El Naggar and Beal, 1980; Boros and Ster-

mitz, 1990; Gonzalez and Dieck, 1996; Peng et al., 1999). No iridoid glucosides were

detected in the other species analyzed.

, ,

2.2.2. Asperuloside (1)1H NMR (400 MHz, D2O) d: 5.75 (d, J ¼ 2:2 Hz, H-1), 7.37 (d, J ¼ 1:8 Hz,

H-3), 3.60–3.10 (m, H-5), 5.68 (br d, J ¼ 7 Hz, H-6), 5.90 (br s, H-7), 3.60–3.10

(m, H-9), 4.85 (br s, H-10), 2.10 (s, AcO), 4.70 (d, 9.0 Hz, H-10), 4.20–3.15 (m,

H-20, H-30, H-40 and 50), 3.90–3.68 (m, H-60).

Table 1

Species of Psychotrieae investigated for alkaloids and iridoid glucosides contents

Species
Site of collection Voucher number
Palicourea rigida H.B.K.
Bom Jardim de Minas, MG Sobral 8180
Psychotria alba Ruiz et Pav.
Blumenau, SC Sobral 9054
P. barbiflora DC.
P. brachyceras Mull.Arg.
Porto Alegre, RS Sobral e Kerber 7899—
UFRGS—08/1995

P. carthagenensis Jacq.
Porto Alegre, RS Sobral 7901
P. deflexa DC.
P. hancorniifolia Benth.
Juiz de Fora, MG Sobral 8216
P. kleinii L.B.Sm. et Downs
P. leiocarpa Cham. Et Schltdl.
Porto Alegre, RS Sobral 7898
P. longipes Mull.Arg.
P. myriantha Mull.Arg.
Tenente Portela, RS Sobral 9044
P. nuda (Cham. Et Schltdl.) Wawra
P. pleiocephala Mull.Arg.
Juiz de Fora, MG Sobral 8213
P. pubigera Schltdl.
P. suterella Mull.Arg.
Santo Antonio da Patrulha, RS Sobral 8336
P. umbellata Vell.
Morretes, PR Hatschbach (MBM 48571)
Rudgea heurckii Mull.Arg.
Peruıbe, SP Sobral 7320
R. jasminoides (Cham.) Mull.Arg.
Torres, RS Sobral 9052
R. recurva Mull.Arg.
ES, Espırito Santo; MG, Minas Gerais; PR, Parana; RS, Rio Grande do Sul; SC, Santa Catarina; SP,

Sao Paulo.


13C NMR (100 MHz, D2O) d: 99.1 (C-1), 150.1 (C-3), 105.2 (C-4), 36.4 (C-5),86.5 (C-6), 128.5 (C-7), 142.9 (C-8), 44.1 (C-9), 61.5 (C-10), 173.5 (C-11), 93.3(C-10), 73.2 (C-20), 76.9 (C-30), 70.2 (C-40), 76.3 (C-50), 61.5 (C-60), 173.7(CO–AcO).

2.2.3. Deacetylasperuloside (2)1H NMR (400 MHz, D2O) d: 5.70 (d, J ¼ 2 Hz, H-1), 7.33 (d, J ¼ 2 Hz, H-3),

3.55–3.15 (m, H-5), 5.66 (br d, J ¼ 7 Hz, H-6), 5.70 (br s, H-7), 3.55–3.15 (m, H-9),4.15 (br s, H-10), 4.80 (d, 8.0 Hz, H-10), 4.45–3.10 (m, H-20, H-30, H-40 and 50).

13C NMR (100 MHz, D2O) d: 99.3 (C-1), 152.4 (C-3), 106.2 (C-4), 37.2 (C-5),87.1 (C-6), 129.2 (C-7), 147.7 (C-8), 44.9 (C-9), 61.9 (C-10), 174.3 (C-11), 93.1(C-10), 73.6 (C-20), 77.0 (C-30), 70.6 (C-40), 76.1 (C-50), 61.5 (C-60).

2.2.4. Loganin (3)1H NMR (400 MHz, D2O) d: 5.35 (d, J ¼ 4:0 Hz, H-1), 7.34 (s, H-3), 3.05 (m,

H-5), 2.10–1.70 (m, H-6), 4.08 (m, H-7), 2.10 (m, H-9), 1.05 (d, J ¼ 7:5 Hz, H-10),4.60 (d, 8.0 Hz, H-10), 4.45–3.15 (m, H-20, H-30, H-40 and 50), 3.90–3.68 (m, H-60),3.65 (s, OMe).

13C NMR (100 MHz, D2O) d: 97.4 (C-1), 151.7 (C-3), 114.1 (C-4), 31.0 (C-5),41.3 (C-6), 75.8 (C-7), 40.8 (C-8), 46.0 (C-9), 13.2 (C-10), 171.2 (C-11), 99.8 (C-10),74.0 (C-20), 77.0 (C-30), 70.2 (C-40), 77.5 (C-50), 61.5 (C-60), 53.0 (OMe).

2.2.5. Alkaloid detectionThe crude alkaloid extracts of dried leaves, obtained by classical acid/base par-

tition, were directly evaluated by HPLC/PDA detector. Aliquots of these extractswere loaded onto a Waters 2690 HPLC system (Millipore Corp.) fitted with a 3:9�150 mm Nova-Pak C18-4 lm column (Waters), preceded by a guard-column,eluted at 1 ml min�1 from 0 to 15 min with a linear gradient solvent systemfrom 50:50 (V/V) methanol/water to methanol. The analysis continued iso-cratically for other 20 min. Eluting compounds were monitored with a WatersMillenium (version 2.15.01) and a Waters 996 photodiode array detector, whichmeasured absorbance (200–400 nm) every 1.8 s with 4.8 nm resolution. Thepresence of indole UV spectra (instead of the indoline chromophore from thepolyindolinic alkaloids) was observed and taken as indicative for the presence ofmonoterpene indole alkaloids.

From some species, the alkaloids were isolated and identified: Psychotriaumbellata yielded umbellatine (4) and the derivatives 3,4-dehydro-18,19-b-epoxy-umbellatine, N4-[1-(2a-hydroxypropyl)]-umbellatine and N4-[1-(2a-hydro-xypropyl)]-umbellatine (Kerber, 1999); from Psychotria brachyceras, brachycerine(5) was isolated (Kerber et al., 2001); lyaloside (6) and strictosamide (7) were foundin Psychotria suterella (De Santos et al., 2001); myrianthosines A (8) and B (9)were isolated from Psychotria myriantha (Farias, 2004) while Psychotria leiocarpaafforded N,b-d-glucopyranosil vincosamide (10) (Lopes, 1998).


, , ,

, , ,

3. Results and discussion

The main metabolites found in pantropical Psychotria (subgenus Psychotria) arealkaloids of polyindoline type and one of the most frequently reported is quad-rigemine A (11). This group of compounds is derived from condensation of severalN-methyltryptamine moieties, and they seem to be characteristic of the speciesfrom subgenus Psychotria (De Santos et al., 2001). With the exception of P. color-ata (Elisabetsky et al., 1997; Verotta et al., 1998) and P. glomerulata (Solıs et al.,1995), these alkaloids have not been detected in neotropical Psychotria (subgenusHeteropsychotria) which have, instead, yielded monoterpene indole alkaloids (Solıset al., 1993; Achenbach et al., 1995; Lopes, 1998; Kerber, 1999; De Santos et al.,2001; Kerber et al., 2001). The two species cited above were formerly included inthe genus Cephaelis, which is considered by some authors (Steyermark, 1972;Taylor, 1996; Solıs et al., 1997) as a synonym of Psychotria (subgenus Hetero-


psychotria). However, the main type of alkaloids of Cephaelis, such as emetine andcephaeline (Southon and Buckinghan, 1989), derive from tyrosine, instead of tryp-tophan. Thus, from the chemosystematic viewpoint, the merging of these two gen-era seems to be unlikely.

Only a few species of Palicourea, an exclusively neotropical genus, have beenanalyzed for alkaloid content. P. dominguensis (Ripperberger, 1982), P. fendlerii(Nakano and Martin, 1976), P. alpina (Woo Ming and Stuart, 1975) and P. ovalis(Garcia et al., 1997) present polyindolines. The compounds described are dimers incontrast with the ones found in Psychotria, which contain several degrees of poly-merization. Both genera also produce monoterpene indole alkaloids, derived fromtryptamine (Morita et al., 1989; Valverde et al., 1999), in addition to the polyindo-line alkaloids.

The total alkaloids fraction of the leaves of 15 Brazilian Psychotria species, threeRudgea species and Palicourea rigida were analyzed by HPLC/PDA and peaks pre-senting UV spectra characteristic for the indole chromophore (223 and 280 nm)were detected in 14 Psychotria and three Rudgea species. This chromophore wasnot detected in Psychotria carthagenensis and Palicourea rigida. In P. suterella, itwas found substances with the extended indol chromophore (237, 289 and 334 nm)characterizing b-carbolines (De Santos et al., 2001). In P. umbellata together withthe indole group it was also observed compounds with UV spectra for 5-6-dihydro-b-carboline (236 and 320 nm) (Kerber, 1999). No alkaloids were detected in P. car-thagenensis (Lopes et al., 2000) and Palicourea rigida, and only traces of alkaloidswere found in Psychotria hancorniifolia. Glucoside monoterpene indole alkaloidswere isolated from P. brachyceras (Kerber et al., 2001), P. leiocarpa (Lopes, 1998),P. suterella (De Santos et al., 2001) and P. umbellata (Kerber, 1999). This type ofalkaloid was firstly found in the neotropical Heteropsychotria subgenus in the spe-cies P. dichroa (Cephaelis dichroa) (Solıs et al., 1993) and Psychotria correae(Cephaelis correae) (Achenbach et al., 1995). Palicourea alpina (Stuart and Woo-


Ming, 1974), P. markgravii (Morita et al., 1989) and P. adusta (Valverde et al.,1999) also contain monoterpene indole alkaloids and such data could corroboratewith the inclusion of part of Palicourea in the subgenus Heteropsychotria. There-fore, alkaloids can be a useful tool to distinguish groupings within a complex genussuch as Psychotria, as previously proposed (Solıs et al., 1995). No chemical investi-gations of Rudgea species were previously reported.

In addition to the alkaloids, iridoid glucosides such as asperuloside (1) and dea-cetylasperuloside (2) were located in some species of Psychotria. Inouye et al.(1988) reported the occurrence of these compounds in P. rubra (Lour) Poir., P. ser-pens L. and P. manillensis Bartl. Ex D.C. In addition, Gonzalez and Dieck (1996)isolated asperuloside from aerial parts of a plant named by them as ‘‘Psichotriamariniana’’. In the ethanolic extract of the leaves of Psychotria leiocarpa, the men-tioned iridoids asperuloside and deacetylasperuloside were found only in smallamounts. On the other hand, Palicourea rigida presents high concentrations ofloganin (3), never found in species of Psychotria, although it is one of the inter-mediates in the biosynthetic pathway of monoterpene indole alkaloids. This pro-duct presents the same configuration (8b) of the iridoids frequently isolated fromspecies of subfamily Rubioideae. This is the first report of iridoids in Palicourea.The results are summarized in Table 2.

In conclusion, our findings reinforce the proposition from Solıs et al. (1995) thatalkaloids are important taxonomic markers and Taylor’s (1996) hypothesis that the

Table 2

Alkaloids and iridoids in Psychotria, Palicourea and Rudgea species

Species I
ridoid glucosides Alkaloids
Palicourea rigida H.B.K. L
oganin (3) n.d.
Psychotria alba Ruiz et Pav. n
.d. i.c.
P. barbiflora DC. n
.d. i.c.
P. brachyceras Mull.Arg. n
.d. Brachycerine (5)
P. carthagenensis Jacq. n
.d. n.d.
P. deflexa DC. n
.d. i.c.
P. hancorniifolia Benth. n
.d. i.c.
P. kleinii L.B.Sm. et Downs n
.d. i.c.
P. leiocarpa Cham. Et Schltdl. A
speruloside (1);
deacetylasperuloside (2)

N,b-d-Glucopyranosil

vincosamide (10)

P. longipes Mull.Arg. n
.d. i.c.
P. myriantha Mull.Arg. n
.d. Myrianthosines A (8) and B (9)
P. nuda (Cham. Et Schltdl.) Wawra n
.d. i.c.
P. pleiocephala Mull.Arg. n
.d. i.c.
P. pubigera Schltdl. n
.d. i.c.
P. suterella Mull.Arg. n
.d. Lyaloside (6); strictosamide (7)
P. umbellata Vell. n
.d. Umbellatina (4)
and derivatives

Rudgea heurckii Mull.Arg. n
.d. i.c.
R. jasminoides (Cham.) Mull.Arg. n
.d. i.c.
R. recurva Mull.Arg. n
.d. i.c.
n.d., not detected; i.c., indole chromophore (detected by HPLC/PDA).


American Psychotria, together with Palicourea, could be joined to form the genusHeteropsychotria. Additionally, the data achieved are in agreement with the state-ments of Nepokroeff et al. (1999) that some groups previously assigned to Psycho-tria (i.e. subg. Heteropsychotria plus Palicourea) are more closely related to othergenera in the Psychotrieae than they are to other species of Psychotria.

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Taxonomic significance of alkaloids and iridoid glucosides in the tribe Psychotrieae (Rubiaceae)