Chemistry of the Convolvulaceae: Secondary Metabolites and their Contribution to the Classification (Chemotaxonomy) (Literature included until June 2007; will be updated stepwise !)
Eckart Eich
Secondary metabolites are relatively small chemical constituents (small molecules) that perform nonessential functions in the plant. However, due to their varied range of bioactive potency in dependency on their chemical structure they may provide advantages, e.g., toxic defense against pathogens (viruses, bacteria, fungi) or herbivores/predators. In contrast to the (essential) primary metabolism of plants, which is more or less ubiquitously the same in the plant kingdom (only few exceptions), each individual species shows a specific secondary metabolism. Thus, the occurrence of certain secondary metabolites or a group of such and even (though limited) their absence may contribute – like, e.g., morphological characters – to a more or less specific characterization of certain plant taxa. Furthermore, phylogenetically related species often share a similar profile of more or less specific secondary metabolites or groups of such. Therefore, they may be also useful as additional characters/markers in plant systematics/classification (chemotaxonomy). Chemotaxonomic significance implicates comparison with at least two taxa (e.g., species, genera). Consequently, singular occurrence of constituents in only one species is not a topic of this synopsis; such cases will be added to the corresponding species site.
However, it must be taken into account that – to date – only 31 out of altogether 58 convolvulaceous genera are documented in the literature with at least one report on structurally elucidated secondary metabolites. Furthermore, it should be taken cognizance of the fact that such papers include only 281 out of altogether ~1,880 species (15%). Thus, 85% of the “convolvulaceous continent” represents uncharted territory with regard to precise information on secondary metabolism. This situation still offers considerable potential for research in the future.
Secondary metabolites may be accumulated in all parts of the plant or only in specific organs, e.g., seeds or roots. Alkaloids, particularly bioactive low-molecular N-containing secondary metabolites biogenetically derived from certain amino acids (e.g., L-ornithine, L-tryptophan), form a characteristic and huge class of constituents in many families of the plant kingdom. This is also true for certain types of such metabolites in the Convolvulaceae family. Members of certain subclasses of alkaloids (e.g., tropanes, ergolines) not only contribute to the characterization of taxa but also show taxonomic relevancy. Phenylpropanoids including flavonoids as well as terpenoids, further significant classes of metabolites in the plant kingdom, are constituents of this family, too. Thus, they may be of use for the characterization of certain species or genera. However, they are mostly of rather limited taxonomic value. In contrast, resin glycosides, characteristic constituents of complex resins occurring usually with lactifers, are of high chemotaxonomic relevancy since they represent unique metabolites in the plant kingdom confined to the Convolvulaceae.
Ornithine-derived Alkaloids
This type of alkaloids is generally shared with the sister family Solanaceae though frequently with individual variations with regard to their chemical structure (qualitative difference). However, usually such constituents were accumulated in convolvulaceous species in lower concentrations than in their solanaceous sisters (quantitative difference).
Simple Pyrrolidines(e.g., hygrine, cuskhygrine)
Distribution among the family: detected in 9 tribes, in all 23 genera checked, and even in almost all of the corresponding species checked [143 out of 150 species (95%)]; the remaining 3 tribes (Humbertieae, Cardiochlamyeae, Poraneae) were not checked to date.
Chemotaxonomic relevancy: significant marker for the family; plesiomorphic characters shared with the sister family Solanaceae; further (erratic) occurrence: different unrelated families throughout the plant kingdom, e.g., Erythroxylaceae, Brassicaceae, Rhizophoraceae
Nicotinoids (predominantly nicotine)
Distribution among the family: detected in 99 out of 150 convolvulaceous species (66%) throughout the family in almost all tribes (Humbertieae, Cardiochlamyeae, Poraneae not checked), almost all genera, and even almost all sections of large genera (Ipomoea, Merremia)
Chemotaxonomic relevancy: nicotine (in very low concentration) is a marker for the family; plesiomorphic character shared with the sister family Solanaceae; further (erratic) occurrence in single species of a number of unrelated families, e.g., Erythroxylaceae, Asteraceae
Calystegines (Polyhydroxy-nortropanes) [e.g., calystegine A3]
Distribution among the family: detected in taxa of all tribes (i.e., including Humbertieae, Cardiochlamyeae, Poraneae) except Cuscuteae and in a wide array of genera (22 out of 29 genera checked); genera with a remarkable percentage of calystegine-positive species: Erycibe (5 spp. + out of 5 spp. checked), Argyreia (6/7), Calystegia (4/5), Convolvulus (10/18), Ipomoea (20/41); calystegine-negative genera with >1 species checked: Polymeria (5 spp.), Operculina (5), Cuscuta (5). Chemotaxonomic relevancy: significant markers for the family; plesiomorphic characters shared with the sister family Solanaceae; further (erratic) occurrence: Erythroxylaceae, Brassicaceae, Rhizophoraceae
Simple Tropanes (e.g., 3α-hydroxytropane [syn.: tropine]; 3β-hydroxytropane [syn.: pseudotropine]
Distribution among the family: detected in 9 tribes; the remaining 3 tribes (Humbertieae, Cardiochlamyeae, Poraneae) were not checked; however, since taxa of these 3 tribes turned out to be calystegine-positive they also must be able to synthesize simple tropanes, because the latter are biogenetic precursors of calystegines. Thus, simple tropanes are metabolites in certain taxa of all 12 tribes (25 out of the 28 genera checked). Furthermore, at least one simple tropane was detected in <95% of all species checked
Chemotaxonomic relevancy: significant markers for the family; plesiomorphic characters shared with the sister family Solanaceae; no consistent genus-typical trait for Polymeria and Operculina; further (erratic) occurrence: Erythroxylaceae, Brassicaceae, Rhizophoraceae
3α-Acyloxytropane/-nortropanes:
Aliphatic esters (e.g., 3α-acetoxytropane/-nortropane)
Distribution among the family: frequent in the tribes Erycibeae, Dichondreae, Cresseae; present in one third of the Ipomeeae species checked; rather rare in the Convolvuleae and Merremieae
Chemotaxonomic relevancy: more or less suitable markers for certain tribes (no consistent genus- typical trait); plesiomorphic characters shared with the sister family Solanaceae
Simple aromatic esters (e.g., convolamine/convolvine)
Distribution among the family: erratic occurrence in the genera Erycibe, Bonamia, Maripa, Jacquemontia, Calystegia, Merremia, Ipomoea; more frequent: Convolvulus
Chemotaxonomic relevancy: apomorphic characters, unique occurrence in the plant kingdom {exception: datumetine [singular further occurrence in Datura metel (Solanaceae)]}; no consistent genus-typical trait; for details see genus page Convolvulus
Prenylated aromatic esters (merresectines) [merresectine C]
Distribution among the family: erratic occurrence in the genera Dichondra, Bonamia, Evolvulus, Convolvulus, Ipomoea; more frequent: Merremia
Chemotaxonomic relevancy: apomorphic characters, unique occurrence in the plant kingdom; no consistent genus-typical trait; for details see genus pages Merremia (sections Cissoides, Vitifolia allies), Convolvulus
Phenylpropanoid esters (e.g., 3α-feruloyloxytropane)
Distribution among the family: erratic occurrence in the tribes Dichondreae, Cresseae, Maripeae, Convolvuleae
Chemotaxonomic relevancy: plesiomorphic characters shared with the sister family Solanaceae; no consistent family-typical trait; further (erratic) occurrence: Erythroxylaceae, Brassicaceae
3β-Acyloxytropanes/-nortropanes:
Aliphatic esters (e.g., 3β-acetoxytropane)
Distribution among the family: erratic occurrence in the tribes Dichondrae, Maripeae, Convolvuleae, Merremieae, Ipomoeae; frequent: Cresseae
Chemotaxonomic relevancy: plesiomorphic characters shared with the sister family Solanaceae; no consistent family-typical trait
Simple aromatic esters (e.g., concneorine)
Distribution among the family: erratic occurrence in the genera Bonamia, Evolvulus, Maripa, Merremia, Ipomoea; more frequent: Convolvulus
Chemotaxonomic relevancy: apomorphic characters, unique occurrence in the plant kingdom [exception: tropacocaine (singular further occurrence in Erythroxylaceae)]; no consistent genus-typical trait; for details see genus page Convolvulus
Prenylated aromatic esters (β-merresectines)
Distribution among the family: erratic occurrence in the genera Dichondra, Bonamia, Jacquemontia, Convolvulus, Ipomoea; more frequent: Merremia
Chemotaxonomic relevancy: apomorphic characters, unique occurrence in the plant kingdom; no consistent genus-typical trait; for details see genus page Merremia (sections Cissoides, Vitifolia allies)
Phenylpropanoid esters (e.g., 3β-feruloyloxytropane):
Distribution among the family: erratic occurrence in the tribes Dichondreae, Cresseae, Maripeae, Convolvuleae; extremely rare in Merrimeae and Ipomoeae
Chemotaxonomic relevancy: plesiomorphic characters shared with the sister family Solanaceae; no consistent family-typical trait; further (erratic) occurrence: Erythroxylaceae, Brassicaceae
Indolizidines (e.g., ipalbidine)
Distribution among the family: only discovered in three Ipomoea spp.; not detectable in 150 further convolvulaceous species from altogether 24 genera checked
Chemotaxonomic relevancy: apomorphic characters, unique occurrence in the plant kingdom; for details see genus page Ipomoea (section Calonyction)
Pyrrolizidines:
Ipangulines and Minalobines
Distribution among the family: apparently confined to genus Ipomoea subgenus Quamoclit sect. Mina; not detectable in 150 further convolvulaceous species from altogether 23 genera checked
Chemotaxonomic relevancy: apomorphic characters, unique occurrence in the plant kingdom; consistent section-typical trait; for details see genus page Ipomoea (section Mina)
Retronecine Derivatives (e.g., lycopsamine)
Distribution among the family: only discovered in two Merremia spp.; not detectable in 150 further convolvulaceous species from altogether 23 genera checked
Chemotaxonomic relevancy: see genus page Merremia (section Cissoides); further occurrence: different unrelated families throughout the plant kingdom, e.g., Boraginaceae, Asteraceae, Apocynaceae
Tryptophan-derived Alkaloids
β-Carbolines (e.g., harman)
Distribution among the family: erratic occurrence detected in a few species of different genera
Chemotaxonomic relevancy: none (further occurrence in many families)
Ergolines (e.g., agroclavine, ergine, ergobalansine)
These metabolites are no direct constituents of certain convolvulaceous species, since their biosynthesis and accumulation turned out to occur in a mutualistic association between the higher plant and an epibiotic clavicipitaceous fungus of the genus Periglandula (for details and references see the tribe page Ipomoeeae). A similar situation with different fungi of the Clavicipitaceae is given for certain grasses (Poaceae) also characterized by the occurrence of ergolines
Distribution among the family: Unambigous occurrence is confined to four genera belonging to the tribe Ipomoeeae (Argyreia, Ipomoea, Stictocardia, Turbina)
Chemotaxonomic relevancy: no consistent tribe- and genus-typical trait; Ipomoea: infrageneric distribution of limited value (for details see genus page Ipomoea); unambiguous occurrence of ergolines in higher plants is confined to the Convolvulaceae and Poaceae
Phenylalanine-derived Metabolites / Phenylpropanoids
This class/group of metabolites is of less systematic interest; however, there are certain specific individual compounds of rare or even unique occurrence in the plant kingdom
Cyanogenic glycosides (e.g., prunasin)
Distribution among the family: no frequent constituents; detected in some Merremia and Ipomoea species as well as in one Stictocardia species (see the corresponding genus pages); for unique constituents see the species page Merremia dissecta
Chemotaxonomic relevancy: none (widely distributed in the plant kingdom)
Phenylmethanoids (e.g., benzoic acid, salicylic acid)
Distribution among the family: presumably frequent constituents; detected in a number of species of different genera
Chemotaxonomic relevancy: none (further occurrence in many families)
Hydroxycoumarins (e.g., umbelliferone, aeculetin, scopoletin)
Distribution among the family: frequent constituents; detected in many species of numerous genera
Chemotaxonomic relevancy: none (common constituents of plants)
Hydroxycinnamate conjugates / caffeic acid derivatives (e.g., chlorogenic acid)
Distribution among the family: distinctive tendency to accumulate such metabolites; detected in many species of different genera; for unique constituents see the species page Ipomoea obscura
Chemotaxonomic relevancy: taxonomic markers within Cuscuta (see the corresponding species page); no further relevancy (common constituents in many families)
Flavonoids(e.g. quercetin, anthocyanins)
Distribution among the family: frequent constituents; detected in many species of numerous genera; anthocyanins (pigments) only checked within the genera Calystegia, Evolvulus, Ipomoea
Chemotaxonomic relevancy: taxonomic markers within the genus Cuscuta (see the corresponding page); no further relevancy (common constituents in many families)
Lignans and Neolignans (e.g., arctigenin and virolongin A, respectively)
Distribution among the family: erratic occurrence of lignans in the genera Hewittia, Ipomoea, Jacquemontia, Merremia, Operculina (neolignans: Bonamia spectabilis)
Chemotaxonomic relevancy: none (widely distributed in the plant kingdom)
Terpenoids (Isoprenoids)
This class/group of metabolites is of less systematic interest; however, there are certain specific individual compounds of rare or even unique occurrence in the plant kingdom
Sesquiterpenoids (C15 Isoprenoids) (e.g. ipomeamarone)
Distribution among the family: compounds of this type were discovered in some species as normal constituents (“constitutive sesquiterpenoids”) [see species pages Humbertia madagascariensis, Ipomoea asarifolia, certain Merremia spp.(see genus page)] or may be formed by a plant species only after a contact with a fungus (“sesquiterpenoid phytoalexins”) [see species page Ipomoea batatas])
Chemotaxonomic relevancy: none (sesquiterpenoids are widely distributed in the plant kingdom; however, one subtype (furanosesquiterpenoids) found in this family is rather rare)
Diterpenoids (C20 Isoprenoids) (e.g, gibberellin A27)
Distribution among the family: novel gibberellins could be identified in Ipomoea nil (see species page) and I. alba (see species page), respectively, whereas novel kaurane-type diterpenoids were discovered in Turbina corymbosa (see species page) and Merremia aurea (see species page), respectively.
Chemotaxonomic relevancy: none (widely distributed in the plant kingdom)
Triterpenoids (C30 Isoprenoids) (e.g., α- or β-amyrin)
Distribution among the family: occurrence detected in Cressa cretica, Ipomoea batatas, Ipomoea mauritiana, Ipomoea quamoclit, Argyreia capitata, Argyreia speciosa (see species pages)
Chemotaxonomic relevancy: none (further occurrence in many families)
Tetraterpenoids/Carotenoids (C40 Isoprenoids)
Distribution among the family: occurrence of carotenoids reported from Cuscuta australis, Cuscuta salina, Cuscuta subinclusa, Ipomoea aquatica, Ipomoea batatas, Ipomoea hederifolia, Ipomoea pes-caprae (see species pages)
Chemotaxonomic relevancy: none (widely distributed in the plant kingdom)
Fatty Acids and Their Derivatives
Fatty Acids as Components of Lipids
Distribution among the family: seeds of species from this family usually contain about 10% fatty oil; occurrence reported on a remarkable number of taxa
Chemotaxonomic relevancy: none (widely distributed in the plant kingdom)
Fatty Acid Amides {e.g., palmitoylamide [16:0], erucamide [22:1 (n-9)]}
Distribution among the family: occurrence detected in Evolvulus glomeratus, Ipomoea plebeia, Operculina riedeliana (see species pages); not detectable in 150 further convolvulaceous species from altogether 23 genera checked
Chemotaxonomic relevancy: none
Aliphatic N-Mono- and N,N-Dimethylamines (e.g., N,N-dimethyldodecylamine)
Distribution among the family: erratic occurrence detected in the tribes Cresseae (1 Evolvulus sp.), Aniseieae (1 Odonellia sp.), Convolvuleae (1 Calystegia sp., 5 Convolvulus spp.), Merremieae (1 Merremia sp., 1 Operculina sp.), Ipomoeeae (4 Ipomoea spp.) (see genus pages); not detectable in 136 further convolvulaceous species from altogether 23 genera checked
Chemotaxonomic relevancy: none
Resin Glycosides (Glycoresins) [e.g. Tricolorin C]
General characterization: Resin glycosides are complex molecules characterized in general by three components: (i) a hydroxy fatty acid, (ii) an oligosaccharide (formed by 2 – 7 specific monosaccharide units, e.g., glucose, rhamnose), and (iii) short-chain aliphatic acids (vast majority of cases) and/or long-chain fatty acids (the latter may lack). (i) and (ii) are linked to each other forming a so-called glycosidic acid which as a consequence of specific cyclization provides a macrolactone. Regularly monosaccharide units of such a macrolactone are additionally acylated by (iii). A single resin glycoside may show a very varying molecular magnitude (618 Da – 1771 Da) as a monomer. However, sometimes dimeric congeners were found in the same plant species; even oligomers (up to heptamers) seem to occur (up to 25,000 Da). Moreover, convolvulaceous resins turned out to consist of a more or less complex mixture of several single resin glycosides, i.e., there are a number of congeners with related chemical structure. They may be considered as the most specific constituents of the convolvulaceous secondary metabolism, since they are confined to and occur frequently in this family; furthermore, they show a rather broad intrafamilial distribution (tribes Cuscuteae, Convolvuleae, Merremieae, Ipomoeae).
Distribution among the family: occurrence proved including structural elucidations of components in 34 species belonging to the genera (see genera pages) Calystegia (1 species), Convolvulus (3 spp.), Cuscuta (2 spp.), Merremia (3 spp.), Operculina (2 spp.), Ipomoea (23 spp.). It should be added that there are reports on the occurrence of resins in several further Convolvulus spp. and Ipomoea spp., respectively, without any structural elucidation of their components (resin glycosides ?)
Chemotaxonomic relevancy: significant markers for the family; apomorphic characters
References (Reviews):
1 Eich, E. (2008) Solanaceae and Convolvulaceae: Secondary Metabolites – Biosynthesis, Chemotaxonomy, Biological and Economic Significance (A Handbook). Springer-Verlag Berlin – Heidelberg, 637 pp.
2 Leistner, E. & Steiner U. (2009) Fungal origin of ergoline alkaloids present in dicotyledonous plants (Convolvulaceae). In: Anke, T. & Weber D. (eds.) Physiology and Genetics, 1st ed., The Mycota XV, Springer-Verlag Berlin – Heidelberg, pp. 197-208
3 Pereda-Miranda, R., Rosas-Ramírez, D. & Castañeda-Gómez, J. (2010) Resin glycosides from the morning glory family. In: Kinghorn A.D. Falk, H. & Kobayashi J. (eds.) Fortschritte der Chemie organischer Naturstoffe / Progress in the Chemistry of Organic Natural Products, vol. 92:77-153