Confluentic acid is an

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...

belonging to the chemical class known as

depside A depside is a type of polyphenolic compound composed of two or more monocyclic aromatic units linked by an ester bond. Depsides are most often found in lichens, but have also been isolated from higher plants, including species of the Ericaceae, L ...

s. It serves as a

secondary metabolite Secondary metabolites, also called specialised metabolites, toxins, secondary products, or natural products, are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the norm ...

in certain lichens and plays a role in distinguishing closely related species within the genus ''

Porpidia ''Porpidia'' is a genus of crustose lichens in the family Lecideaceae. Taxonomy ''Porpidia'' was circumscribed by the German lichenologist Gustav Wilhelm Körber in 1855, with ''Porpidia trullisata'' designated as the type species. Species , ...

''. In 1899,

Friedrich Wilhelm Zopf Friedrich (or Friederich) Wilhelm Zopf (12 December 1846 – 24 June 1909) was a well-known German botanist and mycologist. He dedicated to his whole life with fungal biology, particularly in classification of fungi and dye production in fungi an ...

isolated a compound from '' Lecidea confluens'', which he initially named confluentin and noted for its

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...

of 147–148 °C. This substance demonstrated the ability to turn

litmus paper Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. It is a purple dye that is extracted fro ...

red and, when interacting with

alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...

, decomposed into

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

and

phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it req ...

-like compounds. Zopf subsequently revised the chemical formula and melting point of the compound. Siegfried Huneck renamed it confluentinic acid in 1962, characterising it as optically inactive, with distinct colour reactions and

solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...

properties, and determined its

molecular formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

as C₂₈H₃₆O₈. Researchers typically identify the presence of confluentic acid using methods such as

thin-layer chromatography Thin-layer chromatography (TLC) is a chromatography technique used to separate non-volatile mixtures. Thin-layer chromatography is performed on a sheet of an inert substrate such as glass, plastic, or aluminium foil, which is coated with a t ...

and

high-performance liquid chromatography High-performance liquid chromatography (HPLC), formerly referred to as high-pressure liquid chromatography, is a technique in analytical chemistry used to separate, identify, and quantify each component in a mixture. It relies on pumps to pa ...

. Additionally, an alternative visual detection method involves examining the lichen's

thallus Thallus (plural: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. Many of these organisms wer ...

apothecium An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are ...

(fruiting body) under a microscope on a slide treated with

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...

, which reveals oil droplets indicative of confluentic acid. Several

structural analog A structural analog (analogue in modern traditional English; Commonwealth English), also known as a chemical analog or simply an analog, is a compound having a structure similar to that of another compound, but differing from it in respect to a ce ...

ues of confluentic acid have been isolated from a variety of lichen species.

History

In 1899,

reported isolating a substance from '' Lecidea confluens'', which he named confluentin, characterised by a

of . He also found that this substance turns

red, reacts with FeCl₃ to produce a red-brown colour, and decomposes into

, a volatile substance, and a

-like compound with a melting point of 52°C upon interaction with

. Zopf initially proposed the formula C₃₇H₅₀O₁₀ for this compound before revising it to C₂₆H₃₆O₇, noting the updated melting point as . In his 1962 report of his chemical investigations into the substance, German chemist Siegfried Huneck proposed naming it 'confluentinic acid' due to the presence of the

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

functional group, aligning with the naming conventions of other

lichen product Lichen products, also known as lichen substances, are organic compounds produced by a lichen. Specifically, they are secondary metabolites. Lichen products are represented in several different chemical classes, including terpenoids, orcinol deriva ...

s. Huneck described the substance as optically inactive and noted its poor

petroleum ether Petroleum ether is the petroleum fraction consisting of aliphatic hydrocarbons and boiling in the range 35–60 °C, and commonly used as a laboratory solvent. Despite the name, petroleum ether is not classified as an ether; the term is used ...

ethyl acetate Ethyl acetate ( systematically ethyl ethanoate, commonly abbreviated EtOAc, ETAC or EA) is the organic compound with the formula , simplified to . This colorless liquid has a characteristic sweet smell (similar to pear drops) and is used in glues ...

, and

acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscib ...

, but found it readily soluble in

ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...

, and

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...

. He noted the following colour reactions: weak brownish with alcoholic FeCl₃ solution, blue, green, and finally violet with

and

chloroform Chloroform, or trichloromethane, is an organic compound with chemical formula, formula Carbon, CHydrogen, HChlorine, Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to ...

upon heating, orange to orange-red with tetrazotised

benzidine Benzidine (trivial name), also called 1,1'-biphenyl-4,4'-diamine (systematic name), is an organic compound with the formula (C6H4NH2)2. It is an aromatic amine. It is a component of a test for cyanide. Related derivatives are used in the produc ...

, and gray-violet with ''p''-phenylenediamine; no colouration was observed with

barium hydroxide Barium hydroxide is a chemical compound with the chemical formula Ba(OH)2. The monohydrate (''x'' = 1), known as baryta or baryta-water, is one of the principal compounds of barium. This white granular monohydrate is the usual commercial form. P ...

. Huneck used

elemental analysis Elemental analysis is a process where a sample of some material (e.g., soil, waste or drinking water, bodily fluids, minerals, chemical compounds) is analyzed for its elemental and sometimes isotopic composition. Elemental analysis can be qualita ...

and

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

determination by

titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...

to determine the molecular formula of confluentinic acid as C₂₈H₃₆O₈. The Zeisel determination for

methoxy In organic chemistry, a methoxy group is the functional group consisting of a methyl group bound to oxygen. This alkoxy group has the formula . On a benzene ring, the Hammett equation classifies a methoxy substituent at the ''para'' position as ...

l group analysis indicated two methoxyl groups per molecule. John Elix and Brian Ferguson's proposal for the

total synthesis Total synthesis is the complete chemical synthesis of a complex molecule, often a natural product, from simple, commercially-available precursors. It usually refers to a process not involving the aid of biological processes, which distinguishes ...

of confluentic acid in 1978 marked a significant advancement in understanding of this lichen substance, enabling scientists to better study and understand the compound's structure and biological activity without relying solely on natural extraction. The synthesis began with the direct

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

of suitably substituted aromatic carboxylic acids and phenols, using

dicyclohexylcarbodiimide ''N'',''N''′-Dicyclohexylcarbodiimide (DCC or DCCD) is an organic compound with the chemical formula (C6H11N)2C. It is a waxy white solid with a sweet odor. Its primary use is to couple amino acids during artificial peptide synthesis. The low ...

. Key precursors involved were specially prepared

benzoic acid Benzoic acid is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, wh ...

s, with protective measures for reactive groups. The process included steps like

bromination In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, ...

alkylation Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting ...

, and the strategic use of

protecting group A protecting group or protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis. In many ...

s for the phenol and

carboxyl In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

functionalities. The synthesis culminated in the removal of protecting groups and

hydrogenolysis Hydrogenolysis is a chemical reaction whereby a carbon–carbon or carbon–heteroatom single bond is cleaved or undergoes lysis (breakdown) by hydrogen.Ralph Connor, Homer Adkins. Hydrogenolysis Of Oxygenated Organic Compounds. J. Am. Chem. Soc. ...

over palladised carbon to yield the desired depsides including confluentic acid. In 1993, G. Fegie and colleagues introduced a standardised high-performance liquid chromatographic method that enabled the separation and detection of hundreds of lichen products, confluentic acid included.

Properties

Confluentic acid is a member of the class of chemical compounds called

s. Its

IUPAC name In chemical nomenclature, a preferred IUPAC name (PIN) is a unique name, assigned to a chemical substance and preferred among the possible names generated by IUPAC nomenclature. The "preferred IUPAC nomenclature" provides a set of rules for cho ...

is 4- -hydroxy-4-methoxy-6-(2-oxoheptyl)benzoylxy-2-methoxy-6-pentylbenzoic acid. The

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

absorbance maxima (λ_max) has two peaks at 268 and 304 nm. In the

infrared spectrum Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

, significant peaks indicative of the carboxylic acid

functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...

occur at 1700 cm^-1 (C=O stretching in

carbonyl group In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...

s) and within the broad range of 2600 to 3100 cm^-1 (O-H stretching). The broad band at 3100 is due to

hydrogen bonding In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...

, while the peak at 3500 is the COOH stretching band. Confluentic acid's

is C₂₈H₃₆O₈; it has a

molecular mass The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quanti ...

of 500.57 grams per mole. In its purified form, it exists as crystalline needles with a

of .

Occurrence

The (fungal partner) of the lichen '' Lecidea tessellata'' has been shown to produce confluentic acid when cultured without its algal partner. Confluentic acid has also been reported from mycobiont cultures of '' Parmelina carporrhizans''. Confluentic acid is produced by almost all species of the genus '' Immersaria'', which is usually accompanied by 2'-''O''-methylmicrophyllinic acid. The absence of confluentic acid distinguishes '' Inoderma nipponicum'', from others in genus '' Inoderma'', which typically contain this chemical. The only character reliably distinguishing '' Porpidia contraponenda'' and the morphologically similar '' Porpidia cinereoatra'' is their secondary chemistry: the former contains 2'-''O''-methylmicrophyllinate and the latter has confluentic acid. A is a set of biosynthetically related compounds produced by a lichen. The confluentic acid chemosyndrome was identified in several lichens in the family

Lecideaceae The Lecideaceae are a family of lichens in the order Lecideales (Ascomycota, class Lecanoromycetes). Genera According to a recent (2022) estimate, the Lecideaceae comprise 29 genera and about 260 species. The following list indicates the genus n ...

; it contains confluentic acid as the major metabolite, and minor amounts of 2'-''O''-methylperlatolic acid, olivetonide monomethyl ether, and 2'-''O''-methylmicrophyllinic acid. Not just limited to lichen-forming fungi, confluentic acid has also been reported from the Brazilian plant '' Himatanthus sucuuba'', highlighting the compound's broader biological distribution. Christmas Lichen - Flickr - pellaea

A study on ''Cryptothecia rubrocincta'' reveals distinct biochemical compositions in various parts of its thallus, suggesting specialised roles for the compounds present. Specifically, confluentic acid was found exclusively in localised brown flecks within the red and pink zones of the thallus, alongside

calcium oxalate Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalic acid with the chemical formula . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydrate ...

monohydrate. This distribution is in contrast to other thallus areas, such as the white zone containing only calcium oxalate dihydrate and the dark red zone with chiodectonic acid,

chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...

, beta-carotene, and additional calcium oxalate dihydrate in the pink sub-zone. The presence of confluentic acid in specific areas without beta-carotene and chiodectonic acid—both known UV protectants—suggests that confluentic acid plays a different role in the lichen's survival strategy. While the exact function of confluentic acid in these localised brown flecks remains unclear, it is indicated that it is not required for radiation protection. The study also highlights a transition within the lichen from calcium oxalate dihydrate to the more stable monohydrate form, associated with the ageing process and possibly the metabolic activities involving confluentic acid.

Detection

Alan Fryday (1991) outlined a technique for the detection of confluentic acid in lichen samples. This method involves placing a

section Section, Sectioning or Sectioned may refer to: Arts, entertainment and media * Section (music), a complete, but not independent, musical idea * Section (typography), a subdivision, especially of a chapter, in books and documents ** Section sign ...

of the lichen's

(fruiting body) on a microscope slide, which is then saturated with a 10% potassium hydroxide (KOH) solution. When examined under a

compound microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsco ...

at 40x magnification, a distinctive 'halo' of small oil droplets or bubbles emanating from the tissue section indicates the presence of confluentic acid. The oil droplets generated during this detection process consist of 4-''O''-methylolivetonide, a compound that is insoluble in potassium hydroxide solution. This substance forms as a result of confluentic acid undergoing

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

in the presence of potassium hydroxide. This test is particularly useful in distinguishing between morphologically similar yet chemically distinct species within the genus ''

'', aiding accurate identification and study.

Related compounds

The chemical diversity within lichens includes a variety of compounds related to confluentic acid, reflecting the complex biosynthetic capabilities of these symbiotic organisms and their significance in lichen taxonomy and ecology. In 1987,

Chicita Culberson Chicita Frances Culberson (born Chicita Frances Forman, November 1, 1931 in Philadelphia, Pennsylvania) is an American lichenologist. Education She graduated with a B.S. from the University of Cincinnati in 1953, where she also met her future h ...

and colleagues reported the use of high-performance liquid chromatography to isolate and identify additional higher-carbon analogue substances in the "confluentic series", including hyperconfluentic acid, superconfluentic acid, and subconfluentic acid. These substances were isolated from the lichen '' Pseudobaeomyces pachycarpa''. The structure of subconfluentic acid (4- '-hydroxy-4'-methoxy-6'-(2"-oxopentyl)benzoyloxy2-methoxy-6-pentylbenzoic acid) was later established by synthesis. The compound 4-''O''-demethylsuperconfluentic acid, structurally similar to confluentic acid, was isolated from '' Stirtonia ramosa''. Another analogue, 2-''O''-methylconfluentic acid, was identified from ''

Lecidea fuscoatra ''Lecidea'' is a genus of crustose lichens with a carbon black ring or outer margin (exciple) around the fruiting body disc (apothecium), usually (or always) found growing on (saxicolous) or in (endolithic) rock.Field Guide to California Lichens, ...

''. Gowan (1989) suggested a close chemical and biosynthetic relationship between methyl 2'-''O''-methylmicrophyllinate and confluentic acid, noting that the

biosynthetic pathway Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

s leading to these compounds primarily differ in the length of the acetyl-polymalonyl segment. This means that the two compounds are synthesised through similar processes, differing mainly in the size of a specific chain within the molecule. Additionally, there is only a minor variation in their methylation patterns. Gowan further suggested that methyl 2'-''O''-methylmicrophyllinate likely originated from an ancestor that already produced confluentic acid.

Notes

References

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