Alkylresorcinols (ARs), also known as resorcinolic lipids, are amphiphilic

phenolic lipid Phenolic lipids are a class of natural products composed of long aliphatic chains and phenolic rings. Phenolic lipids occur in plants, fungi and bacteria. Types * Alkylcatechols * Alkylphenols (nonylphenol, cardanol) * Alkylresorcinols * Anaca ...

s characterised by a non-polar odd-numbered alkyl side chain with up to 27 carbon atoms attached to a polar

resorcinol Resorcinol (or resorcin) is a phenolic compound. It is an organic compound with the formula C6H4(OH)2. It is one of three isomeric benzenediols, the 1,3-isomer (or ''meta- (chemistry), meta''-isomer). Resorcinol crystallizes from benzene as co ...

(1,3-dihydroxybenzene) ring.

Natural sources of alkylresorcinols

Alkylresorcinols are relatively rare in nature and are reported to be found in fungi, bacteria, and some lower and higher plants.

DB-2073 DB-2073 is an alkylresorcinol antibiotic isolated from the broth culture of ''Pseudomonas ''Pseudomonas'' is a genus of Gram-negative bacteria belonging to the family Pseudomonadaceae in the class Gammaproteobacteria. The 348 members of the ge ...

is an antibiotic isolated from the broth culture of ''

Pseudomonas ''Pseudomonas'' is a genus of Gram-negative bacteria belonging to the family Pseudomonadaceae in the class Gammaproteobacteria. The 348 members of the genus demonstrate a great deal of metabolic diversity and consequently are able to colonize a ...

'' sp. They are also the main constituents of the outer shell of the cyst of ''

Azotobacter ''Azotobacter'' is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts (and also has hard crust) and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes that play an impo ...

''. Among the plant sources, the shell oil of cashew nut ( ''Anacardium occidentale'' L.) has the highest amount of ARs, which consists of 20% phenolic lipids. Moreover, ARs were found in the peels and pulp of peas ( ''Pisum sativum'' L.), pulp and leaves of ginkgo ( ''Ginkgo biloba'' L.), pulp and peels of mango ( ''Mangifera indica'' L.), and in some cereals. In the case of cereals, the hyaline layer, inner pericarp, and testa showed the highest amounts of AR. DB-2073

Occurrence in cereals

The alkylresorcinols alkyl chain, present in cereals, ranges from 15 to 25 carbon atoms. ARs have been reported to be present in high amounts in

rye Rye (''Secale cereale'') is a grass grown extensively as a grain, a cover crop and a forage crop. It is grown principally in an area from Eastern and Northern Europe into Russia. It is much more tolerant of cold weather and poor soil than o ...

wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...

, and

triticale Triticale (; × ''Triticosecale'') is a hybrid of wheat (''Triticum'') and rye (''Secale'') first bred in laboratories during the late 19th century in Scotland and Germany. Commercially available triticale is almost always a second-generation ...

, and in low concentrations in

barley Barley (), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains; it was domesticated in the Fertile Crescent around 9000 BC, giving it nonshattering spikele ...

maize Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout grass that produces cereal grain. It was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. Native American ...

oat The oat (''Avena sativa''), sometimes called the common oat, is a species of cereal grain grown for its seed, which is known by the same name (usually in the plural). Oats appear to have been domesticated as a secondary crop, as their seeds ...

, and

millet Millets () are a highly varied group of small-seeded grasses, widely grown around the world as cereal crops or grains for fodder and human food. Most millets belong to the tribe Paniceae. Millets are important crops in the Semi-arid climate, ...

, while no information is at present available for

Khorasan wheat Khorasan wheat or Oriental wheat (''Triticum turgidum'' ssp. ''turanicum'' also called ''Triticum turanicum'') is a tetraploid wheat species. The grain is twice the size of modern-day wheat, and has a rich, nutty flavor. Taxonomy Original bo ...

. They are most abundant in the bran fractions (2600-4100 μg/g; 0.1-0.3% of dry weight), whereas they are in trace amounts in strachy

endosperm The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the Embryo#Pla ...

and germ. They can also be found in

rice Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...

, though not in the edible parts of the rice plant. Their presence in the

(the part of

cereal A cereal is a grass cultivated for its edible grain. Cereals are the world's largest crops, and are therefore staple foods. They include rice, wheat, rye, oats, barley, millet, and maize ( Corn). Edible grains from other plant families, ...

grain that is used to make white

flour Flour is a powder made by Mill (grinding), grinding raw grains, List of root vegetables, roots, beans, Nut (fruit), nuts, or seeds. Flours are used to make many different foods. Cereal flour, particularly wheat flour, is the main ingredie ...

), means that alkylresorcinols can be used as 'biomarkers' for people who eat foods containing wholegrain wheat and rye, rather than cereal products based on white flour. Moreover, they were thought to have anti-nutritive properties (e.g. decreasing growth of pigs and chickens fed rye), but this theory has been discredited, and a number of animal studies have demonstrated that they have no obvious negative effect on animals or humans.

Biomarkers of a whole grain diet

Increasing evidence from human intervention trials suggests that they are the most promising biomarker of

whole grain A whole grain is a grain of any cereal and pseudocereal that contains the endosperm, germ, and bran, in contrast to refined grains, which retain only the endosperm. As part of a general healthy diet, consumption of whole grains is associated ...

wheat and rye intake. Alkylresorcinol metabolites, 3,5-dihydroxybenzoic acid (DHBA) and 3,5-dihydroxyphenylpropionoic acid (DHPPA) were first identified in urine and can be quantified in urine and plasma, and may be an alternative, equivalent biomarker of whole grain wheat intake. The average intake of alkylresorcinols in the UK is around 11 mg/person/day, and in Sweden is around 20 mg/person/day. This varies widely depending on whether people normally consume wholegrain/wholemeal/brown bread, which is high in alkylresorcinols (300-1000 μg/g), or white wheat bread, which has very low concentrations of alkylresorcinols (<50 μg/g).

Biomarkers of cereal presence in archaeological pottery

Recently, alkylresorcinols have been widely recognised as a biomarker for the presence of cereals in archaeological pottery. They were previously found in a well-preserved Bronze Age wooden container from Switzerland, and coarse ware vessels from a Roman cavalry barrack at

Vindolanda Vindolanda was a Roman auxiliary fort ('' castrum'') just south of Hadrian's Wall in northern England, which it pre-dated. Archaeological excavations of the site show it was under Roman occupation from roughly 85 AD to 370 AD. Located near th ...

. A study demonstrated that the survival of ARs is highly dependent on the cooking procedures and burial conditions. However, if recoverable, analysis of these phenolic lipids in archaeological contexts is valuable as it can help explain the uptake and spread of cereal processing of past communities in particular regions.

Possible biological activities

In vitro ''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...

studies have shown that alkylresorcinols may prevent cells turning

cancerous Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...

, but that they do not have any effect on cells that are already cancerous. Alkylresorcinols also increase gamma-

tocopherol Tocopherols (; TCP) are a class of organic compounds comprising various methylated phenols, many of which have vitamin E activity. Because the vitamin activity was first identified in 1936 from a dietary fertility factor in rats, it was named ''t ...

levels in rats when fed in high amounts (0.2% of total diet and above). The alkylresorcinols in '' Grevillea banksii'' and Grevillea 'Robyn Gordon' are responsible for

contact dermatitis Contact dermatitis is a type of acute or chronic inflammation of the skin caused by exposure to chemical or physical agents. Symptoms of contact dermatitis can include itchy or dry skin, a red rash, bumps, blisters, or swelling. These rashes ...

Trivial names of some resorcinolic lipids

olivetol Olivetol is an organic compound with the formula . Several isomers exist; olivetol has the two hydroxyl and the pentyl groups located at the 1,3, and 5 positions on the benzene ring. It is a colorless solid that is soluble in a variety of organi ...

* persoonol * grevillol * ardisinol I * ardisinol II * adipostatin A * adipostatin B *

bilobol Bilobol is an alkylresorcinol from ''Ginkgo biloba''. Chemically, it is similar in structure to urushiol, the irritant found in poison ivy; it is a strong skin irritant itself. Natural occurrences Bilobol can be found in ''Ginkgo biloba ...

hexylresorcinol 4-Hexylresorcinol is an organic compound with local anaesthetic, antiseptic, and anthelmintic properties. As an antiseptic, it is marketed as S.T.37 by Numark Laboratories, Inc. (in a 0.1% solution) for oral pain relief and as a topical antisep ...

* hydrobilobol * cardol * rucinol * irisresorcinol * panosialin * stemphol * R- leprosol * α- leprosol * merulinic acid * xenognosin

Derivatives

Sorgoleone is a hydrophobic root exudate of ''

Sorghum bicolor ''Sorghum bicolor'', commonly called sorghum () and also known as great millet, broomcorn, guinea corn, durra, imphee, jowar, or milo, is a species in the grass genus ''Sorghum'' cultivated for its grain. The grain is used as food by humans, ...

''.

References

{{Reflist