alkaloid

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(ăl'kə-loid') pronunciation
n.
Any of various organic compounds normally with basic chemical properties and usually containing at least one nitrogen atom in a heterocyclic ring, occurring chiefly in many vascular plants and some fungi. Many alkaloids, such as nicotine, quinine, cocaine, and morphine, are known for their poisonous or medicinal attributes.

[ALKAL(I) + -OID.]

alkaloidal al'ka·loi'dal (-loid'l) adj.


Basic ( base) organic compounds of plant origin, containing combined nitrogen. Alkaloids are amines, so their names usually end in ine (e.g., caffeine, nicotine, morphine, quinine). Most have complex chemical structures of multiple ring systems. They have diverse, important physiological effects on humans and other animals, but their functions in the plants that produce them are poorly understood. Some plants (e.g., opium poppy, ergot fungus) produce many different alkaloids, but most produce only one or a few. Certain plant families, including the poppy family (Papaveraceae) and the nightshade family (Solanaceae), are particularly rich in them. Alkaloids are extracted by dissolving the plant in dilute acid.

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A cyclic organic compound that contains nitrogen in a negative oxidation state and is of limited distribution among living organisms. Over 10,000 alkaloids of many different structural types are known; and no other class of natural products possesses such an enormous variety of structures. Therefore, alkaloids are difficult to differentiate from other types of organic nitrogen-containing compounds.

Simple low-molecular-weight derivatives of ammonia, as well as polyamines and acyclic amides, are not considered alkaloids because they lack a cyclic structure in some part of the molecule. Amines, amine oxides, amides, and quaternary ammonium salts are included in the alkaloid group because their nitrogen is in a negative oxidation state (the oxidation state designates the positive or negative character of atoms in a molecule). Nitro and nitroso compounds are excluded as alkaloids. The almost-ubiquitous nitrogenous compounds, such as amino acids, amino sugars, peptides, proteins, nucleic acids, nucleotides, prophyrins, and vitamins, are not alkaloids. However, compounds that are exceptions to the classical-type definition (that is, a compound containing nitrogen, usually a cyclic amine, and occurring as a secondary metabolite), such as neutral alkaloids (colchicine, piperine), the β-phenyl-ethylanines, and the purine bases (caffeine, theophylline, theobromine), are accepted as alkaloids.

Alkaloids often occur as salts of plant acids such as malic, meconic, and quinic acids. Some plant alkaloids are combined with sugars, for example, solanine in potato (Solanum tuberosum) and tomatine in tomato (Lycopersicum esculentum). Others occur as amides, for example, piperine from black pepper (Piper nigrum), or as esters, for example, cocaine from coca leaves (Erythroxylum coca). Still other alkaloids occur as quaternary salts or tertiary amine oxides.

While most alkaloids have been isolated from plants, a large number have been isolated from animal sources. They occur in mammals, anurans (frogs, toads), salamanders, arthropods (ants, millipedes, ladybugs, beetles, butterflies), marine organisms, mosses, fungi, and certain bacteria.

Many alkaloids exhibit marked pharmacological activity, and some find important uses in medicine. Atropine, the optically inactive form of hyoscyamine, is used widely in medicine as an antidote to cholinesterase inhibitors such as physostigmine and insecticides of the organophosphate type; it is also used in drying cough secretions. Morphine and codeine are narcotic analgesics, and codeine is also an antitussive agent, less toxic and less habit-forming than morphine. Colchicine, from the corms and seeds of the autumn crocus, is used as a gout suppressant. Caffeine, which occurs in coffee, tea, cocoa, and cola, is a central nervous system stimulant; it is used as a cardiac and respiratory stimulant and as an antidote to barbiturate and morphine poisoning. Emetine, the key alkaloid of ipecac root (Cephaelis ipecacuanha), is used in the treatment of amebic dysentery and other protozoal infections. Epinephrine or adrenaline (see structure), produced in most animal species by the adrenal medulla, is used as a

bronchodilator and cardiac stimulant and to counter allergic reactions, anesthesia, and cardiac arrest.


Naturally occurring organic bases which have marked pharmacological actions in man and other animals. Many are found in plant foods, including potatoes and tomatoes (the Solanum alkaloids), or as the products of fungal action (e.g. ergot), although they also occur in animal foods (e.g. tetrodotoxin in puffer fish, tetramine in shellfish).


A group of nitrogen-containing substances that are produced by plants and can have potent effects on body functions. Many alkaloids are medicinally important drugs, including morphine, quinine, and atropine.

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A member of a group of nitrogen-containing compounds that include cocaine, morphine, and nicotine.

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alkaloid, any of a class of organic compounds composed of carbon, hydrogen, nitrogen, and usually oxygen that are often derived from plants. Although the name means alkalilike, some alkaloids do not exhibit alkaline properties. Many alkaloids, though poisons, have physiological effects that render them valuable as medicines. For example, curarine, found in the deadly extract curare, is a powerful muscle relaxant; atropine is used to dilate the pupils of the eyes; and physostigmine is a specific for certain muscular diseases. Narcotic alkaloids used in medicine include morphine and codeine for the relief of pain and cocaine as a local anesthetic. Other common alkaloids include quinine, caffeine, nicotine, strychnine, serotonin, and LSD. Aconitine is the alkaloid of aconite. Cinchonine and quinine are derived from cinchona, coniine is found in poison hemlock, and reserpine is an extract of rauwolfia roots. Emetine is an alkaloid of ipecac.



A plant compound containing nitrogen that, possibly because of its bitter taste, defends plants against predators. Alkaloids also include plant poisons or medicines. Nicotine, cocaine, and quinine are examples of plant alkaloids.


GRAS ingredients that fall into this category, like caffeine and theo-bromine, must be used sparingly. By definition, alkaloids are very complex compounds that can have a biological effect on the user. The term alkaloid derived from alkali-like refers specifically to the presence of nitrogen in the molecule. Theobromine (demethylated caffeine) is found in chocolate. Caffeine (methyl theobromine) is found in many items including chocolate, coffee, tea, and some common extracts. Some alkaloids are highly toxic. See Caffeine, Theobromine, Chart 21.


any member of a broad group of nitrogen-containing basic organic compounds present in various dicotyledonous plants and some fungi. Although only about 5% of the world's plant species have so far been examined, they have yielded over 2000 different alkaloids. Heterocyclic alkaloids derived from amino acids are termed true alkaloids. Alkaloids with and without heterocyclic rings and not derived from amino acids are termed pseudoalkaloids; in these the carbon skeleton is usually isoprenoid derived. Alkaloids often have marked and specific pharmacological activity.

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One of a large group of small organic compounds, mainly derived from amino acids, and containing nitrogen, found in plants. They are water-soluble, usually bitter in taste and are characterized by powerful physiological activity. Examples are morphine, cocaine, atropine, quinine, nicotine and caffeine. The term is also applied to synthetic substances that have structures similar to plant alkaloids, such as procaine. When treated with acids they are converted to water-soluble salts. In cases of poisoning by alkaloids the recommended antidote is tannic acid, but heavy metal salts and iodine also precipitate them. Includes pyrrolizidine and solanaceous alkaloids.

(al′kəloid)
n

Any one of the many nitrogen-containing organic bases derived from plants. The alkaloids are bitter and physiologically active. A number are useful therapeutic agents.

The first individual alkaloid, morphine, was isolated in 1804 from poppy (Papaver somniferum).[1]

Alkaloids are a group of naturally occurring chemical compounds that contain mostly basic nitrogen atoms. This group also includes some related compounds with neutral[2] and even weakly acidic properties.[3] Also some synthetic compounds of similar structure are attributed to alkaloids.[4] In addition to carbon, hydrogen and nitrogen, alkaloids may also contain oxygen, sulfur and more rarely other elements such as chlorine, bromine, and phosphorus.[5]

Alkaloids are produced by a large variety of organisms, including bacteria, fungi, plants, and animals, and are part of the group of natural products (also called secondary metabolites). Many alkaloids can be purified from crude extracts by acid-base extraction. Many alkaloids are toxic to other organisms. They often have pharmacological effects and are used as medications, as recreational drugs, or in entheogenic rituals. Examples are the local anesthetic and stimulant cocaine; the psychedelic psilocin; the stimulant caffeine; nicotine;[6] the analgesic morphine; the antibacterial berberine; the anticancer compound vincristine; the antihypertension agent reserpine; the cholinomimeric galatamine; the spasmolysis agent atropine; the vasodilator vincamine; the anti-arhythmia compound quinidine; the anti-asthma therapeutic ephedrine; and the antimalarial drug quinine. Although alkaloids act on a diversity of metabolic systems in humans and other animals, they almost uniformly invoke a bitter taste.[7]

The boundary between alkaloids and other nitrogen-containing natural compounds is not clear-cut.[8] Compounds like amino acid peptides, proteins, nucleotides, nucleic acid, amines, and antibiotics are usually not called alkaloids.[2] Natural compounds containing nitrogen in the exocyclic position (mescaline, serotonin, dopamine, etc.) are usually attributed to amines rather than alkaloids.[9] Some authors, however, consider alkaloids a special case of amines.[10][11][12]

Contents

Naming

The article that introduced the concept of "alkaloid".

The name "alkaloids" (German: Alkaloide) was introduced in 1819 by the German chemist Carl F.W. Meissner,[13] and is derived from late Latin root Latin: alkali (which, in turn, comes from the Arabic al-qalwī – "ashes of plants") and the suffix Greek: -οειδής – "like".[nb 1] However, the term came into wide use only after the publication of a review article by O. Jacobsen in the chemical dictionary of Albert Ladenburg in the 1880s.[14]

There is no unique method of naming alkaloids.[15] Many individual names are formed by adding the suffix "ine" to the species or genus name.[16] For example, atropine is isolated from the plant Atropa belladonna, strychnine is obtained from the seed of Strychnine tree (Strychnos nux-vomica L.).[5] If several alkaloids are extracted from one plant then their names often contain suffixes "idine", "anine", "aline", "inine", etc. There are also at least 86 alkaloids containing the root "vin" (extracted from the Vinca plant).[17]

History

Friedrich Sertürner, the German chemist who first isolated morphine from opium.

Alkaloid-containing plants have been used by humans since ancient times for therapeutic and recreational purposes. For example, medicinal plants have been known in the Mesopotamia at least around 2000 BC.[18] The Odyssey of Homer referred to a gift given to Helen by the Egyptian queen, a drug bringing oblivion. It is believed that the gift was an opium-containing drug.[19] A Chinese book on houseplants written in 1st–3rd centuries BC mentioned a medical use of Ephedra and opium poppies.[20] Also, coca leaves have been used by South American Indians since ancient times.[21]

Extracts from plants containing toxic alkaloids, such as aconitine and tubocurarine, were used since antiquity for poisoning arrows.[18]

Studies of alkaloids began in the 19th century. In 1804, the German chemist Friedrich Sertürner isolated from opium a "soporific principle" (Latin: principium somniferum), which he called "morphium" in honor of Morpheus, the Greek god of dreams; in German and some other Central-European languages, this is still the name of the drug. The term "morphine", used in English and French, was given by the French physicist Joseph Louis Gay-Lussac).

A significant contribution to the chemistry of alkaloids in the early years of its development was made by the French researchers Pierre Joseph Pelletier and Joseph Bienaimé Caventou, who discovered quinine (1820) and strychnine (1818). Several other alkaloids were discovered around that time, including xanthine (1817), atropine (1819), caffeine (1820), coniine (1827), nicotine (1828), colchicine (1833), sparteine (1851), and cocaine (1860).[22]

The first complete synthesis of an alkaloid was achieved in 1886 by the German chemist Albert Ladenburg. He produced coniine by reacting 2-methylpyridine with acetaldehyde and reducing the resulting 2-propenyl pyridine with sodium.[23][24] The development of the chemistry of alkaloids was accelerated by the emergence of spectroscopic and chromatographic methods in the 20th century, so that by 2008 more than 12,000 alkaloids had been identified.[25]

Bufotenin, an alkaloid from some toads, contains an indole core and is produced in living organisms from the amino acid tryptophan.

Classification

The nicotine molecule contains both pyridine (left) and pyrrolidine rings (right).

Compared with most other classes of natural compounds, alkaloids are characterized by a great structural diversity and there is no uniform classification of alkaloids.[26] First classification methods have historically combined alkaloids by the common natural source, e.g., a certain type of plants. This classification was justified by the lack of knowledge about the chemical structure of alkaloids and is now considered obsolete.[5][27]

More recent classifications are based on similarity of the carbon skeleton (e.g., indole-, isoquinoline-, and pyridine-like) or biogenetic precursor (ornithine, lysine, tyrosine, tryptophan, etc.).[5] However, they require compromises in borderline cases;[26] for example, nicotine contains a pyridine fragment from nicotinamide and pyrrolidine part from ornithine[28] and therefore can be assigned to both classes.[29]

Alkaloids are often divided into the following major groups:[30]

  1. "True alkaloids", which contain nitrogen in the heterocycle and originate from amino acids.[31] Their characteristic examples are atropine, nicotine, and morphine. This group also includes some alkaloids that besides nitrogen heterocycle contain terpene (e.g., evonine[32]) or peptide fragments (e.g. ergotamine[33]). This group also includes piperidine alkaloids coniine and coniceine[34] although they do not originate from amino acids.[35]
  2. "Protoalkaloids", which contain nitrogen and also originate from amino acids.[31] Examples include mescaline, adrenaline and ephedrine.
  3. Polyamine alkaloids – derivatives of putrescine, spermidine, and spermine.
  4. Peptide and cyclopeptide alkaloids.[36]
  5. Pseudalkaloids – alkaloid-like compounds that do not originate from amino acids.[37] This group includes, terpene-like and steroid-like alkaloids,[38] as well as purine-like alkaloids such as caffeine, theobromine and theophylline.[39] Some authors classify as pseudoalkaloids such compounds such as ephedrine and cathinone. Those originate from the amino acid phenylalanine, but acquire their nitrogen atom not from the amino acid but through transamination.[39][40]

Some alkaloids do not have the carbon skeleton characteristic of their group. So, galantamine and homoaporphines do not contain isoquinoline fragment, but are, in general, attributed to isoquinoline alkaloids.[41]

Main classes of monomeric alkaloids are listed in the table below:

Class Major groups Main synthesis steps Examples
Alkaloids with nitrogen heterocycles (true alkaloids)
Pyrrolidine derivatives[42]
Pyrrolidine structure.svg
Ornithine or arginineputrescine → N-methylputrescine → N-methyl-Δ1-pyrroline [43] Cuscohygrine, hygrine, hygroline, stachydrine[42][44]
Tropane derivatives[45]
Tropane numbered.svg
Atropine group
Substitution in positions 3, 6 or 7
Ornithine or arginineputrescine → N-methylputrescine → N-methyl-Δ1-pyrroline [43] Atropine, scopolamine, hyoscyamine[42][45][46]
Cocaine group
Substitution in positions 2 and 3
Cocaine, ecgonine [45][47]
Pyrrolizidine derivatives[48]
Pyrrolizidine.svg
Non-esters In plants: ornithine or arginineputrescine → homospermidine → retronecine [43] Retronecine, heliotridine, laburnine [48][49]
Complex esters of monocarboxylic acids Indicine, lindelophin, sarracine [48]
Macrocyclic diesters Platyphylline, trichodesmine[48]
1-aminopyrrolizidines (lolines) In fungi: L-proline + L-homoserineN-(3-amino-3-carboxypropyl)proline → norloline[50][51] Loline, N-formylloline, N-acetylloline[52]
Piperidine derivatives[53]
Piperidin.svg
Lysinecadaverine → Δ1-piperideine [54] Sedamine, lobeline, anaferine, piperine [34][55]
Octanoic acid → coniceine → coniine [35] Coniine, coniceine [35]
Quinolizidine derivatives[56][57]
Quinolizidine.svg
Lupinine group Lysinecadaverine → Δ1-piperideine [58] Lupinine, nupharidin [56]
Cytisine group Cytisine [56]
Sparteine group Sparteine, lupanine, anahygrine[56]
Matrine group Matrine, oxymatrine, allomatridine[56][59][60]
Ormosanine group Ormosanine, piptantine[56][61]
Indolizidine derivatives[62]
Indolizidine.svg
Lysine → δ-semialdehyde of α-aminoadipic acidpipecolic acid → 1 indolizidinone [63] Swainsonine, castanospermine [64]
Pyridine derivatives[65][66]
Pyridine.svg
Simple derivatives of pyridine Nicotinic acid → dihydronicotinic acid → 1,2-dihydropyridine [67] Trigonelline, ricinine, arecoline [65][68]
Polycyclic noncondensing pyridine derivatives Nicotine, nornicotine, anabasine, anatabine [65][68]
Polycyclic condensed pyridine derivatives Actinidine, gentianine, pediculinine [69]
Sesquiterpene pyridine derivatives Nicotinic acid, isoleucine [12] Evonine, hippocrateine, triptonine [66][67]
Isoquinoline derivatives and related alkaloids [70]
Isoquinoline numbered.svg
Simple derivatives of isoquinoline [71] Tyrosine or phenylalaninedopamine or tyramine (for alkaloids Amarillis) [72][73] Salsoline, lophocerine [70][71]
Derivatives of 1- and 3-isoquinolines [74] N-methylcoridaldine, noroxyhydrastinine [74]
Derivatives of 1- and 4-phenyltetrahydroisoquinolines [71] Cryptostilin [71][75]
Derivatives of 5-naftil-isoquinoline [76] Ancistrocladine [76]
Derivatives of 1- and 2-benzyl-izoquinolines [77] Papaverine, laudanosine, sendaverine
Cularine group[78] Cularine, yagonine [78]
Pavines and isopavines [79] Argemonine, amurensin [79]
Benzopyrrocolines [80] Cryptaustoline [71]
Protoberberines [71] Berberine, canadine, ophiocarpine, mecambridine, corydaline [81]
Phthalidisoquinolines [71] Hydrastine, narcotine (Noscapine) [82]
Spirobenzylisoquinolines [71] Fumaricine [79]
Ipecacuanha alkaloids[83] Emetine, protoemetine, ipecoside [83]
Benzophenanthridines [71] Sanguinarine, oxynitidine, corynoloxine [84]
Aporphines [71] Glaucine, coridine, liriodenine [85]
Proaporphines [71] Pronuciferine, glaziovine [71][80]
Homoaporphines [86] Kreysiginine, multifloramine [86]
Homoproaporphines [86] Bulbocodine [78]
Morphines[87] Morphine, codeine, thebaine, sinomenine [88]
Homomorphines [89] Kreysiginine, androcymbine [87]
Tropoloisoquinolines [71] Imerubrine [71]
Azofluoranthenes [71] Rufescine, imeluteine [90]
Amaryllis alkaloids[91] Lycorine, ambelline, tazettine, galantamine, montanine [92]
Erythrite alkaloids[75] Erysodine, erythroidine [75]
Phenanthrene derivatives [71] Atherosperminine [71][81]
Protopins [71] Protopine, oxomuramine, corycavidine [84]
Aristolactam [71] Doriflavin [71]
Oxazole derivatives[93]
Oxazole structure.svg
Tyrosinetyramine [94] Annuloline, halfordinol, texaline, texamine[95]
Isoxazole derivatives
Isoxazole structure.png
Ibotenic acidMuscimol Ibotenic acid, Muscimol
Thiazole derivatives[96]
Thiazole structure.svg
1-Deoxy-D-xylulose 5-phosphate (DOXP), tyrosine, cysteine [97] Nostocyclamide, thiostreptone [96][98]
Quinazoline derivatives[99]
Quinazoline numbered.svg
3,4-Dihydro-4-quinazolone derivatives Anthranilic acid or phenylalanine or ornithine [100] Febrifugine[101]
1,4-Dihydro-4-quinazolone derivatives Glycorine, arborine, glycosminine[101]
Pyrrolidine and piperidine quinazoline derivatives Vazicine (peganine) [93]
Acridine derivatives[93]
Acridine.svg
Anthranilic acid [102] Rutacridone, acronicine[103][104]
Quinoline derivatives[105][106]
Quinoline numbered.svg
Simple derivatives of quinoline derivatives of 2 – quinolones and 4-quinolone Anthranilic acid → 3-carboxyquinoline [107] Cusparine, echinopsine, evocarpine[106][108][109]
Tricyclic terpenoids Flindersine[106][110]
Furanoquinoline derivatives Dictamnine, fagarine, skimmianine[106][111][112]
Quinines Tryptophantryptamine → strictosidine (with secologanin) → korinanteal → cinhoninon [73][107] Quinine quinidine cinchonine, cinhonidine [110]
Indole derivatives[88]
Indole numbered.svg
Non-isoprene indole alkaloids
Simple indole derivatives [113] Tryptophantryptamine or 5-hydroxitriptofan [114] Serotonin, psilocybin, dimethyltryptamine (DMT), bufotenin [115][116]
Simple derivatives of β-carboline [117] Harman, harmine, harmaline, eleagnine [113]
Pyrroloindole alkaloids [118] Physostigmine (eserine), etheramine, physovenine, eptastigmine[118]
Semiterpenoid indole alkaloids
Ergot alkaloids[88] Tryptophan → chanoclavine → agroclavine → elimoclavine → paspalic acid → lysergic acid [118] Ergotamine, ergobasine, ergosine[119]
Monoterpenoid indole alkaloids
Corynanthe type alkaloids[114] Tryptophantryptamine → strictosidine (with secologanin) [114] Ajmalicine, sarpagine, vobasine, ajmaline, yohimbine, reserpine, mitragynine,[120][121] group strychnine and (Strychnine brucine, aquamicine, vomicine [122])
Iboga-type alkaloids[114] Ibogamine, ibogaine, voacangine[114]
Aspidosperma-type alkaloids[114] Vincamine, vincotine, aspidospermine[123][124]
Imidazole derivatives[93]
Imidazole structure.svg
Directly from histidine[125] Histamine, pilocarpine, pilosine, stevensine[93][125]
Purine derivatives[126]
9H-Purine.svg
Xanthosine (formed in purine biosynthesis) → 7 methylxantosine → 7-methyl xanthinetheobrominecaffeine [73] Caffeine theobromine theophylline saxitoxin [127][128]
Alkaloids with nitrogen in the side chain (protoalkaloids)
β-Phenylethylamine derivatives[80]
Phenylethylamine numbered.svg
Tyrosine or phenylalanine → dioxyphenilalanine → dopamineadrenaline and mescaline tyrosinetyramine phenylalanine → 1-phenylpropane-1,2-dione → cathinoneephedrine and pseudoephedrine [12][40][129] Tyramine, ephedrine, pseudoephedrine, mescaline, cathinone, catecholamines (adrenaline, noradrenaline, dopamine)[12][130]
Colchicine alkaloids [131]
Colchicine.svg
Tyrosine or phenylalaninedopamine → autumnaline → colchicine [132] Colchicine, colchamine[131]
Muscarine [133]
Muscarine.svg
Glutamic acid → 3-ketoglutamic acid → muscarine (with pyruvic acid)[134] Muscarine, allomuscarine, epimuscarine, epiallomuscarine[133]
Benzylamine[135]
Benzylamine.svg
Phenylalanine with valine, leucine or isoleucine[136] Capsaicin, dihydrocapsaicin, nordihydrocapsaicin [135][137]
Polyamines alkaloids
Putrescine derivatives[138]
Putrescine.svg
ornithineputrescinespermidinespermine[139] Paucine [138]
Spermidine derivatives[138]
Spermidine.svg
Lunarine, codonocarpine[138]
Spermine derivatives[138]
Spermine.svg
Verbascenine, aphelandrine [138]
Peptide (cyclopeptide) alkaloids
Peptide alkaloids with a 13-membered cycle [36][140] Numularine C type From different amino acids [36] Numularine C, numularine S [36]
Ziziphin type Ziziphin A, sativanine H [36]
Peptide alkaloids with a 14-membered cycle [36][140] Frangulanine type Frangulanine, scutianine J [140]
Scutianine A type Scutianine A [36]
Integerrine type Integerrine, discarine D [140]
Amphibine F type Amphibine F, spinanine A [36]
Amfibine B type Amphibine B, lotusine C [36]
Peptide alkaloids with a 15-membered cycle [140] Mucronine A type Mucronine A [33][140]
Pseudoalkaloids (terpenes and steroids)
Diterpenes [33]
Isoprene.svg
Lycoctonine type Mevalonic acid → izopentenilpyrophosfate → geranyl pyrophosphate [141][142] Aconitine, delphinine [33][143]
Steroids[144]
Cyclopentenophenanthrene.svg
Cholesterol, arginine[145] Solasodine, solanidine, veralkamine[146]

Properties

Head of a lamb born by a sheep that ate leaves of the corn lily plant. The cyclopia in the calf is induced by the alkaloid cyclopamine present in the plant.

Most alkaloids contain oxygen in their molecular structure; those compounds are usually colorless crystals at ambient conditions. Oxygen-free alkaloids, such as nicotine[147] or coniine,[23] are typically volatile, colorless, oily liquids.[148] Some alkaloids are colored, like berberine (yellow) and sanguinarine (orange).[148]

Most alkaloids are weak bases, but some are amphoteric, for example theobromine and theophylline.[149] Most alkaloids are poorly soluble in water but readily dissolve in organic solvents, such as diethyl ether, chloroform and 1,2-dichloroethane. However, caffeine dissolves well in boiling water.[149] With acids, alkaloids form salts of various strengths. Those salts are usually soluble in water and alcohol and poorly soluble in most organic solvents. Exceptions include scopolamine hydrobromide, which is soluble in organic solvents and water-soluble quinine sulfate.[148]

Most alkaloids have a bitter taste. It is believed that plants evolved the ability to produce these bitter substances, many of which are poisonous, in order to protect themselves from animals; however, animals in turn evolved the ability to detoxify alkaloids.[150] Some alkaloids can produce developmental defects in the offspring of animals that consume them but cannot detoxify them. A characteristic example is the alkaloid cyclopamine, which is present in the leaves of corn lily. During the 1950s, up to 25% lambs born by sheep that had grazed on corn lily suffered serious facial defects. Those defects ranged from deformed jaws to cyclopia (see picture). After decades of research, in 1980s, the substance that was responsible for the deformities was identified as the alkaloid 11-deoxyjervine, which was renamed cyclopamine.[151]

Distribution in nature

Strychnine tree. Its seeds are rich in strychnine and brucine.

Alkaloids are generated by various living organisms, especially by higher plants – about 10 to 25% of those contain alkaloids.[152][153] Therefore, in the past the term "alkaloid" was associated with plants.[154]

The alkaloids content in plants is usually within a few percent and is inhomogeneous over the plant tissues. Depending on the type of plants, the maximum concentration is observed in the leaves (black henbane), fruits or seeds (Strychnine tree), root (Rauwolfia serpentina) or bark (cinchona).[155] Furthermore, different tissues of the same plants may contain different alkaloids.[156]

Beside plants, alkaloids are found in certain types of fungi, such as psilocybin in the fungus of the genus Psilocybe, and in animals, such as bufotenin in the skin of some toads.[15] Many marine organisms also contain alkaloids.[157] Some amines, such as adrenaline and serotonin, which play an important role in higher animals, are similar to alkaloids in their structure and biosynthesis and are sometimes called alkaloids.[158]

Extraction

Crystals of piperine extracted from black pepper.

Because of the structural diversity of alkaloids, there is no single method of their extraction from natural raw materials.[159] Most methods exploit the property of most alkaloids to be soluble in organic solvents but not in water, and the opposite tendency of their salts.

Most plants contain several alkaloids. Their mixture is extracted first and then individual alkaloids are separated.[160] Plants are thoroughly ground before extraction.[159][161] Most alkaloids are present in the raw plants in the form of salts of organic acids.[159] The extracted alkaloids may remain salts or change into bases.[160] Base extraction is achieved by processing the raw material with alkaline solutions and extracting the alkaloid bases with organic solvents, such as 1,2-dichloroethane, chloroform, diethyl ether or benzene. Then, the impurities are dissolved by weak acids; this converts alkaloid bases into salts that are washed away with water. If necessary, an aqueous solution of alkaloid salts is again made alkaline and treated with an organic solvent. The process is repeated until the desired purity is achieved.

In the acidic extraction, the raw plant material is processed by a weak acidic solution (e.g., acetic acid in water, ethanol, or methanol). A base is then added to convert alkaloids to basic forms that are extracted with organic solvent (if the extraction was performed with alcohol, it is removed first, and the remainder is dissolved in water). The solution is purified as described above.[159][162]

Alkaloids are separated from their mixture using their different solubility in certain solvents and different reactivity with certain reagents or by distillation.[163]

Biosynthesis

Biological precursors of most alkaloids are amino acids, such as ornithine, lysine, phenylalanine, tyrosine, tryptophan, histidine, aspartic acid, and anthranilic acid.[164] Nicotinic acid can be synthesized from tryptophan or aspartic acid. Ways of alkaloid biosynthesis are too numerous and cannot be easily classified.[73] However, there are a few typical reactions involved in the biosynthesis of various classes of alkaloids, including synthesis of Schiff bases and Mannich reaction.[164]

Synthesis of Schiff bases

Schiff bases can be obtained by reacting amines with ketones or aldehydes.[165] These reactions are a common method of producing C=N bonds.[166]

Schiff base formation.svg

In the biosynthesis of alkaloids, such reactions may take place within a molecule,[164] such as in the synthesis of piperidine:[29]

Schiff base formation intramolecular.svg

Mannich reaction

An integral component of the Mannich reaction, in addition to an amine and a carbonyl compound, is a carbanion, which plays the role of the nucleophile in the nucleophilic addition to the ion formed by the reaction of the amine and the carbonyl.[166]

Mannich.png

The Mannich reaction can proceed both intermolecularly and intramolecularly:[167][168]

Mannich reaction intramolecular.svg

Dimer alkaloids

In addition to the described above monomeric alkaloids, there are also dimeric, and even trimeric and tetrameric alkaloids formed upon condensation of two, three, and four monomeric alkaloids. Dimeric alkaloids are usually formed from monomers of the same type through the following mechanisms:[169]

The biological role

The role of alkaloids for living organisms that produce them is still unclear.[170] It was initially assumed that the alkaloids are the final products of nitrogen metabolism in plants, as urea in mammals. It was later shown that alkaloid concentrations varies over time, and this hypothesis was refuted.[8]

Most of the known functions of alkaloids are related to protection. For example, aporphine alkaloid liriodenine produced by the tulip tree protects it from parasitic mushrooms. In addition, presence of alkaloids in the plant prevents insects and chordate animals from eating it. However, some animals adapted to alkaloids and even use them in their own metabolism.[171] Such alkaloid-related substances as serotonin, dopamine and histamine are important neurotransmitters in animals. Alkaloids are also known to regulate plant growth.[172]

Applications

In medicine

Medical use of alkaloid-containing plants has a long history, and, thus, when the first alkaloids were isolated in the 19th century, they immediately found application in clinical practice.[173] Many alkaloids are still used in medicine, usually in the form of salts, including the following:[8][174]:

Alkaloid Action
Ajmaline antiarrhythmic
Atropine, scopolamine, hyoscyamine anticholinergic
Vinblastine, vincristine antitumor
Vincamine vasodilating, antihypertensive
Codeine cough medicine
Cocaine anesthetic
Colchicine remedy for gout
Morphine analgesic
Reserpine antihypertensive
Tubocurarine Muscle relaxant
Physostigmine inhibitor of acetylcholinesterase
Quinidine antiarrhythmic
Quinine antipyretics, antimalarial
Emetine antiprotozoal agent
Ergot alkaloids sympathomimetic, vasodilator, antihypertensive

Many synthetic and semisynthetic drugs are structural modifications of the alkaloids, which were designed to enhance or change the primary effect of the drug and reduce unwanted side-effects.[175] For example, naloxone, an opioid receptor antagonist, is a derivative of thebaine that is present in opium.[176]

In agriculture

Prior to the development of a wide range of relatively low-toxic synthetic pesticides, some alkaloids, such as salts of nicotine and anabasine, were used as insecticides. Their use was limited by their high toxicity to humans.[177]

Use as psychoactive drugs

Preparations of plants containing alkaloids and their extracts, and later pure alkaloids, have long been used as psychoactive substances. Cocaine and cathinone are stimulants of the central nervous system.[178][179] Mescaline and many of indole alkaloids (such as psilocybin, dimethyltryptamine and ibogaine) have hallucinogenic effect.[180][181] Morphine and codeine are strong narcotic pain killers.[182]

There are alkaloids that do not have strong psychoactive effect themselves, but are precursors for semi-synthetic psychoactive drugs. For example, ephedrine and pseudoephedrine are used to produce methcathinone and methamphetamine.[183]

See also

Notes

  1. ^ In the penultimate sentence of his article [W. Meissner (1819) "Über Pflanzenalkalien: II. Über ein neues Pflanzenalkali (Alkaloid)" (On plant alkalis: II. On a new plant alkali (alkaloid)), Journal für Chemie und Physik, vol. 25, pp. 377–381] Meissner wrote "Überhaupt scheint es mir auch angemessen, die bis jetzt bekannten Pflanzenstoffe nicht mit dem Namen Alkalien, sondern Alkaloide zu belegen, da sie doch in manchen Eigenschaften von den Alkalien sehr abweichen, sie würden daher in dem Abschnitt der Pflanzenchemie vor den Pflanzensäuren ihre Stelle finden." (In general, it seems appropriate to me to impose on the known plant substances not the name "alkalis" but "alkaloids", since they differ greatly in some properties from the alkalis; among the chapters of plant chemistry, they would therefore find their place before plant acids [since "Alkaloid" would precede "Säure" (acid)].)

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