Alkaloids: Definition, types and classification

• Alkaloids are natural products that have potent physiological effects.
• They are therapeutic agents used to treat malaria, cancer and other diseases.
• These can be extremely poisonous, with examples including:
  • Curare extract, which is an tubocurarine alkaloid that is an arrow poison
  • Hemlock (coniine), which killed Socrates
  • Belladonna (atropine), which is used as a murdering agent
  • Egyptian henbane (hyoscyamine), which killed Cleopatra

Classification

• Classification can be based on pharmacological action, chemical structure, biochemical origin, or taxonomical origin.

Classification Based on Chemical Structure

• Non-heterocyclic alkaloids are atypical, also known as protoalkaloids.
• Heterocyclic alkaloids are typical - subgroups separated by ring structure.

Hegnauer's Classification

• True alkaloids contain nitrogen in a heterocyclic ring and are derived from amino acids. Examples of these include:
  • Nicotine
  • Atropine
• Proto-alkaloids have nitrogen, but not in a heterocyclic ring and are derived from amino acids. An example is:
  • Ephedrine
• Pseudoalkaloids have nitrogen in a heterocyclic ring, but are derived from non-amino acids. An example is:
  • Coniine

Distribution

• Flowering plants (Angiosperms) are major source
• These are found more recently in animals, fungi & marine organisms
• True alkaloids are rare in lower plants like fungi, for example, ergot alkaloids
• Alkaloids are not usually found in Salicales and Cucurbitales orders
• Families rich in alkaloids include:
  • Monocotyledons (Liliaceae & Amaryllidaceae)
  • Dicotyledons (Solanaceae, Papaveraceae, Compositae)
• Animal alkaloids include:
  • Those found in amphibian skin, which are potent neurotoxins
  • Indole and isoquinoline derivatives, plus themammalian morphine, found in mammals

Nomenclature

• Trivial names end in "ine"
• Names may be based on a plant's:
  • Genus (atropine from Atropa belladonna)
  • Species (cocaine from Erythroxylon coca)
  • Common name (ergotamine from ergot)
  • Discoverer (pelletierine from Pelletier)
  • Physiological action (emetine from "emetic")
  • Physical character (hygrine from "hygroscopic")

Prefixes

• "Nor-" means N-demethylation or N-demethoxylation, for example, norpseudo-ephedrine and nornicotine
• "Apo-" means dehydration (apomorphine), while "Iso-, pseudo-, neo-, &epi-" means different types of isomers

Suffixes

• "-dine" indicates isomerism (Cinchona alkaloids), for example:
  • Quinidine is an optical isomer of quinine
  • Cinchonidine is an optical isomer of cinchonine
• “-inine” denotes a lower pharmacological activity, which makes ergotaminine less potent than ergotamine

Nitrogen Atoms

• Alkaloids usually have 1 N atom, but can contain 2-5 N atoms
• Nicotine contains 2 N
• Ergotamine contains 5N

Types of Amino Groups

• Primary amino group NH2 e.g. nor-pseudoephedrine
• Secondary amino group NH e.g. ephedrine
• Tertiary amino group N e.g. nicotine & atropine
• Quaternary ammonium ion N+ e.g. tubocurarine

Basicity

• Basicity results from a lone pair of electrons on the nitrogen atom; alkaloids resemble ammonia.
• Alkaloids form salts with acids without liberating water.
• These occur as free bases, salts or N-oxides (N → O).

Factors Affecting Basicity

• Unsaturation decreases basicity, for example, piperidine alkaloids are more basic than pyridine alkaloids.
• Electron donating groups (alkyl gps) increase basicity
• Electron withdrawing groups (carbonyl gps) decrease basicity
• Some alkaloids are amphoteric due to acidic groups, for example:
  • Phenolic alkaloids like morphine, psychotrine, & cephaline
  • Alkaloids with a carboxylic group like narceine

Physical Properties

• Most are crystalline solids, but some are liquids like nicotine and coniine
  • Volatile examples of alkaloids are Nicotine and Coniine
  • Non-volatile examples include pilocarpine and hyoscine
• Majority are colorless
• Some are colored:
  • Colchicine and berberine are yellow
  • Canadine is orange
  • Salts of sanguinarine are copper-red
• Alkaloidal bases and salts are soluble in alcohol

Solubility

• Bases are soluble in organic solvents and insoluble in water.
• Salts are soluble in water and insoluble or sparingly soluble in organic solvents.
• Salts of weak bases decompose easily in solution without alkalinization; the bases are then extracted using organic solvents.

Exceptions

• Bases like caffeine, ephedrine, codeine, colchicine dissolve in water
• Morphine and psychotrine dissolve in ether, or theobromine and theophylline in benzene are insoluble or sparingly soluble in organic solvents
• Quinine sulphate does not dissolve in water
• Lobeline and apoatropine hydrochlorides do dissolve in chloroform

Optical Activity

• Many alkaloids are optically active due to one or more asymmetric C atoms (chiral centers).
• Active isomers show different physiological activities.
• The l (-) isomer is more active than the d (+) isomer, for example:
  • l-ephedrine is 3.5 times more active than d-ephedrine
  • l-ergotamine is 3-4 times more active than d-ergotamine

Exceptions

• d-Tubocurarine is more active than the corresponding l-form
• Both quinine (l-form) and quinidine (its d-isomer) are active
• Racemic dl-atropine is physiologically active

Chemical Properties

• Alkaloids are amphoteric, very weak, or acidic:
  • Oxygenated alkaloids (morphine)
  • Very weak alkaloids (caffeine, unstable salt)
  • Acidic alkaloids (ricinine)
• Most contain oxygen, but few are oxygen-free (nicotine & coniine).
• Alkaloids + acids form salts due to basic character:
  • Strong bases form salts with very weak acids
  • Weak bases form salts with stronger acids
  • Very weak bases form unstable salts (Colchicine, piperine, capsaicin, papaverine, narcotine & caffeine)
• Dibasic alkaloids may form 2 series of salts.
• Amphoteric alkaloids (containing phenolic or carboxylic groups) make salts with both acids & alkalis.
• Acidic alkaloids don't make salts with acids e.g. ricinine.

Stability

• Exposure to light, heat, oxygen, acids & alkalis should be avoided
• Alkaloids are decomposed by heat, except caffeine, which sublimes without decomposition.
• Most 3ary amine alkaloids turn into N-oxides when exposed to light & oxygen at elevated temperature.
• N-oxides are usually water-soluble, and have delayed release, low toxicity & low addictive properties compared to parent alkaloids.
• Dilute or conc. mineral acids may cause dehydration, anhydro- or apo-alkaloids:
  • Morphine to apomorphine
  • Atropine to apoatropine
  • O-demethylation to phenolic alkaloids from codeine + HI
  • Hydrolysis of ester alkaloids (atropine & reserpine) & glucoalkaloids (solanine).

Alkalines

• NH3 liberates most alkaloids from their salts.
• NaHCO3 creates salts with alkaloids containing carboxylic group (e.g. narceine yielding its sodium salt).
• Aqueous NaOH and KOH create salts with phenolic alkaloids, and hot alkalis cause hydrolysis of ester alkaloids like atropine and cocaine

Detection and Identification

• Precipitation reactions can be used
  • Precipitating reagent plus neutral or slightly acidic aqueous solution creates alkaloidal salts and crystalline precipitates
  • Reagents, containing heavy metals (Hg, Pt, Bi, Cd) create double salts with most alkaloids
  • Reactions can be used for extraction and purification

Precautions

• Certain alkaloids don't react, for example, caffeine with Mayer’s reagent does not create a ppt.
• False positive responses may arise because most reagents also precipitate proteins, tannins, coumarins & certain flavonoids
• Color reagents + solid free bases create characteristic colored solutions.
• Reagents generally contain conc. H2SO4 + oxidizing agent.
• Reagents create colors with most alkaloids or may be specific.

Van-Urk's Test

• Testing for ergot alkaloids involves their reaction with para-dimethylamino-benzaldehyde (PDAB) in conc H2SO4, which results in a blue color

Vitalis' test

• Testing for solanaceous alkaloids and conc. HNO3 and alcoholic KOH results in a violet color.

Common Reagents for Detection of Alkaloids

• Mayer's reagent contains Potassium-mercuric iodide, and its precipitate of alkaloids are creamy white (+ve with caffeine & dil.ephedrine)
• Wagner’s reagent contains Iodine / potassium iodide, and its precipitate of alkaloids are reddish brown
• Hager's reagent contains Saturated solution of picric acid, and its precipitate of alkaloids are yellow
• Dragendorff’s reagent contains Potassium bismuth iodide, and its precipitate of alkaloids are orange-reddish brown
• Marmé’s reagent contains Potassium cadmium iodide, and its precipitate of alkaloids are yellow
• Reactions with Froehd's reagent (Ammoniummolybdate/conc. H2SO4), Mandalin's reagent (Ammonium vanadate / conc. H2SO4), Marquis' reagent (Formaldehyde / conc. H2SO4), Erdmann's reagent (Conc. nitric acid / conc. H2SO4), Mecke's reagent (Selenious acid / conc. H2SO4), Shaer's reagent (Hydrogen peroxide / conc. H2SO4), Rosenthaler's reagent (Potassium arsenate / conc. H2SO4), and Dragendorff's reagent, (Potassium bismuth iodide) can be used and colors are characteristic, sensitive for micro amounts and used for colorimetric estimation

Extraction, Purification & Separation

• Acid base purification processes can be used

Acid Base Purification

• Powdered plant material can use petroleum ether or other methods

Ammonia Use

• Can be used as an alkali of choice that is sufficiently basic to liberate most alkaloids
• Volatile and completely removed after extraction

Isolation methods

• Volatile alkaloids like nicotine & coniine can be isolated by steam distillation by the process:
  1. Immerse plant material in water, then ad NaOH or Na2CO3 to liberate a free base Steam distillation of the liberated alkaloid is required

Isolation of Sublimable Alkaloids

• Methods for Caffeine include direct sublimation from the dried powdered plant

Quantitative Analysis for Alkaloids

• Determination of genuineness of raw material.
• Evaluation of plant material for marketing and determination of site of biosynthesis.
• Selecting the best stage for plant collection and evaluating stability & activity of a preparation.
• Preventing overdose & intoxication.
• Determining bioavailability in different organs & tissues.

Quantitative Analysis: Method Selection

• Proximate Assays that determine all alkaloids in plant material.
• Ultimate Assays determining individual alkaloids after separation by crystallization, distillation and chromatography

Measurement Methods

• Volumetric Assay
  • Aqueous titration
  • Non-aqueous titration
• Gravimetric Assay uses: -Very Weak alkaloid
• Colorimetric Assay
  • Atropine

Volumetric methods

• Aqueous titration is defined by: Direct titration of alcoholic solution of alkaloid residue with standard acid. Dissolving residue in standard acid and dissolving residual acid against standard alkali

Non-aqueous titration

• Determining purified residue that is dissolved in glacial acetic acid or chloroform with standard perchloric acid

Gravimetric Methods

• Used for very weak bases, which cannot be determined by volumetric methods like caffeine & colchicine, and mixtures of alkaloids Procedures used include: weighing the purified alkaloidal residue and calculate the %, and precipitation of total alkaloids Drawbacks include: insensitivity to microamounts of alkaloids and are hard to use when calculating thermolabile constituents.

Colorimetric Determinations

• Based on color reactions of alkaloids with specific reagents.
• Solanaceous alkaloids plus vitali-Morin's reagent yield a violet color.
• Colorimetric, spectrophotometric, fluorimetric, polarimetric & different chromatographic methods.