ALKALOIDASiti Rofida

Alkaloids: Introductionsmall organic molecules (secondary metabolites) ofplants which contain nitrogen (ussually in a ring)alkaloids, protoalkaloids, pseudoalkaloidsstructurally diverse: 12,000+ structures20% of all plant speciesconcentrated in specific plant taxa (families, genera,species)biosynthetically diverse (families/types)strong biological effects (-> 3 ecological examples)

Historical and human importance:pharmaceutically significant (often neurological)effect of Nbiological effects & uses: analgesic - pain killers (morphine)paralysis, anesthesia (tubocurarine, coniine)stimulants (caffeine, nicotine)antitussive (codeine)emetic (emetine)anti-cancer drugs (taxol, vinblastine)toxins and antibiotics (quinine, sanguinarine)

Alkaloid Families and Biosynthesisfamilies classified by ring structurederived from amino acidsExamples:tyrosine -> morphine (opium alkaloids)tryptophan -> quinine (quinoline)vinblastine (indole)glutamate (via ornithine) -> cocaine (tropane)- >nicotine (tobacco alk)- > senecionine (pyrrollizidine)aspartate -> nicotinexanthine -> caffeine, theobromine (purine)

Classification:True (Typical) alkaloids that are derived from amino acids and have nitrogen in a heterocyclic ring. e.g AtropineProtoalkaloids that are derived from amino acids and do not have nitrogen in a heterocyclic ring. e.g EphedrinePseudo alkaloids that are not derived from amino acids but have nitrogen in a heterocyclic ring. e.g Caffeine

Terpenoid Indole Alkaloidsantikanker anti-malaria anti-aritmia .

Benzylisoquinoline AlkaloidsAnalgesik,gout, blocker neuromuskuler antimikroba

Tropane Alkaloids and NicotineFamilia Solanaceaeantikolinergik

Purine AlkaloidsXanthosine7-methylxanthosine7-methylxanthine,N-methylationstheobrominecaffeineN-methylationsN-methylationscaffeine synthase / N-methyltransferase caffeine synthase / N-methyltransferase

Tests for AlkaloidsMost alkaloids are precipitated from neutral or slightly acidic solution by Mayer's reagent (potassiomercuric iodide solution) Cream coloured precipitate. Dragendorff's reagent (solution of potassium bismuth iodide) orange coloured precipitate. Wagners reagent (iodine in potassium iodide) red-brown precipitateHagers reagent (picric acid) yellow precipitate

Caffeine does precipitate

Extraction of AlkaloidsExtraction is based on the basicity of alkaloids and on the fact that they normally occur in plants as salts (i.e.: on the solubility of bases and salts in water and organic solvents).Herbs often contain other materials which can interfere with extraction such as large amounts of fat, waxes, terpenes, pigments and other lipophilic substances (e.g by forming emulsions) avoided by defatting the crushed herb (using petroleum ether and hexane)

Extraction in an Alkaline MediumStep 1: Powdered, defatted herb is mixed with an alkaline aqueous solution.

This displaced alkaloids from their salt combinations.

Free bases are then extracted with organic solvents.

Normally aqueous ammonia is used, but a carbonate solution is used when alkaloids contain fragile elements such as a ester or lactone.

In some cases, e.g. Cinchona bark, a mixture of calcium hydroxide & sodium hydroxide should be used as the alkaloids are bound to tannins.

Organic solvent: chloroform, dichloromethane or ethyl acetate depends on the toxicity, safety, cost & ease of recovery and recycling of the solvent).

The Soxhlet ApparatusIndustry: uses solid-liquid extractors based on the principle of counter-current extraction, such Soxhlet apparatus.

Extraction of Alkaloids: Step IIOrganic solvent containing alkaloids (bases) is separated from residue & concentrated by distillation under pressure if needed.

Solvent is stirred with an acidic aqueous solution: alkaloids go into the solution as salts. Impurities remain in the organic phase.

Repeated until the organic phase no longer contains alkaloids.

Many acids can be used (HCL, Sulfuric, citric, tartaric), but always in very dilute concentrations (1-5%)

Alkaloid Extraction: Step IIIAqueous solution of alkaloid salts is washed with an apolar solvent (hexane)

Alkalinized with a base using an organic solvent not miscible with water.

Alkaloids precipitate and dissolve in the organic phase.

Extraction of aqueous phase continues till all alkaloids have moved into the organic phase (tested when Mayers reaction on the aqueous phase becomes negative).

This purification step may be carried out in a separation funnel or in centrifugal extractors.

Separation Funnel

Final StepOrganic solvent containing alkaloid bases is decanted, freed from water traces (drying over anhydrous salt e.g. sodium sulphate) and evaporated under reduced pressure. A dry residue remains: total basic alkaloids.

Kava Powder

Alkaloid Extraction in Acidic Medium2 Methods possible

Pulverized drug is extracted directly with acidified water

Or

Pulverized drug is extracted with acidified alcoholic or a hydroalcoholic solution. This is then followed by distillation under vacuum (eliminates that alcohol, leaving behind and acidic aqueous solution of alkaloid salts)

Vacuum Distillation

In both cases: Results = aqueous solution of alkaloid salts needing purification.Purification achieved by

Alkalinizing solution & extracting bases with an immiscible organic solvent.

Selectively absorb the alkaloids contained in the solution on an ion exchange resin, then eluting them with a strong acid.

Precipitating the alkaloids as iodomercurates. The resulting alkaloids are recovered by filtration, dissolved in a mixture of water, alcohol and acetone and decomposed by passing through an ion-exchange resin.

Isolation of AlkaloidsAll methods of alkaloid extraction yield impure compounds, so alkaloids therefore have to be separated.TLC and HPLC are most commonly used.Under the best conditions, alkaloids can be obtained by direct crystallization: simply by neutralizing the acidic extraction medium.

Crystals of Hydrastine

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