Botany session 7

© Copyright PCNM 2011
Botany and Pharmacognosy
Session 7

Review
Last week we covered:
• Ethnobotony

This Session
During this session we will
cover:
• Phytochemistry, Part I

Phytochemistry
• Phytochemistry is the study of phytochemicals (plant constituents)
derived from plants. It is considered a sub field of botany and
chemistry.
• The application of phytochemistry is pharmacognosy or the
discovery of new medicines sourced from plants.
• Plant constituents the chemicals in plants that exert a therapeutic
effect.

Phytochemistry
• Metabolic and biochemical pathways
within plants are essential for their
survival.
• A well known biochemical pathway
is photosynthesis. Whereby
chlorophyll in the leaves converts
energy from sunlight into glucose
and oxygen from carbon dioxide and
water from the environment.
• All chemical compounds found in
plants are derived from a few well
studied metabolic pathways using
products of photosynthesis

Phytochemistry
• Plants produce two types of metabolites – primary and secondary.
Primary Metabolites
• These are necessary to sustain the life of the plant and include
enzymes and other proteins, lipids, carbohydrates and chlorophyll.

Phytochemistry
Secondary Metabolites
• These do not appear to be necessary to sustain life but instead act
as protection and defence. Due to the immobility of plants in diverse
and changing environments, along with the possibility of attack by
animals and pathogens, plants have developed these secondary
metabolises for protection.

Phytochemistry
• In recent years particular attention has been paid to the specific
ecological roles of these secondary metabolites, that were once
thought of as waste products, are now being researched for their
medicinal properties.
• Alkaloids are thought to play a defensive role in plants against
herbivores and pathogens.
• Tannins act to preserve the wood in living trees from microbial
decomposition and insects.
• Several classes of secondary metabolites are induced by infection,
wounding or grazing.

Phytochemistry
• The main constituent groups
• Simple Phenols and Glycosides
• Glucosinolates
• Mucilages
• Essential Oils
• Flavonoids
• Tannins and Oligomeric Procyanidins
• Resins
• Bitters
• Pungent Constituents
• Saponins
• Anthraquinones
• Coumarins
• Phytooestrogens
• Alkaloids

Simple Phenols and Glycosides
• Phenols comprise the largest group of plant secondary metabolites.
They range from simple structures with only one benzene ring to
larger molecules such as tannins, anthraquinones, flavonoids and
coumarins.
• They are defined as compounds that have at least one hydroxyl
group attached to a benzene ring.

• Phenols differ in their chemical properties from other tertiary
alcohols because the presence of the benzene ring stabilises the
phenolate ion and therefore are more acidic and more reactive.
• Phenolic acids are a special class of simple phenols which in
addition have at least one carboxylic acid group. Examples include:
• Gallic acid – Witch hazel
• Salicylic acid - Willow
• Caffeic acid – Globe Artichoke
• Vanillic acid – Dong quai
• Some of these compounds are sometimes called pseutannins, as
they have astringent characteristics but lack all the properties of true
tannins.

• Glycosides are secondary metabolites that
yield one or more sugars on hydrolysis.
The most frequently occurring sugar is
glucose (i.e. Glycoside). The non-sugar
component of the molecule, which may be
a simple phenol, flavonoid, anthraquinone,
triterpenoid or many other structures, is
called the aglycone. Glycosides may also
bond to oxygen and carbon molecules.
• The glycoside linkage is usually resistant
to human digestive enzymes and
glycosides are often poorly absorbed from
the digestive tract. As a result they travel
to the large intestines where microbial
activity forms the aglycone, which is then
absorbed into the bloodstream.

Cyanogenic Glycosides
• Cyanogenic glycosides are capable of
generating hydrocyanic acid (cyanide).
Structurally they are glycosides of 2-
hydroxynitriles which can be hydrolysed
by the enzyme beta-glucosidase into
cyanohydrin. This is unstable and
dissociates to hydrocyanic acid.
Examples are:
• Amygdalin – bitter almonds and
peach kernels
• Prunasin – Wild Cherry Bark

• Although hydrocyanic acid is a
violent poison, oral intake of
cyanogenic glycosides is not
necessarily toxic. Hydrolysis of
the glycosides in the digestive
tract or by the liver leads to a
slow release of hydrocyanic acid
which can be readily detoxified by
the body.
Wild Cherry

• Interest arose in the 1970’s in the use of a synthetic cyanogenic
glycoside as an alternative anticancer compound. The theory was
that cancer tissues contain beta-glucosidase and that circulating
cyanogenic glycosides would therefore act as selective cytotoxic
agents. However, amygdalin did not prove to be an effective
anticancer agent, presumably because the beta-glucosidase
activity of cancer cells is quite low.
• Amygdalin given orally to humans at 500mg three times a day
produced no toxic effects and only moderately raised blood
cyanide levels. A study published in the Journal of American
Medical Association.

Cardiac Glycosides
• Are also known as cardioactive
glycosides . They are steroidal
glycosides that are similar to but also
very different from steroidal saponins.
They constitute a well defined and highly
homogeneous group from both a
structural and a pharmacological
perspective.
• Cardiac glycosides have been used as
conventional medicine drugs for 200
years and are still widely used today e.g.
Digoxin.
Digitalis purpurea

Cardiac Glycosides
• There are two types which have
either a steroidal aglycone with
23 carbons (the cardenolide
glycosides) or 24 carbons (the
bufadienolide glycosides). The
sugar part usually consists of 2-4
sugars joined together.

Glucosinolates
• Glucosinolates are sulphur and nitrogen containing glycosides which
are responsible for the pungent properties of horseradish,
nasturtium and mustard. The glucosinolate itself is not pungent.
When it comes into contact with the enzyme myrosinase, stored in
another compartment of the cell, the aglycone is formed which
usually rearranges into the pungent and corrosive isothiocyanate.

Glucosinolates
• Isothiocyanates can also be
formed from glucosinolates by
steam distillation and so are
called mustard oils. For this
reason they are sometimes
classified as essential oils.
Mustard field

Glucosinolates
• The glucosinolates are ionic in
nature and occur in the plant as
potassium salts.
• Glucosinolates are also found in
brassicas such as cabbage,
broccoli and Brussels sprouts.
As such they are frequently
consumed as a normal part of a
human diet.

Checkpoint!

Mucilages
• Mucilages are generally not chemically well defined. They are large,
highly branched polymeric structures built from many different sugar
and uronic acid units. They are hydrophilic and are capable of
trapping water in their cage-like structures to form a gel.
• When a mucilage is mixed with water it swells to many times its
original volume as it absorbs water. The beta configuration of the
saccharide linkages inhibits the breakdown of mucilages by human
digestive enzymes. However they can be partially decomposed by
bowel flora into short chain fatty acids.

Mucilages
• Not only do mucilages soothe the gut but the short chain fatty acids
formed in the colon provide a source of readily absorbed and
assimilated nourishment.
Marshmallow

Mucilages
• Mucilaginous remedies have been
primarily used for topical emollient
and demulcent properties and their
direct benefits in the management of
inflammatory conditions of the
digestive system.
• The protective effect of mucilages
isolated from Plantago major
(greater plantain) and guar-gum
provide an extra barrier against
gastric acid and has been shown to
reverse aspirin induced gastric
ulcers due to an anti-inflammatory
action.
Plantago major

Mucilages
• Mucilages also function as bulk laxatives and are a class of
soluble fibre. Examples are:
• Psyllium
• Linseed
• Fenugreek
• Slippery elm
• Marshmallow root

Mucilages
• They are also used as reflex
demulcents especially to ease
irritable dry coughs. The reflect
effect is postulated to be
mediated by the vagus nerve
whereby the demulcent effect
on the upper digestive tract
soothes the bronchial mucosa.
Similar associations are used
for the use of mucilages in
urinary tract conditions.
Corn silk

Mucilages
• Since mucilages are water soluble and
relatively insoluble in ethanol, liquid
galenical preparations of mucilages are
not appropriate, except for their use as
reflex demulcents). For demulcent
effects in the digestive tract mucilages
are best given as powders or capsules.
Psyllium

Essential Oils
• Essential oils are mixtures of
fragrant compounds which can be
isolated from plants by the
process of steam distillation.
Since they are volatile in steam
and usually have pronounced
aromas, essential oils are often
referred to as volatile oils.
• Essential oils are important items
of commerce being used for
perfumes, food flavourings,
personal care products and
pharmaceutical products. They
are also used for aromatherapy.

Essential Oils
• Essential oils are water insoluble oily liquids which are usually
colourless. They are not chemically related to lipid oils or
hydrocarbon oils. They will slowly evaporate if left in an open
container and placing a drop of oil on blotting paper can be used as
a simple technique to test for adulteration with a fixed oil. If a fixed
oil is present an oily smear will remain on the paper a few days later.

Essential Oils
• Adulteration is an important issue in essential oils and many
techniques have been developed to imitate and extend essential
oils. For example oil of wintergreen the trade in the synthetic oil has
completely supplanted the natural product.
• This issues does not apply for oils prescribed as part of the whole
plant extract as used by herbalists.

Essential Oils
• The synthesis and accumulation of
essential oils are generally associated
with the presence of specialised
structures in the plant which are often
located on or near the surface.
• For example the delicate glandular
hairs of the Lamiaceae (mint)
family.

Essential Oils
• Essential oil composition can very quite dramatically within a
species and often distinct chemotypes are recognised. This means
the same species can produce quite different oils in terms of
chemistry, pharmacology and toxicology. The components of
essential oils can be classified into two major groups:
• Terpenoids
• Phenylpropanoids

Essential Oils
Terpenoids
• Terpenoids are constructed from a five carbon isoprene unit. Plants
produce terpenoids using the mevalonic acid biosynthetic pathway.
• Naming of terpenoids is based on multiples of 10 carbons (two
isoprene units). Hence molecules with 10 carbons are called
monoterpenes, those with 15 are called sesquiterpenes (sesqui
means one and a half).
• Only mono and sesquiterpenes are found in essential oils. Higher
terpenoids are too large and are not volatile in steam. However,
diterpenes occur in resins as resin acids and triterpenes are found in
saponins.
• Not even all mono and sesquiterpenes are volatile in steam, as the
iridoids in eyebright are one example.

Essential Oils
• Examples of monoterpenes in essential oils include
• Limonene – citrus fruit
• Geraniol - Rose and citronella
• Borneol - Camphor
• Thujone – Thuja and wormwood

Essential Oils
• Phenylpropanoids are far less common as components of essential
oils. Their basic chemical skeleton is a three carbon chain attached
to a benzene ring. They are formed by the shikimic acid biosynthetic
pathway. Examples are:
• Anethole - Aniseed and fennel
• Eugenol – Clove and nutmeg

Essential Oils
• Essential oil components can
also be classified according to
their functional groups. The
most common compounds
found in essential oils are
hydrocarbons, alcohols,
aldehydes, ketones, phenols,
oxides and esters. These
functional groups play a large
part in determining the
pharmacology and toxicology of
essential oil components.

Essential Oils
• Essential oil components
often exhibit optical
isomerism (where the two
isomers are mirror images of
each other). For example
(+)-carvone isolated from
caraway oil has a caraway
like odour and (-)-carvone
isolated from spearmint oil
has a spearmint like odour.

Summary
Today we covered
• Phytochemistry, Part I

Next Session
We will cover:
• Phytochemistry, Part II

Preparation
Brief Notes
• Don’t forget to log on to the LMS and download and print off your
brief notes and handouts for the next session.

Botany session 7

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