Botany session 8

© Copyright PCNM 2011
Botany and Pharmacognosy
Session 8

Review
Last session we covered:
• Phytochemistry, Part I

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

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

Flavonoids
• Flavonoids are extremely common and widespread in the plant
kingdom. They function as plant pigments, responsible for the
colours of flowers and fruits. Being abundant in plants flavonoids
are commonly consumed in the human diet, especially a diet rich in
fruits and vegetables.

Flavonoids
• The word ‘flavonoid’ is derived
from the Latin word ‘flavus’
meaning yellow and many
flavonoids are yellow in colour.
Many others are white and the
special flavonoid related
anthocyanidins are red, blue or
purple.
• Flavonoids are also present in
leaves where they are said to
protect the plant tissue against
the damaging effects of ultraviolet
radiation.

Flavonoids
• Flavonoids consist of a single benzene ring joined to a benzo-
gamma-pyrone structure. They are formed from three acetate units
and a phenylpropane unit through the shikimic acid pathway. More
than 2000 are known with nearly 500 occurring in the free
(aglycone) state and the rest as O- or C-glycosides. Flavonoid
glycosides are generally water soluble. There are three main types,
classified according to the state of oxygenation at carbon 3. These
are flavones, flavonols and flavonones.

Tannins and Oligomeric Procyanidins
• Tannins are defined as
vegetable substances capable
of tanning animal hides to
produce leather. This is used
as a method to preserve the
hide and at a molecular level is
effected via the cross linking
by the tannins of hide proteins.
Powered hide is still used as a
phytochemical test for tannins.

• Like flavonoids, tannins are
polyphenolic compounds which
have an affinity for proteins.
However the higher number of
phenolic groups and the larger
molecular size of tannins mean
that they are capable of binding
strongly to proteins at several
sites and can precipitate them
from solution.
Chickweed

• The phytochemical classification of tannins can be complex but two
main groups are usually recognized:
• Hydrolysable tannins
• Condensed tannins
• Hydrolysable tannins usually consist of a central glucose molecule
linked to molecules of gallic acid (gallitannins) or
hexahydroxydiphenic acid (ellagitannins). They are readily
hydrolysed, hence their name. Ellagitannins are found in:
• Cranesbill
• Oak bark - also contains condensed tannins
• Meadowsweet

• Condensed tannins are polymeric
flavans which are not readily
hydrolysable. They often consist
of molecules of catechin and
epicatechin joined by carbon-
carbon bonds. Hence catechin
and epicatechin are referred to as
monomers and molecules
containing 2-4 monomers are
referred to as oligomeric
procyanidins.

• Protein-binding capacity increases
markedly with the degree of
polymerization, hence dimers are
much less astringent than
hexamers. From a pharmacological
perspective oligomeric
procyanidins and their monomers
behave much like flavonoids and
therefore are sometimes classified
with them.

• Hydrolysable tannins generally
decompose slowly when kept in
aqueous solution. They may also be
hydrolysed by acids or enzymes such
as tannase into their component
molecules.
• Condensed tannins are more resistant
to decomposition into their monomeric
components. However, they are
readily oxidised over time as shown
by their colour change to purplish
pink. These oxidized tannins are
responsible for the reddish colour of
many barks and roots.

• Long storage of solutions of
condensed tannins (such as tinctures
and fluid extracts) induces extensive
precipitation of condensate products
known as phlobaphenes or
phlobatannins. Glycerol was
traditionally added to liquid galenicals
to decrease this effect.
• Tannins complex with and precipitate
alkaloids and herbal extracts
containing tannins should not be
mixed with alkaloid-containing
extracts.

Resins
• Resins are sticky, water-insoluble substances often exuded by the
plant. The term is used in several contexts. When certain plants are
damaged, either by incision or naturally due to the action of animals
or the environment, they secrete a viscous fluid which soon hardens.
This is likely to be a form of protection.
• The exudate is a mixture of chemicals which softens on heating.
Such resins are often associated with essential oils (oleoresins),with
gums (gum resins) or with oil and gum (oleo-gum resins). The resin
components which mainly comprise diterpenes known as resin
acids, resin alcohols and resin phenols are soluble in alcohol and
ether but insoluble in water and hexane.

Resins
• Myrrh is an oleo-gum resin with
astringent and antimicrobial
properties. The former actions are
probably entirely due to the resin and
the combined effect from the resin
and the essential oil.
• Resins have also been applied to
inflammatory conditions of the upper
digestive tract with some benefit. This
probably reflects on their astringent
property.
Myrrh resin

Resins
• Important resin containing herbs
include:
• Propolis (not actually a herb but
collected by bees from resinous
plants)
• Grindelia
• Calendula
• Guggul
• Boswellia
• Juniper
Boswellia

Bitters
• Bitters are substances capable of
strongly stimulating the bitter receptors
in the taste buds at the back of the
tongue. The taste stimulus triggered by
an intramolecular bonding with these
receptors. Each taste bud contains
approximately 20-30 sensory cells
whose microvilli extend to the bud
opening. The taste receptors lie in the
membrane of these microvilli and
consist of glycoproteins.
• Given that bitters are defined
physiologically the bitter compounds
actually come from a number of
different phytochemical classes.

Bitters
• Monoterpenes, sesquiterpenes, diterpenes, flavonoids and
triterpenes can exhibit bitter properties.
• The most notable bitter compounds are the monoterpene secoiridoid
glycosides of gentian (particularly amarogentin), centaury and
bogbean, and the sesquiterpene lactone dimers of wormwood
(absinthin). These compounds are among the most bitter
substances known.

Checkpoint!

Pungent Constituents
• Like bitters pungent constituents is a physiological classification
rather than a phytochemical one. The most commonly used pungent
spices are:
• Cayenne pepper - capsicum
• Black pepper - piperine
• Ginger - gingerols
• Although the above constituents are chemically distinct it has been
suggested that they act upon a common group of nerve cell
receptors.

Pungent Constituents
• Capsaicin and piperine are alkaloids based on homovanillic acid,
whereas gingerols are substituted alkylphenols.

Saponins
• Saponins are phytochemicals which
produce a foam when dissolved in
water. Their name derives from the
same root as the word soap; Latin
sapo = soap
• Like soap, saponins are large
molecules which contain a hydrophilic
part at one end and a lipophilic part at
the other.
• In aqueous solution the saponin
molecules align themselves vertically
on the surface with their hydrophobic
ends oriented away from the water.
This has the effect of reducing the
surface tension of the water, causing it
to foam.
Wild yam

Saponins
• As saponins can reduce the surface
tension they are classified as
surface-active agents.
• Saponin molecules can align to
form a spherical configuration in
water, creating micelles. Micelles
have a lipophilic centre and this
creation of a fat-loving compartment
explains why detergents can
dissolve grease and oils.

Saponins
• Saponins are glycosides with the sugar
part comprises the hydrophilic ends. There
are two classes based on the structure of
their aglycone:
• Steroidal saponins –have a
characteristic four ringed steroid
nucleus
• Triterpenoid saponins – have a five
ringed structure
• Both of these classes have a glycosidal
linkage usually at carbon 3 and have a
common biosynthetic origin via the
mevalonic acid pathway.

Saponins
• Steroidal saponins are mainly found in the monocotyledons.
• Triterpenoid saponins are by far the most common.
Steroidal saponin
Triterpenoid saponin

Saponins
• There are a few
exceptions to the rule.
For example
ginsenosides in ginseng
are grouped with the
triterpenoid saponins,
even though they exhibit
a steroidal structure.
Panax ginseng

Saponins
• Steroidal saponins typically
contain extra furan and pyran
heterocyclic rings, which is not a
feature of ginsenosides.
• Furans and pyrans are five and
six membered rings, respectively,
containing oxygen.
• Saponins are consumed in many
common foods and beverages
including oats, spinach, quinoa,
asparagus, legumes, peanuts,
tea and beer.

Anthraquinones
• Anthraquinones as the name suggests, are phytochemicals based
on anthracene; three benzene rings joined together. At each apex of
the central ring is a carbonyl group (carbon double-bonded to
oxygen), which is the quinone part.
• Not all anthraquinones are strictly quinones. For example the
sennosides are dianthrones which consist of two anthrone units,
each bearing only one carbonyl group.

Anthraquinones
• Anthraquinones usually occur in
plants as glycosides. For example
the sennosides from senna are O-
glycosides while the aloins from aloe
are C-glycosides. The unusual
molecules of cascarosides are C,O-
glycosides. Meaning they have one
glucose linked to a central anthrone
via a carbon atom and a second
glucose linked via oxygen.
Cascaroside

Anthraquinones
• If aglycones do occur in dried
herbs, they are always
anthraquinones, as anthrones are
too unstable in the free state.
Dianthrone glycosides such as the
sennosides are not found in the
living plant. Instead they are
formed on harvesting and drying
from monomeric anthrone
glycosides.

Coumarins
• Coumarins name comes from ‘coumarin’
which was the common name for the
tonka bean, from which the
phytochemical coumarin was first isolated
in 1820.
• Coumarins are benzo-alpha-pyrones
(latones of o-hydroxycinnamic acid)
formed via the shikimic acid pathway.
Except for a few rare cases including
coumarin itself which is unsubstituted, all
plant coumarins contain a hydroxyl or
methoxy group in position 7. These
substituted simple coumarins such as
scopoletin, aesculetin and umberlliferone
are common and widespread in higher
plants and often occur as glycosides.

Coumarins
• Simple coumarin has a pleasant vanilla-like odour. It is probably not
present in the intact plant but is rather formed by enzymatic activity
from a glycoside of o-hydroxycinnamic acid after harvesting and
drying. This accounts for the odour of newly mowed grass, which is
not present in the undamaged plants.
• Courmain is used to perfume pipe tobacco and can be sometimes
found as an adulterant in commercial vanilla flavouring.

Coumarins
• The furanocoumarins are closely related furano derivatives of
coumarin, which are commonly found in the Rutaceae (rue) family
and Umbelliferae (celery) family.
• Linear furanocoumarins are often called psoralens and act as
photosensitizing agents.

Coumarins
• The furanochromones such as
khellin from Ammi visnaga are
structural derivatives of benzo-
gamma-pyrone and therefore are
related to flavonoids as
coumarins. Meaning the
carbonyl group is opposite the
oxygen rather than adjacent to it.
However they are usually
classified as coumarins.

Coumarins
• The wide spread nature of
coumarins means that they are
consumed in the human diet e.g.
Carrots, celery, parsnip.
Coumarins are fluorescent
compounds and this property is
widely utilised in a number of
biochemical techniques.
• Simple substituted coumarins are
also used as pigments in
sunscreen.

Phytooestrogens
• Phytooestrogens are
phytochemicals that have
oestrogenic activity. Compounds
belonging to several
phytochemical classes interact
with oestrogen receptors but
research has focused on
isoflavones and lignans.
• Oestrogenic isoflavones, which
include genistein, daidzein and
their glycosides are mainly found
in members of the Leguminosae
(pea) family e.g. Red clover and
soya bean.

Phytooestrogens
• Linseed is the richest source of the oestrogenic lignans enterodiol
and enterolactone, which are formed by bacterial action on the
precursor secoisolariciresinol diglucoside, found in the seed.

Alkaloids
• It is difficult to consistently define
alkaloids because there is a vast and
diverse group of archetypal plant
constituents.
• The word alkaloid derives from the
term ‘vegetable alkali’ which refers to
the alkaline nature of these
compounds, which results from the
fact that they always contain
nitrogen.
• The names of alkaloids all end with
the letter –ine.
Nicotine

Alkaloids
• Alkaloids are alkaline (basic) nitrogen-
containing heterocyclic compounds derived
from higher plants and often exhibiting
marked pharmacological activity.
• However there are several exceptions to this
definition. The nitrogen in ephedrine is not
part of a ring, so it is not heterocyclic.
Therefore is it sometimes referred to as a
protoalkaloid. Berberine is not alkaline and
some alkaloids are derived from animals
and microorganisms.
• Generally alkaloids are white but some are
highly coloured e.g. Berberine is yellow and
sanguinarine is red.

Alkaloids
• Alkaloids were the first chemical drugs to be derived from plants, a
mixture of morphine and narcotine was isolated from opium in 1803.
• They have maintained an important role in conventional drug
therapy. Examples of modern drug medicine that are alkaloids
include:
• Codeine
• Morphine
• Atropine
• Quinine
• Pilocarpine
• Theophylline
• Pseudoephedrine
• Vincristine

Alkaloids
• These drugs are potent pharmacological agents and their properties
are well described in conventional pharmacology texts. They are
also associated with a high risk of adverse reactions.
• While phytotherapists do rely on plants containing alkaloids, their
activity is at the milder end of the pharmacological spectrum and are
not consisted as important as what conventional medicine places on
them.

Alkaloids
• Most alkaloids are synthesised by the plant from amino acids. If they
are not, they are called pseudoalkaloids. Most of the known
examples of pseudoalkaloids are isoprenoids and are referred to as
terpenoid alkaloids.
• Aconitine from various aconite species is an example of a
diterpene alkaloid and is one of the most toxic substances
known.

Alkaloids
• Steroidal alkaloids are also found in plants. Some are combined as
glycosides e.g. Solanine from potato shoots.
• Little is known about the function of alkaloids in plants. They may
have a defensive role or may be a by product of primary
metabolism.

Summary
Today we covered
• Phytochemistry, Part II

Next Session
We will cover:
• Ethnopharmacology, Part I

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 8

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Botany session 8

Similar to Botany session 8 (20)

More from Fluke Fox

More from Fluke Fox (18)

Recently uploaded

Recently uploaded (20)

Botany session 8