Chemistry of lipids ii

LIPIDS
In pursuit of quality education
DR. USMAN SALEEM
Pharm.D, M.Phil. Scholar, R.Ph,
Lecturer Pharmaceutical Biochemistry
Rashid Latif College of Pharmacy

CHAPTER OUTLINES
 Chemistry of Fatty acids and Lipids
 Classification
 Saponifiable and non-saponifiable lipids
 Simple, Complex and Derived lipids
 Reactions of Fatty acids and other Lipids,
 Essential fatty acids,
 Biological and pharmaceutical importance of lipids.

1. Define and List the Functions of Lipids
Learning
Objective

DEFINITION
 The lipids are a heterogeneous group of compounds
related to fatty acids and include fats, oils, waxes and
other related substances.
 These are oily or greasy organic substances, relatively insoluble in
water, and considerably soluble in organic solvents like ether,
chloroform and benzene.
 The term ‘lipid’ was first used by the German biochemist Bloor
in 1943 for a major class of tissue components and foodstuffs.

FUNCTIONS OR BIOLOGICAL IMPORTANCE
1. Storage form of energy (triacylglycerol)
2. Structural components of biomembranes (phospholipids and cholesterol)
3. Metabolic regulators (steroid hormones and prostaglandins)
4. Act as surfactants, detergents and emulsifying agents (amphipathic lipids)
5. Act as electric insulators in neurons

FUNCTIONS OR BIOLOGICAL IMPORTANCE
6. Provide insulation against changes in external temperature (subcutaneous fat)
7. Give shape and contour to the body
8. Protect internal organs by providing a cushioning effect (pads of fat)
9. Help in absorption of fat soluble vitamins (A, D, E and K)
10. Improve taste and palatability of food.

2. Classify Lipids as Simple, Complex and
Derived, and as Saponifiable or Nonsaponifiable.
Learning
Objective

CLASSIFICATION OF LIPIDS
 Two commonly used classification systems exist for lipids:
 A system based on chemical composition of lipids (Bloor’s Classification), and
 A system based on particular chemical reaction (Saponification) that lipids
undergo.

 Bloor has proposed the following classification of lipids based on their chemical
composition.
1. Simple lipids: These are esters of fatty acids with various alcohols.
a) Neutral Fats (Triacylglycerol,TAG):
 These are esters of fatty acids with trihydroxy alcohol, glycerol. Oils are fats in the
liquid state.
b) Waxes:
 These are esters of fatty acids with higher molecular weight monohydric alcohols.

2. Compound lipids: These are esters of fatty acids with alcohol and possess
additional group(s) also.
a) Phospholipids:
 These are lipids containing, in addition to fatty acids and glycerol, a phosphoric acid,
a nitrogen base and other substituents. For example, in glycerophospholipids the
alcohol is glycerol and in sphingophospholipids, the alcohol is sphingosine.
b) Glycolipids:
 These are lipids containing a fatty acid, sphingosine, and carbohydrate.

3. Derived Lipids: These are compounds obtained by hydrolysis of simple of
compound lipids. These include fatty acids, alcohols, mono- and diglycerides, steroids,
terpenes and carotenoids.
 Glycerides and cholesterol esters, because of their uncharged nature, are also
called neutral lipids.

 Lipids are grouped into two main classes in the chemical reaction (saponification)
classification system:
1) Saponifiable lipids
2) Nonsaponifiable lipids.
 Saponification refers to the process in which esters are hydrolyzed under
basic conditions.

Saponifiable Lipids
 Saponifiable lipids are esters that
undergo hydrolysis in basic solution to
yield two or more smaller product
molecules.
 Triglycerides, waxes, phospholipids, and
sphingolipids are all belong to this class.
Non-Saponifiable Lipids
 Nonsaponifiable lipids are not esters
and cannot be hydrolyzed into smaller
components.
 Steroids and prostaglandins belong to
this class.

3. Describe chemistry and classification of
fatty acids.
Learning
Objective

FATTY ACIDS
 Fatty acids are long, unbranched hydrocarbon chains with a carboxylic acid group at
one end.
 It is represented by a chemical formula R-COOH, where R stands for hydrocarbon
chain.
 Fatty acids are amphipathic compounds because the carboxyl group is hydrophilic
and the hydrocarbon tail is hydrophobic.
 The carboxyl group can ionize as the carboxylate anion (–COO- ) under the proper
conditions of physiological pH.

FIGURE: The molecular structure of fatty acid. A. Lauric acid and B. a simplified
diagram of a fatty acid with a nonpolar tail and a polar head.

FATTY ACIDS
• In aqueous solution, the ions of fatty
acids associate to form spherical
clusters, called micelles.
• In micelles, the nonpolar chains extend
toward the interior of the structure
away from water, and the polar
carboxylate groups face outward in
contact with the water.

FATTY ACIDS
 The fatty acids found in natural lipids have several characteristics in common:
1. They are usually straight-chain carboxylic acids (no branching).
2. The sizes of most common fatty acids range from 10 to 20 carbons.
3. Fatty acids usually have an even number of carbon atoms (including the carboxyl
group carbon).
4. Fatty acids can be saturated (containing no double bonds between carbons) or
unsaturated (containing one or more double bonds between carbons).

FATTY ACIDS
5. In terms of carbon chain length, fatty acids are characterized as:
 Long-chain fatty acids (C12 to C26),
 Medium chain fatty acids (C8 and C10), or
 Short-chain fatty acids (C4 and C6).
6. Fatty acids are rarely found free in nature but rather occur as part of the structure
of more complex lipid molecules.

CLASSIFICATION OF FATTY ACIDS
 The hydrocarbon chain of a fatty acid may or may not contain carbon–carbon double
bonds.
 On the basis of this consideration, fatty acids are classified as:
1. Saturated fatty acids (SFAs),
2. Unsaturated Fatty Acids
3. Monounsaturated fatty acids (MUFAs), and
4. Polyunsaturated fatty acids (PUFAs).

1. Saturated Fatty Acids
 A saturated fatty acid is a fatty acid with a carbon chain in which all carbon–
carbon bonds are single bonds.
 Examples include:
 Propionic acid
 Palmitic acid
 Stearic acid

2. Unsaturated Fatty Acids
 They are classified further according to degree of unsaturation.
a) Monounsaturated Fatty Acids (MUFA’s)
b) Polyunsaturated Fatty Acids (PUFA’s)

a) Monounsaturated Fatty Acids
 A monounsaturated fatty acid
is a fatty acid with a carbon chain
in which one carbon–carbon
double bond is present.
 For example, Oleic acid is a
monounsaturated fatty acid, that is
found in nearly all fats.

b) Polyunsaturated Fatty Acids
 A polyunsaturated fatty acid is a fatty acid with a carbon chain in which two
or more carbon–carbon double bonds are present.
 Up to six double bonds are found in biochemically important PUFAs; they
include:
• Dienoic acids series have two double bonds, e.g. linoleic acid
• Trienoic acids series have three double bonds, e.g. linolenic acid
• Tetraenoic acid series with four double bonds, e.g. arachidonic acid

NOMENCLATURE OF FATTY ACIDS
 The systematic nomenclature of the fatty acids is based on the Genevan system.
 According to this system, the fatty acid is named after the hydrocarbon with the
same number of carbon atoms.
 The suffix -oic is written in place of the final letter e in the name of the hydrocarbon.
The names of saturated fatty acids end with the suffix –anoic e.g., Octadecanoic
acid.
The names of unsaturated fatty acids end with the suffix –enoic e.g.,
Octadecenoic acid (Oleic acid).

 The position of carbon atoms in the fatty
acid chain is indicated either:
 By numbering - in which case the
carboxyl carbon is numbered as C1, the
carbon adjacent to C1 as C2 and so on;
or
 By the use of Greek letters - in which
case C2 is denoted as a-carbon, C3 as b-
carbon and so on, while the terminal
methyl (-CH3) carbon is known as w-
carbon.

 The notation most commonly used for fatty acids indicates the:
• Number of carbon atoms, and
• Number of double bonds.
 The notation 18:0 denotes an C18 fatty acid with no double bonds, while 18:1
signifies an C18 fatty acid with one double bond.

 To specify double-bond positioning within the carbon chain of an unsaturated fatty
acid, the preceding notation is expanded by adding the Greek capital letter delta (D)
followed by one or more superscript numbers.
 The notation 18:3(D9,12,15) denotes a C18 PUFA with three double bonds at locations
between carbons 9 and 10, 12 and 13, and 15 and 16.

SELECTED FATTY ACIDS OF BIOLOGICAL IMPORTANCE
Saturated Fatty Acids
Common Name Structural
Notation
Formula Common Sources
Lauric Acid 12:0 CH3(CH2)10COOH Laurel oil, Spermaceti
Myristic Acid 14:0 CH3(CH2)12COOH Butter and wool fat
Palmitic Acid 16:0 CH3(CH2)14COOH Palm Oil
Stearic Acid 18:0 CH3(CH2)16COOH Animal and plant fats
Arachidic Acid 20:0 CH3(CH2)18COOH Peanut Oil

SELECTED FATTY ACIDS OF BIOLOGICAL IMPORTANCE
Unsaturated Fatty Acids
Common
Name
Structural
Notation
Formula Common
Sources
Oleic Acid 18:1 – D9
CH3(CH2)7CH=CH(CH2)7COOH
Animal and plant
fats
Linoleic Acid 18:2 – D9,12
CH3(CH2)4CH=CH(CH2)CH=CH(CH2)7COOH
Peanut &
cottonseed Oil
Linolenic Acid 18:3 – D9,12,15
CH3(CH2CH=CH)3(CH2)7COOH Linseed Oil
Arachidonic Acid 20:4 – D5,8,11,14
CH3 (CH2)4(CH=CHCH2) 4(CH2)2COOH Animal fats

ISOMERISM IN FATTY ACIDS
 The unsaturated fatty acids exhibit geometric (or cis-trans) isomerism at the double
bonds.
 In biochemically important MUFAs, the configuration about the double bond is nearly
always cis rather than trans.
 ‘Cis’ form is comparatively unstable and is more reactive.
 For example, Oleic acid can exist in two forms: cis-oleic acid and trans-oleic acid which is
also called elaidic acid.

ISOMERISM IN FATTY ACIDS
 ‘Cis’ configuration creates a rigid
30o
bend, or kink, in the fatty acid
chain that is not found in
saturated fatty acids.
 Such a bend affects the physical
properties of a fatty acid.

ESSENTIAL FATTY ACIDS
 Essential fatty acids are those polyunsaturated fatty acids that cannot be synthesized
by our cells and we should obtain them from plants through diet.
 Linoleic acid and linolenic acid are the only essential fatty acids for animals.
 Other polyunsaturated fatty acids can be synthesized from these essential fatty acids.
 For example, arachidonic acid can be synthesized only from linoleic acid.
 Therefore, in deficiency of linoleic acid, arachidonic acid also becomes an essential
fatty acids.

IMPORTANCE OF ESSENTIAL FATTY ACIDS
 Linoleic acid is the starting material for the biosynthesis of arachidonic acid.
 Arachidonic acid is the major starting material for eicosanoids, substances that help
regulate blood pressure, clotting, and several other important body functions.
 Linolenic acid is the starting material for the biosynthesis of two additional omega-3
fatty acids.
Linolenic acid (18:3) → EPA (20:5) → DHA (22:6)

IMPORTANCE OF ESSENTIAL FATTY ACIDS
 EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are important
constituents of the communication membranes of the brain and are necessary for
normal brain development.
 EPA and DHA are also active in the retina of the eye.

NEUTRAL FATS
(TRIACYLGLYCEROLS (TAG) OR TRIGLYCERIDES)
 Triacylglycerol, also called triglycerides, are composed of three fatty acids bonded
by an ester linkage to glycerol.

TYPES OF NEUTRAL FATS
 Triacylglycerol are of two types:
1. SimpleTriacylglycerol
2. Mixed Triacylglycerol
 Naturally occurring simple triacylglycerols are rare.
 Most biochemically important triacylglycerols are mixed triacylglycerols.

1. SimpleTriacylglycerol
 These types of triacylglycerol contain
the same types of fatty acids at the
three carbon atoms.
 The triacylglycerol produced from
glycerol and three molecules of
stearic acid is an example of a simple
triacylglycerol.

ACTIVITY
Draw the structural formula of the triacylglycerol produced from the
reaction between glycerol and three molecules of Lauric acid.

2. MixedTriacylglycerol
 These types of triacylglycerol contain
more than one kind of fatty acid
molecule.
 Figure shows the structure of a
mixed triacylglycerol in which one
fatty acid is saturated, another
monounsaturated, and the third
polyunsaturated.

FATS AND OILS
FATS
 A fat is a triacylglycerol mixture that is a
solid or a semi-solid at room temperature
(25◦C).
 Generally, fats are obtained from animal
sources.
 All fats, even highly saturated fats,
contain some unsaturated fatty acids.
OILS
 An oil is a triacylglycerol mixture that is a
liquid at room temperature (25 ◦C).
 Generally, oils are obtained from plant
sources.
 All oils, even polyunsaturated oils,
contain some saturated fatty acids.

5. Write key reactions for fats and oils.
Learning
Objective

CHEMICAL REACTIONS OF TRIACYLGLYCEROLS
1. Hydrolysis
 Fats can be hydrolyzed in the presence of an acid or a base.
 Under acidic conditions, the hydrolysis products are glycerol and fatty acids.
 Under basic conditions, the hydrolysis products are glycerol and fatty acid salts.
 Within the human body, triacylglycerol hydrolysis occurs during the process of
digestion by the specific fat-splitting enzymes called lipases, for example pancreatic
lipase.

 Triacylglycerol is sequentially hydrolyzed to diacylglycerol and monoacylglycerol and
finally glycerol plus 3 fatty acids.

2. Saponification
 Hydrolysis of a fat by an alkali such as sodium hydroxide or potassium hydroxide is
called saponification.
 For fats and oils, the products of saponification are glycerol and fatty acid salts.

3. Hydrogenation
 Hydrogenation involves hydrogen addition across carbon–carbon double bonds,
which increases the degree of saturation as some double bonds are converted to
single bonds.

 Many food products are produced via partial hydrogenation.
 In partial hydrogenation some, but not all, of the double bonds present are converted
into single bonds.
 In this manner, liquids (usually plant oils) are converted into semi-solid materials.
 This is the basis of Banaspati (Dalda) manufacture, where inedible and cheap oils
like cotton seed oil are hydrogenated and converted to edible solid fat.

4. Oxidation
 Fats very rich in unsaturated fatty acids such as linseed oil undergo spontaneous oxidation
at the double bond forming aldehyde and carboxylic acid products.
 The short-chain aldehydes and carboxylic acids so produced often have objectionable
odors, and fats and oils containing them are said to have become rancid.
 To avoid this unwanted oxidation process, commercially prepared foods containing
fats and oils always contain antioxidants.
 Two naturally occurring antioxidants are vitamin C and vitamin E.

5. Rancidity
 The unpleasant taste and odor developed by most natural fats on aging is refereed to as
rancidity.
 Hydrolytic rancidity is due to partial hydrolysis of the fats due to traces of hydrolytic
enzymes present in naturally occurring fats and oils.
 Oxidative rancidity is the result of partial oxidation of unsaturated fatty acids with
resultant formation of epoxides and peroxides of small molecular weight fatty acids by
peroxides and free radicals.
 The same process, if it occurs in vivo will affect the integrity of biomembranes, leading to
cell death.

 Many natural vegetable fats and oils may contain antioxidants like vitamin E which
prevent onset of rancidity.Therefore, vegetable fats can be preserved for a longer
time than animal fats.

CHARACTERIZATION OF FAT OR
IDENTIFICATION OF FATS AND OILS (FAT CONSTANTS)
 Fat constants or numbers are test necessary to:
 Identify a pure fat
 Assess the degree of adulteration
 Determine the proportions of different types of fat in a mixture

1. Saponification Number
 Saponification number is defined as the number of milligrams of potassium
hydroxide required to saponify one gram of fat.
 It is inversely proportional to the molecular weight of fat.
 This value is high in fats containing a short chain fatty acids.
 For example, the saponification number of:
 Butter = 220
 Coconut oil = 260

2. Acid Number
 Acid number is the number of milligrams of KOH required to neutralize the free
fatty acids present in one gram of fat.
 It is used for the detection of hydrolytic rancidity because it measures the amount
of free fatty acids present.
 Acid number is directly proportional to the rancidity.

3. Iodine Number
 Iodine number is the number of grams of iodine absorbed by 100 grams of fat.
 It is an index of the degree of unsaturation and is directly proportional to the
content of unsaturated fatty acids.
 Higher the iodine number, higher is the degree of unsaturation, e.g.
 Butter = 28,
 Sunflower oil = 130

4. Acetyl Number
 Acetyl number is the number of milligrams of KOH needed to neutralize the acetic
acid liberated from hydrolysis of 1gm of acetylated fat.
 The natural fat that contains fatty acids with free hydroxyl groups are converted
into acetylated fat by reaction with acetic anhydride.
 Thus, acetyl number is a measure of the amount of hydroxy fatty acids in fat
content.

 Castor oil because of its high content of ricinoleic acid has a high acetyl number.
 Acetyl number of some oils are:
 Castor oil = 146-150
 Cod Liver oil = 1.1
 Cotton Seed oil = 21-25
 Olive oil = 10.5

4. Reichert Meissl Number
 Reichert-Meissl number is the numbers of milliliters of 0.1N alkali required to
neutralize the volatile acid obtained from 5g of a fat, which has been saponified
then acidified to liberate the fatty acids and then steam distilled.
 It is also known as volatile fatty acid number.
 Reichert Meissl value for:
 Butter = 26
 Coconut oil = 7.

 It is less than one for other edible oils.
 The admixture of certain fats may be used to prepare synthetic butter which may
simulate butter in most of the constants except RM value and hence, can be
detected.

WAXES
 Waxes are esters of long-chain saturated and
unsaturated fatty acids with long-chain monohydroxy
alcohols.
 The fatty acids range in between C14 and C36 and
the alcohols range from C16 to C36.

WAXES
 Beeswax, for example, contains a wax (1-triacontyl palmitate) with the following
structure:

WAXES
 Waxes generally have higher melting points than fats (60 to 100°C) and are harder.
 Animals and plants often use them for protective coatings.
 In plants, waxes are secreted as surface coating to prevent excessive evaporation and
to protect against parasites.
 In animals, waxes are secreted by cutaneous glands as a protective coating to keep
the skin pliable, lubricated and water-proof.

WAXES
 Important waxes include carnauba wax (from a Brazilian palm
tree), lanolin (from lamb’s wool), beeswax, and spermaceti
(from whales).
 These substances are used to make cosmetics, polishes,
candles, and ointments.
 Sperm whale wax (spermaceti) and beeswax are composed
mainly of palmitic acid esterified with either hexacosanol,
CH3(CH2)24.CH2OH or triacontanol, CH3(CH2)28.CH2OH.

6. What are Complex or Compound Lipids?
Learning
Objective

Chemistry of lipids ii

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