Transcript Lipids - University of Nebraska Omaha

Lipids
Classification of Lipids
• Lipids: A variety of naturally occurring organic
compounds classified together on the basis of
common solubility properties.
• insoluble in water.
• soluble in aprotic organic solvents including diethyl
ether, dichloromethane and acetone.
• Lipids include
•
•
•
•
•
triglycerides (fats and oils)
cholesterol, steroid hormones, and bile acids
phospholipids
prostaglandins
fat-soluble vitamins
Saturated fat
Unsaturated fat
Triglycerides
• Triglyceride: An ester of glycerol with three
fatty acids.
• Saturated triglycerides are more commonly
known as fats.
• Unsaturated triglycerides are more commonly
known as oils.
Fatty Acids
• Fatty acid: A long, unbranched chain carboxylic
acid.
• Nearly all have an even number of carbon atoms,
most between 12 and 20 in an unbranched chain.
• The three most abundant are palmitic acid (16:0),
stearic acid (18:0), and oleic acid (18:1).
• (#C : #double bonds)
• In most naturally occurring unsaturated fatty acids,
the cis isomer predominates; the trans isomer is rare.
• Unsaturated fatty acids have lower melting points
than their saturated counterparts; the greater the
degree of unsaturation, the lower the melting point.
• At room temperature most saturated fatty acids are
solids and most unsaturated fatty acids are liquids.
• The greater the degree of unsaturation, the
lower the melting point.
• Note these naturally occurring unsaturated fatty acids
have cis geometry around their double bonds.
Triglycerides (again)
• The physical properties of triglycerides depend
on the properties of their fatty acid components.
• Melting point increases as the number of carbons in
their hydrocarbon chains increases and as the
number of double bonds decreases.
• Triglycerides rich in unsaturated fatty acids are
generally liquid at room temperature and are called
oils.
• Triglycerides rich in saturated fatty acids are generally
semisolids or solids at room temperature and are
called fats.
• The lower melting points of triglycerides rich in
unsaturated fatty acids are related to differences
in their three-dimensional shape.
• Hydrocarbon chains of saturated fatty acids can lie
parallel to each other with strong dispersion forces
between their chains. Their pack into well-ordered,
compact crystalline forms and melt above room
temperature.
• Because of the cis configuration of the double bonds
in unsaturated fatty acids, their hydrocarbon chains
have a less ordered structure and dispersion forces
between them are weaker. These triglycerides have
melting points below room temperature.
Reduction of Triglycerides
• The process of converting oils to fats is called
hardening and involves catalytic reduction of
some or all of an oil’s carbon-carbon double
bonds.
• In practice, the process is controlled to
produce a fat of a desired consistency.
• The resulting fats are sold for cooking (Crisco,
Spry, and others).
H2
Ni
Unsaturated fat (oil)
Saturated fat (solid)
Reduction of Triglycerides
• Margarine and other butter substitutes are
produced by partial hydrogenation of
polyunsaturated oils derived from corn,
peanuts, and soybeans.
• Catalytic hydrogenation is to some degree
reversible, hardening results in the
isomerization of some cis fatty acids to transfatty acids resulting in the hardening of an oil.
• Current nutrition science states that trans fatty
acids are to be avoided as much as possible.
Soaps and Detergents
• Natural soaps are prepared by boiling lard or
other animal fat with NaOH (lye), in a reaction
called saponification (Latin, sapo, soap) to
have the fat undergo base-catalyzed ester
hydrolysis.
• Soap is the (sodium) salt of a fatty acid.
• Soaps clean by acting as emulsifying agents.
• The long hydrophobic hydrocarbon chains of soaps
are insoluble in water and tend to cluster in such a
way as to minimize their contact with water.
• The polar hydrophilic carboxylate groups remain in
contact with the surrounding water molecules.
• Driven by these two forces, soap molecules
spontaneously cluster into micelles.
• Micelle: A spherical arrangement of organic
molecules in water clustered so that their hydrophobic
parts are buried inside the sphere and their
hydrophilic parts are on the surface of the sphere and
in contact with water.
• When soap is mixed with water-insoluble grease, oil,
and fat stains, the nonpolar parts of the soap micelles
“dissolve” nonpolar dirt molecules and they are
carried away in the rinse water.
• Soaps form water-insoluble salts when used in
water containing Ca2+, Mg2+, and Fe3+ ions (hard
water).
• These insoluble salts are sometimes known as
“soap scum”.
Detergents
• The design criteria for a synthetic detergent are
• a long hydrocarbon tail of 12 to 20 carbons.
• a polar head group that does not form insoluble salts with
Ca2+, Mg2+, or Fe3+ ions.
• The most widely used synthetic detergents are the linear
alkylbenzenesulfonates (LAS).
• Also added to detergent preparations are foam stabilizers,
bleaches, and optical brighteners.
• Soaps and detergents are part of class of
compounds called surfactants.
• Surfactants affect the properties of a solution by
lowering surface tension. (for example, between
the solution and grease)
• The properties of a surfactant depend on the
charge on the molecule. There are three broad
categories of surfactants.
• Anionic surfactants
• Cationic surfactants
• Nonionic surfactants
Anionic Surfactants
Cationic Surfactants
O
-
O S O Na
CH3
+
N CH3
O
CH3
sodium dodecyl sulfate
hexadecyltrimethylammonium bromide
O
-
S O Na+
O
sodium dodecylbenzenesulfonate
LAS (linear alkyl sulfonate)
Non-ionic Surfactants
OH
O
O
OH
O
O
O
O
polyethylene glycol
OH
OH
pentaerythrityl palmitate
O
Br
OH
Phospholipids
• Phospholipids are the second most abundant
group of naturally occurring lipids.
• They are found almost exclusively in plant and animal
membranes, which typically consist of 40% to 50%
phospholipids and 50% to 60% proteins.
• The most abundant phospholipids are derived from
phosphatidic acid, which is glycerol esterified with two
fatty acids and one phosphoric acid.
• Further esterification with a low-molecular weight
alcohol with a quaternary amine gives a phospholipid.
• The three most abundant fatty acids in phosphatidic
acids are palmitic acid (16:0), stearic acid (18:0), and
oleic acid (18:1).
• A phosphatidate and a phospholipid
Lipid Bilayer
• When placed in aqueous solution, phospholipids
spontaneously form a lipid bilayer.
• Polar head groups lie on the surface, giving the
bilayer an ionic coating.
• Nonpolar fatty acid hydrocarbon chains lie buried
within the bilayer.
• This self-assembly is driven by two noncovalent
forces.
• Hydrophobic effects, which result when nonpolar
hydrocarbon chains cluster to exclude water
molecules.
• Electrostatic interactions, which result when polar
head groups interact with water and other polar
molecules in the aqueous environment.
Biological Membranes
• Fluid mosaic model: A biological membrane
consists of a phospholipid bilayer with proteins,
carbohydrates, and other lipids embedded on
the surface and in the bilayer.
• Fluid signifies that the protein components of
membranes “float” in the bilayer and can move freely
along the plane of the membrane.
• Mosaic signifies that the various components of the
membrane exist side by side, as discrete units rather
than combining to form new molecules and ions.
Steroids
• Steroids: A group of plant and animal lipids that
have the tetracyclic ring structure shown below.
• Features common to steroids.
• The fusion of rings is trans and each atom or group at
a ring junction is axial.
• The pattern of atoms or groups along the ring
junctions is nearly always trans-anti-trans-anti-trans.
• The steroid system is nearly flat and quite rigid.
• Most have axial methyl groups at C-10 and C-13.
Examples of Steroids
Cholesterol
Testosterone
Progesterone
Androsterone
Estrone (estrogen)
Cortisone
Prostaglandins
• Prostaglandins: A family of compounds that
have the 20-carbon skeleton of prostanoic acid.
• Note the 5-membered ring in the middle.
• Prostaglandins are not stored in tissues as such,
but are synthesized from membrane-bound 20carbon polyunsaturated fatty acids in response
to specific physiological triggers.
• One such polyunsaturated fatty acid is arachidonic
acid.
• Among the prostaglandins synthesized
biochemically from arachidonic acid are
PGE2 and PGF2.
• Both hormones are used to induce labor in
pregnant women.
Fat-Soluble Vitamins
• Vitamins are divided into two broad classes on
the basis of their solubility.
• Those that are fat soluble (and hence classified as
lipids.
• Those that are water soluble.
• The fat-soluble vitamins include A, D, E, and K.
• Vitamin A (Retinol)
• Used in vision.
• Vitamin D
• Needed for absorption
of minerals.
• Vitamin E
• Antioxidant
• Vitamin K
• Needed for
coagulation