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Organic Chemistry II / CHEM
252
Chapter 22-25 – Natural
Compounds
Bela Torok
Department of Chemistry
University of Massachusetts Boston
Boston, MA1
Carbohydrates
2
Classification of Carbohydrates
• general formula Cx(H2O)y
• Carbohydrates are polyhydroxy aldehydes or ketones or substances that
hydrolyze to yield polyhydroxy aldehydes and ketones
• Monosaccharides cannot be hydrolyzed to simpler carbohydrates
– Disaccharides can be hydrolyzed to two monosaccharides
– Oligosaccharides yield 2 to 10 monosaccharides
– Polysacccharides yield >10 monosaccharides
Carbohydrates are synthesized in
plants by photosynthesis
Carbohydrates act as a repository
of solar energy
Carbohydrates
3
• Monosaccharides
– Classification: (1) The number of carbon atoms (2) aldehyde or ketone
– D and L Designations of Monosaccharides
• The simplest: glyceraldehyde (chiral) and dihydroxyacetone (achiral)
(+)-glyceraldehyde - (D); (-)-glyceraldehyde - (L)
Carbohydrates
4
– Structural Formulas
• Fischer projections
• Glucose exists in cyclic hemiacetal forms
α-anomer: C1 OH trans to the -CH2OH
β-anomer: C1 OH cis to the -CH2OH
• The flat representation is called a
Haworth formula; glucose actually exists
in the chair form
• Mutarotation
Carbohydrates
5
• The D Family of Aldoses - Most biologically important aldoses are D
Carbohydrates
6
• Disaccharides
– Sucrose (sugar) is a disaccharide formed from D-glucose and D-fructose
– Sucrose is a nonreducing sugar because of its acetal linkage
- Lactose (milk sugar) – galactose+glucose
Carbohydrates
7
• Polysaccharides
• Homopolysaccharides vs heteropolysaccharides
• A polysaccharide made up of only glucose is called a glucan
– Three important glucans are starch, glycogen and cellulose
– Starch
• The storage form of glucose in plants is called starch
• The two forms of starch are amylose and amylopectin
• Amylose consists typically of more than 1000 D-glucopyranoside units
connected by α linkages between C1 of one unit and C4 of the next
Amylopectin is similar to amylose but has branching points every 20-
25 glucose units
8
Carbohydrates
– Glycogen
• the major carbohydrate storage molecule in animals
• similar to amylopectin except that glycogen has far more branching
– Branching occurs ever 10-12 glucose units in glycogen
• Glycogen is a very large polysaccharide
– The large size of glycogen prevents if from leaving the storage cell
– The storage of tens of thousands of glucose molecules into one
molecule greatly relieves the osmotic problem for the storage cell
(this would be caused by the attempted storage of many individual
glucose molecules)
– The highly branched nature of glycogen allows hydrolytic enzymes to
have many chain ends from which glucose molecules can be
hydrolyzed
• Glucose is the source of “ready energy” for the body
– Long chain fatty acids of triacylglycerols are used for long term
energy storage
Carbohydrates
9
– Cellulose
• In cellulose, glucose units are joined by β-glycosidic linkages
• Cellulose chains are relatively straight
• The linear chains of cellulose hydrogen bond with each other to give the
rigid, insoluble fibers found in plant cell walls
– The resulting sheets then stack on top of each other
• Humans lack enzymes to cleave the β linkages in cellulose and so cannot
use cellulose as a source of glucose
Carbohydrates
10
– Chitin
• Structural polysaccharide – e.g. crustaceans
• Leathery, could be strengthened by CaCO3 – mauch harder
• Stabilize foods, drugs
• Surgical threads
Lipids
11
• Introduction
• Lipids are compounds of biological origin that dissolve in nonpolar
solvents such as chloroform and diethyl ether
– Lipids are defined by the physical operation used to isolate them
– Lipids include a variety of structural types
Lipids
12
• Fatty Acids and Triglycerides
• Most long-chain carboxylic acids of biological origin are found as
esters (b) of glycerol(a)
– Oils from plants and fats of animal origin are triacylglycerols
• Hydrolysis of tracylglyerols yields fatty acids
• Most natural fatty acids are unbranched with an even number of carbon
– In natural unsaturated fatty acids the double bonds are all cis and
are usually not conjugated
Lipids
13
• Saturated fatty acids have higher melting points than unsaturated fatty
acids - Saturated fatty acids pack well
• Cis double bonds in unsaturated fatty acids put bends in the chain
– Unsaturated fatty acid chains pack poorly and have weaker van der
Waals attractions between molecules than saturated fatty acids
– Unsaturated fatty acids have lower melting points than saturated fatty
acids with the same number of carbons
• Triacylglycerols with a higher content of saturated fatty acids have higher
melting points
– Triacylglycerols in animal fats contain mostly saturated fatty acids
and are solids are room temperature
– Triacylglycerols in oils have a large proportion of unsaturated and
polyunsaturated fatty acids and are therefore liquids are room
temperature
Lipids
14
Lipids
15
– Hydrogenation of Triacylglycerols
• vegetable oils can be partially hydrogenated to yield solid cooking fats
• Partial hydrogenation isomerizes some of the cis double bonds to trans
– “Trans” fats have been associated with increased risk of
cardiovascular disease– Biological Functions of Triacylglycerols
• Triacylglycerols are primarily used as an energy reserve in animals
• more than twice the amount of energy per gram that carbohydrates
• Fats are a form of long-term energy storage, carbohydrates are a source
of rapid-release energy
– Saponification of Triacylglycerols
• Basic hydrolysis of triacylglycerols yields salts of carboxylic acids and
glycerol
Lipids
16
• Salts of long-chain carboxylic acids are called soaps
– In water, soaps exist in soluble spherical clusters called micelles
• Micelles have the hydrophilic carboxylate group of the fatty acid salt on
the outside exposed to water
– The nonpolar hydrophobic hydrocarbon chains of the fatty acids are
oriented toward the interior of the micelle (away from water)
• Soaps clean by incorporating greasy (hydrophobic) dirt molecules into the
hydrophobic alkyl portion of micelles
Lipids
17
• Terpenes and Terpenoids
• Terpenes and terpenoids are found in odoriferous essential oils of plants
– They are lipids that contain 10, 15, 20 or 30 carbons
– Terpenoids are terpenes that contain oxygen
• Terpenes are formally viewed as coming from C5 units called isoprene
– Isoprene itself is not involved in the biosynthesis of terpenes,
however (See Special Topic D)
Lipids
18
Lipids
19
• Steroids
• Steroids are important “biological regulators”
– Structure and Systematic Nomenclature of Steroids
• Steroids have a characteristic tetracyclic ring structure
– The rings are given letter designations A-D
– Steroid carbons are numbered as shown
Lipids
20
– Cholesterol
• Cholesterol is the most widely occurring steroid and the biosynthetic
precursor of all other steroids
• The human body makes sufficient cholesterol for its needs
– Dietary cholesterol usually causes the body to make less of its own
– High levels of blood cholesterol have been implicated in development
of arterioschlerosis (hardening of the arteries) and in heart attacks
Lipids
21
• Cholesterol is found in the body with other lipids and proteins
– These aggregates are called chylomicrons, high-density
lipoproteins (HDLs) and low-density lipoproteins (LDLs) and are
generally in micelle form
– They serve to transport water-insoluble cholesterol and other lipids
• HDLs (“good cholesterol”) carry lipids from the tissues to the liver for
degradation and excretion
• LDLs (“bad cholesterol”) carry biosynthesized cholesterol from the
liver to tissues
• Chylomicrons carry dietary lipids from the intestines to the tissues
Lipids
22
• Phospholipids and Cell Membranes
• Most phospholipids are derived from phosphatidic acid
– A phosphatidic acid is a glycerol molecule esterified at one terminal
hydroxyl group with phosphoric acid and the other hydroxyl groups
esterified with fatty acids (it is a diacylphosphoglycerate)
– Phosphatides
• When the phosphate group of a phosphatidic acid is bound to one of the
following nitrogen-containing groups, a phosphatide is the result
Lipids
23
• Phophatides form lipid bilayers in biological systems
– See the central portion of diagram (b) below
Lipids
24
Lipids
25
• Sphingolipids
• Sphingolipids are derived from sphingosine
• Sphingolipids are components of myelin, the protective coating of axon
nerve fibers
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