Organic Chemistry II / CHEM 252 Chapter 22-25 – Natural ...alpha.chem.umb.edu/chemistry/ch252/files/Overheads/...• Steroids • Steroids are important “biological regulators”

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

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Lipids

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• 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