محاضرات الكيمياء الطبية الدكتورة نوال عبدالله مرتضى
The Chemist’s View of Carbohydrates
Carbohydrates are made of carbon, hydrogen and oxygen atoms.
Carbohydrates (glycans) have the following basic composition:
At the molecular level, most carbohydrates are polyhydroxyaldehydes, polyhydroxyketones, or compounds that yield either of these after hydrolysis. Therefore, the chemistry of carbohydrates is essentially the chemistry of hydroxyl groups and carbonyl groups, and of the acetal bonds formed between these two functional groups.
Classification of Carbohydrates.
A- Number of carbohydrate units
1- Monosaccharides (simple carbohydrates) :one carbohydrate unit .
.2- Disaccharides (complex carbohydrates):two carbohydrate units
. 3- Trisaccharides: three carbohydrate units
. 4- Polysaccharides: many carbohydrate units
B-Position ofcarbonyl group
at C1, carbonyl is an aldehyde: aldose .
at any other carbon, carbonyl is a ketone: ketose .
C-Number of carbons
three carbons: triose ‚
four carbons: tetrose ‚
five carbons: pentose ‚
six carbons : hexose ‚
seven carbons : heptose ‚ etc.
D- Cyclic form
Monosaccharides
Monosaccharrides : are single sugars (most are hexoses). 1- Glucose : serves as the essential energy source, and is commonly known as blood sugar or dextrose.
2- Fructose : is the sweetest, occurs naturally in honey and fruits, and is added to many foods in the form of high-fructose corn syrup.
3- Galactose : rarely occurs naturally as a single sugar.
Disaccharides
Disaccharides : are pairs of monosaccharides, one of which is always glucose Condensation reactions link monosaccharides together. Hydrolysis reactions split molecules and commonly occur during digestion.
Maltose : consists of two glucose units. It is produced during the germination of seeds and fermentation.
Sucrose : is fructose and glucose combined. It is refined from sugarcane and sugar beets, tastes sweet, and is readily available.
Lactose : is galactose and glucose combined. It is found in milk and milk products.
The Complex Carbohydrates
Few (oligosaccharides) or many (polysaccharides) glucose units bound / linked together in straight or branched chains.
1- Glycogen
Storage form of glucose in the body Provides a rapid release of energy when needed . Starches Storage form of glucose in plants Found in grains, tubers, and legumes . A glycogen molecule contains hundreds of glucose units in highly branched chains. Each new glycogen molecule needs a special protein for the attachment of the first glucose (shown here in red).
Dietary fibers provide structure in plants, are very diverse, and cannot be broken down by human enzymes . Soluble fibers are viscous and can be digested by intestinal bacteria ( this property is also known as fermentability ) . These fibers are found in fruits and vegetables. Insoluble fibers are nonviscous and are not digested by intestinal bacteria . These fibers are found in grains and vegetables .
Sugar Nomenclature
For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers . D & L sugars are mirror images of one another. They have the same name, e.g., D-glucose & L-glucose. Other stereoisomers have unique names,
e.g., glucose, mannose, galactose, etc. The number of stereoisomers is 2n, where n is the number of asymmetric centers. The 6 - C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars).
Furan Pyran
Cyclic Forms of Carbohydrates:Pyranose Forms.
Pentoses and hexoses can cyclize as the ketone or aldehyde reacts
with a distal OH. Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring, named after pyran.These representations of the cyclic sugars are called Haworth projections.
Fructose forms either a 6 - member pyranose ring, by reaction of the C2 keto group with the OH on C6, or a 5-member furanose ring, by reaction of the C2 keto group with the OH on C5.
Cyclization of glucose produces a new asymmetric center at C1.
Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1:
a (OH below the ring)
b (OH above the ring).
Because of the tetrahedral nature of carbon bonds, pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar.
The representation above reflects the chair configuration of the glucopyranose ring more accurately than the Haworth projection.
Reactions of Monosaccharidesand Properties
1-Action of acids:
Monosaccharides on treatment with strong concentrated sulphuric acid undergoes dehydration to give furfural or furfural derivatives which on condensation with α – naphthol yield a violet or purple colored complex. Pentoses yield furfural whereas hexoses yield 5- hydroxyl furfural .
+ 3H2O Conc. H2SO4 ------˃+
D-Ribose Furfural
+ Conc. H2SO4 ------˃+3H2O
2- Mutarotation
Mutarotation is defined as the change in specific rotation of optically active solution without any change in other properties . When glucose is dissolved in water , the optical rotation of the solution gradually changes and attains an equilibrium value . The change in optical rotation is called mutarotationMutarotationoccurs due to the cyclization of open chain form of glucose into α or β form with equal probability .This α and β cyclic form of glucose have different optical rotations. They differ in configuration about the anomeric carbon( C1) but have the same configuration at C2, C3, C4, and C5 asymmetric carbons. These cyclic forms are in equilibrium with open chain structure in aqueous solution .such a change from a single form to an equilibrium mixture that includes its other form is called mutarotation.
3- Reducing property
Monosaccharides by virtue free aldehydic or ketonic group in their structure , i.e., presence of free anomeric carbon atom, reduces certainheavy metallic cation, e.g., Cu+2 ions in alkaline solution at high temperature.
So all the reducing sugars will give Benedicts qualitative test and Fehling test positive . The reaction is as follows :
CuSO4↔ Cu++ + SO4--
Reducingsugars + Na2CO3 ------>Enediol form of reducing sugar Cu+2 + Enediols+ high temp.------> Cu+ + Mixture of sugar acidsCu+ + OH- + Δ ------> CuOH
2CuOH + Δ ------> Cu2O ( cuprous oxide ) + H2O
Benedict՚s qualitative reagent contains cupric sulphate , sodium carbonate and sodium citrate whereas Fehling solution contains cupric sulphate , sodium hydroxide and sodium potassium tartrate ( Rochelle salt ) . Benedict՚s qualitative reagent is preferred above Fehling solution because it is stable .
4- Osazone Formation
It involves two reactions . Firstly glucose with phenylhydrazine gives glucosephenylhydrazone by elimination of a water molecule from the functional group. The next step involves reaction of one mole of glucosephenylhydrazone with two moles of phenylhydrazine (excess). First phenylhydrazine is involved in oxidizing the alpha carbon to a carbonyl group, and the second phenylhydrazine involves in removal of one water molecule with the formyl group of that oxidized carbon and forming the similar carbon nitrogen bond . The alpha carbon is attacked here because its more reactive than the others .Osazones are highly coloured and crystalline compounds and can be easily detected . Glucose gives broomstick or needle shaped crystals with this whereas maltose gives sunflower shaped crystals .
5- Action of dilute alkali
Monosaccharides on treatment with dilute alkali undergo a variety of molecular transformation through enediolformation .
The enediols of sugars are good reducing agents and form the basis of reducing action of sugars in alkaline medium . When glucose is treated with dilute alkali for several hours, the resulting mixture obtained contains both fructose and mannose in addition to glucose .A similar mixture of same sugars is optained with any of the other towsugars . Mannose
║
Enediol = fructose = glucose
mannose
║
6- Oxidation
Aldoses are oxidized under variety of conditions to the following :
1- Aldonicacid : Whereby the first carbon atom (C-1) is oxidized to carboxyl group only . The rest of the molecule structure remains unaffected .
2- Uronicacid : Whereby the terminal carbon atom is oxidized to carboxyl group only .
3- Aldaric acid or saccharicacid : Whereby both the first carbon atom, i.e., aldehydic group and the terminal carbon atom , i.e., primary alcoholic group are oxidized to carboxyl group .
7 – Fermentation
Fermentation is the process of breakdown of complex organic substances into smaller substances with the help of Glucose is fermented to ethyl alcohol and carbon dioxide by yeast . Hence this process is called alcoholic fermentation as alcohol is produced .
Lactobacilli
C6H12O6 ------> 2CH2CH.OH.COOH
galactose Sterptococcoi lactic acid
8- Ester formation
Monosaccharides interact with phosphoric acid to give phosphoric sugars, and this plays an important role in the metabolic processes of carbohydrates
9- Glycoside or acetal formation
Glycosides are sugar derivatives in which hydrogen of the hydroxyl group of hemiacetal or hemiketal form of the sugar is replaced by an organic moiety. A molecule of water is eliminated when this reaction takes place . Glycosides are not reducing sugars and do not show mutarotation . I f the organic moiety is derived from another monosaccharide , the product formed is disaccharide. If the organic moiety is a noncarbohydrate, then it is called a glycone. A glycone: The noncarbohydrate portion of the glycoside is called the a glycone or a glucone . Glycosides do not reduce alkaline copper sulphate because sugar group is combined , i.e., aldehyde group is converted to an acetal group .
Gcosides = Carbohydrate + Carbohydrate part or noncarbohydrate part (a glycone).
D - glucose
+
CH3OH
↓
Types of glycosidic bonds in carbohydrate
( 1:4) - α - D-Glucosidic linkage in maltose .
( 1:4) - β - D - Glucosidic linkage in lactose .
α ( 1 ) → β ( 2 ) - D - Glucosidic linkage in sucrose .
α ( 1 → 6 ) - D - glycosidic linkage inisomaltose ( a disaccharide is derived from the branch point of starch ).
α ( 1 , 4 ) and α ( 1 , 6 ) glycosidic linkages in starch.
β ( 1 , 4) glycosidic1inkage in cellulose.
Α ( 1 , 4 ) and α ( 1 , 6 ) glycosidic linkage in glycogen.
β ( 1,4 ) and β ( 1,3 ) glycosidic linkage in hyaluronic acid.
α ( 1,4 ) glycosidic linkage in heparin .
Poly saccharides :
Polysaccharides have two important biological functions :
1- As storage form of fuel (i.e. glycogen of animal origin and starch of plant origin ).
2- As structural components .
Polysaccharides can be divided into two groups :
-Homopolysaccharides .
-Heteropolysaccharides .
Homopolysaccharides .
Starch
Native starch is a mixture of two polysaccharides .
a- Amylose : is a linear unbranched molecule in which D- glucose units are linked by a ( 1 → 4 ) glycosidic linkages . It is water soluble and gives blue color with iodine .
b- Amylopectins :is a branched chain molecule in which D – glucose units in addition to α- ( 1→4 ) linkages are branched by α- ( 1→6 ) glycosidic linkages . This branching occurs on an average of 24 to 30 D- glucose units. It is water insoluble and gives violet color with iodine . Starch is a non reducing polysaccharide, on hydrolysis with dilute mineral acids, i.e. with hydrochloric acid gives glucose only .
Starch
2- Cellulose
Cellulose is a linear polymer of D – glucose units joined together by β (1,4 ) glycosidic linkages. On partial hydrolysis, cellulose yields β – 1,4 disaccharide cellobiose instead of maltose. Cellobiose is a disaccharide with the formula [HOCH2CHO(CHOH)3]2O. is a reducing sugar, consists of two β-glucose molecules linked by a β(1→4) bond. It can be hydrolyzed to glucose enzymatically or with acid. Cellobiose has eight free alcohol (OH) groups, one acetal linkage and one hemiacetal linkage, which give rise to strong inter- and intra-molecular hydrogen bonds. It can be obtained by enzymatic or acidic hydrolysis of cellulose and cellulose rich materials such as cotton, or paper.
3- Glycogen:
Glycogen is the carbohydrate reserve of the body . Glycogen is also called animal starch, because it serves as nutritional reservoir in animal tissues. Glycogen is a highly branched chain molecule in which glucose unit in addition to linear α (1,4) linkages are also linked by α (1,6) at the branched point . This branching repeats after every 8 -10 glucose units. Glycogen is water soluble and has no reducing property. It gives red color with iodine.
4- Dextrins
They are the partial hydrolytic products of starch by α-amylase, β –amylase and acids. All dextrins have free sugar group and accordingly reduced alkaline copper sulphate solution.
b- Heteropolysaccharides
1- Hyaluronic acid
2- Heparin
heparin
3-Chondroitin sulphates.
4-Sialic Acids.
mesotartaric acid
D - glyceraldehyde
- glyceraldehyde (+)
l - glyceraldehyde
- glyceraldehyde(−)
Chemistry of Lipids
Lipids (fats)
Lipids :are one of the large biological molecules (Carbohydrates, fats, proteins and nucleic acids) ,substances such as a fat, oil or wax that dissolves in alcohol , non polar solvents like ether, chloroform, benzene, etc., but not in water . Associated with them are various fat soluble , non- lipid substances which includes carotenoid pigments (are organic pigments that are found in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms, including some bacteria and some fungi. Carotenoids can be produced from fats and other basic organic metabolic building blocks by all these organisms),and certain vitamins, i.e., vitamins A, D, E and K. Lipids contain carbon, hydrogen and oxygenatoms andthey are an important part of living cells which widely distributed throughout both plant and animal kingdom and are essential constituents of cell membrane. Fats are said to be protein sparing because their availability in the diet reduces the need to burn proteins for energy. Lipids consist about 5% of the organic materials used in the structure of a living cell,and there are about 40-50 type of lipid molecules in the cell. The brain cells and especially nerve tissues are rich in complex lipid compounds. Some lipids contain ionic groups as phosphates or cholines but the largest part of the lipid molecule is non polar. Theessential building units of lipids are mostly consist of fatty acids, glycerol, sphingosine,andsteroids.
Lipids biological functions
Lipids have several important biological functions:
1-They serve as the reservoir of energy because of their :
a- High energy content. The calorific value is 9 Kcal / gm as compared to carbohydrates which have calorific value of 4 Kcal / gm. b- Storage in concentrated form in water free state in the tissues as compared to carbohydrates which are highly hydrated and cannot be stored in such concentrated form.
2- As structural components of cell membranes.
3- As transport forms of various metabolic fuel.
4- As protective coating on the surface of many organs such as kidney, against injury.
5- To facilitate the absorption of the fat soluble vitamins A, D, E and K.
Dietry fat can be divided into two types :
a- Visible fat or fat consumed as such, e.g. butter, oil, ghee.
b- Invisible fat or fat present as part of other foods items, e.g. egg, fish, meat, cereal, nuts, etc.
Classification of lipids
1- Neutral lipids( glycerides ) ( ester compounds for fatty acids with glycerol ).
2-Phosphoglycerides (phospholipids):
(ester phosphate for glyceride compounds it may also contains nitrogen compound).
3- Sphingolipids (sphingosine , fatty acid, phosphate group, nitrogen compound).
4-Glycolipids(compounds contain fatty acid, alcohol, sugar).
5- Lipoproteins(compounds contain lipids and proteins).
6- Waxes(ester compounds for fatty acids, mono hydroxyl alcohols).
7- Steroids(derivatives of cyclic alcohol compounds).
8- Terpens(derivatives for polymers contain condensed isoprene units.
Fatty acids
a- Fatty acids in nature as such are not very abundant but are present as ester.
b-Fatty acids are derivatives of lipid because theyinterfere in the formation of various types of lipids.
c- Fatty acids are represented as general formula R— COOH.
General points about fatty acids:
1- They are monocarboxylic acids.
2- Number of carbon atoms are even, though odd number fatty acids exist but are very rare.
3- They may be saturated or may be unsaturated.
If unsaturated they can be monounsaturated acid or polyunsaturated acid. Mammals and plants contain both mono saturated and poly unsaturated fatty acids whereas all the fatty acids containingdouble bonds that are present in bacteria are monounsaturated. Plant and fish fats contain more polyunsaturated fatty acids than animal fats. The double bonds in a polyunsaturated fatty acid are neither adjacent nor conjugated since this would make the structure to easily oxidisable when exposed to environment oxygen. 1- The most common among the saturated fatty acids are palmitic acid (C16), stearic acid (C18) and among the unsaturated fatty acid, oleic acid (C18).
2- Unsaturated fatty acids have lower melting point than saturated fatty acids of same chain length. 3- Fatty acids with odd number of carbon atoms occur in trace amounts in terrestrial and marine animals.
4- Fatty acids with one to eight carbons are liquids at room temperature while those with more carbon atoms are solids. 5- The presence of double bond in the molecule gives rise to geometric isomerism. All naturally occurring unsaturated long chain fatty acids are found in cis isomer. 6- Most plant fats are liquid since they contain a large proportions of unsaturated fatty acids with melting points. 7- Animal fats, on the other hand, contain a high proportion of palmiticand stearic acids, and are solid or semi – solid at room temperature. Milk fat is unusual in containing a high proportion of shorter chain (C4 – C14) fatty acids.
The most common fatty acids in neutral fats are :
Formula / No. of atomsCH3 — (CH2)2 — COOH
CH3 — (CH2)4 — COOH
CH3 — (CH2)10 — COOH
CH3 — (CH2)14 — COOH
CH3 — (CH2)16 — COOH
CH3 — (CH2)7 —CH═CH—(CH2)7—COOH / 4
6
12
16
18
18 / Butyric acidCaproic acid
Lauric acid
Palmitic
Stearic acid
Oleic acid
Naturally occurring straight chain saturated fatty acid
Systematic name / Type of chain / Common name / No. of C atomsn- Ethanoic acid
n- Propanoic acid
n- Butanoic acid
n- Octanoic acid
n- Decanoic acid
n- Dodecanoic acid
n- Tetradecanoic acid
n- Hexadecanoic acid
n- Octadecanoic acid
n- Eicosanoic acid / Short chain
Short chain
Short chain
Medium chain
Medium chain
Long chain
Long chain
Long chain
Long chain
Long chain / Acetic acid
Propionic acid
Butyric acid
Caprylic acid
Capric acid
Lauric acid
Myristic acid
Palmitic acid
Stearic acid
Arachidic acid / 2
3
4
8
10
12
14
16
18
20
Structures
The first three fatty acids are known as the volatile fatty acidsVFA's Acetic Acid (C 2:0):
Propionic Acid (C 3:0):
Butyric Acid (C 4:0):
Caproic Acid (C 6:0):