Mahdi Sharawi

Asma Karamah

6

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Mamon Ahram

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Carbohydrates

Note: Anything written in violet is an addition from my understanding, it might help you understand an

unclear point, studying it is optional.

First of all those are what we going to discuss:

What are Carbohydrates?

- Carbohydrates are polyhydroxy Aldehydes or Ketones.

Polyhydroxy Means they contain more than one Hydroxyl (OH) Group

- Saccharide is another name for a Carbohydrate

Functions: - Source of Energy:

- Carbohydrates are used as fuel in organisms through several mechanisms

such as: Glycolysis.*in the other word* oxidation of carbohydrates is the

central energy-yielding pathway in most nonphotosynthetic cells.

- Structure:

1) Insoluble carbohydrate polymers serve as structural components and protective

elements of several classes of organisms such as Cellulose in tress and chitin.

- Building blocks for bigger structures, or Modifications done to other

molecules, for instance:

Lec6 aims

Chirality and

isomerism

Classifications

of

carbohydrates

Introduction to

cyclic

carbohydrates

Carbohydrates

as a concept

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Lipids Glycolipids

Proteins Proteoglycans

- Cell Recognition and adhesion :

- Cell surface Receptors like those found on Immune cells.

Classification of Carbohydrates:

There are 3 criteria for classification of carbohydrates:

1) Complexity of carbohydrates which reflects the Amount of Sugars involved

2) size of carbon skeleton

**note:

Most carbohydrates are found in their Bound form rather than

Simple Sugars.

**Remember that :

covalent–binding of molecules requires loss of H2O molecules in a

dehydration reaction

1. By Complexity of the Molecule: (Amount of Sugars that are involved)

- Monosaccharides:

For example: A single Glucose molecule

- Basic Formula: (CH2O) n (n is between 3-7 usually).

- At least they contain two hydroxyl groups.

**notice that the essential functional groups involved in carbohydrates structure are:

1) carbonyl group found in both Aldehydes and ketones.

2) multiple hydroxyl groups.

- Disaccharides:

- Formed by joining 2 Monosaccharides together. In a dehydration reaction.

- Oligosaccharides: From 3 – 10 Monosaccharides joined together.

- Polysaccharides : A lot of Monosaccharides joined together like Starch.

Natural Forms of Carbohydrates: (Refer to Slide 3, Page 5)

- Most carbohydrates are found in their Bound form rather than Simple

Sugars.

**NOTES:

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Depending on the location of C=O group a sugar is either aldose or ketose .

The functional group in Aldehydes (Aldoses) is found at the Terminal, and the

functional group of the Ketone (Ketoses) is found inside the chain itself.

The Carbon-Hydroxyl Structure is repeated along the chain to prolong it.

"ose ending" represents sugar ,GlucOSE, FructOSE,…ETC.

Always consider that Aldehyde Sugars become Aldoses. Ketones Sugars become Ketoses .

You should know the functional groups very well.

Both Aldehydes and Ketones have the Carbonyl functional group.

Hence: Dr. Maamon says that we should always keep the Functional

group of the Aldose sugar on top. And usually the Ketose functional

group is carbon number 2.

It's carbon number 2 because as we learned in organic Chemistry we start numbering from its side.

*The following are common Monosaccharides that the doctor mentioned and are

found in the slides:

Glucose: - Mild sweet flavor

- Known as blood sugar

- Essential energy source

- Found in every disaccharide and polysaccharide

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Galactose:

- Hardly tastes sweet & rarely found naturally as a single sugar

Fructose: - Sweetest sugar, found in fruits and honey

- Added to soft drinks, cereals, deserts

Fructose is used in soft drinks as a sweetener and it's harmful for the human body.

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*All of these Aldoses are for Memorization

*These Ketoses are for Memorization

Classification of Carbohydrates: We spoke earlier about the first way of classification which depends on the complexity, now let's discuss the 2nd way of classification:

2. The number of carbon atoms involved in the chain: Triose: Three carbons

Tetrose: Four Carbons

Pentose: Five Carbons

Hexose: Six Carbons

Heptose: Seven Carbons

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Three carbons are the smallest number in a sugar( monosaccharide ) and they're found in: 1- Glyceraldehyde ( Aldose)

(Trioses) 2- DihydroxyAcetone (Ketose)

These are called Trioses Because they contain 3 Carbons only.

So after all of this we can name a sugar depending on its functional group location (Aldehyde or Ketone) and the number of Carbons in that molecule. For instance: - An Aldehyde sugar with 4 carbons is called : Aldotetrose - A Ketone sugar with 5 carbons is called : Ketopentose

Notice :Fructose is a ketohexose because it contains the ketone functional

group and 6 carbons.

Chirality: (Here we see our organic's specialists!!) *the property of nonsuperimposability:

*If we take a look at Glyceraldehyde we can notice that the number 2 carbon is bonded to 4 different groups, we call this Carbon a Chiral Carbon.

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**So a Chiral carbon is: A carbon atom bonded to 4 different groups.*is

an asymmetric atom*

3. The third way to classify sugars is depending on their spatial structure: - Isomers: Molecules that share the same Molecular Formula but have

different structures in space. For instance: C6H12O6 is the Molecular formula for Glucose/Mannose/Galactose , What is different is their Spatial Structure.

- The Question here is how could spatial arrangement ( different structures in space) of sugars parts be a source of their diversity????? Firstly recall that different structural formula leads to formation of

isomers which are classified into types such as stereoisomers (with same bond pattern, same molecular formula, different arrangement in space).

Isomers are divided into:

A. Stereoisomers: Same Molecular Formula, Same bond pattern, different arrangement in space. How can I find out the total number of possible

stereoisomers for a molecule? By spotting all the Chiral Carbons in that molecule, then using the formula, 2𝑛where n denotes the number of CHIRAL CARBONS. **Example: Aldohexoses ( 6 Carbon Atoms, Refer to Slide for structure 13) This sugar has 4 Chiral Carbons, so if we apply the equation we will get 24 which is equal to 16, which means we have 16 Different structures in space. A good practice after you have memorized the structures I mentioned earlier which are required from us, is to go this slide and try to find each of the following molecules from its structural arrangement : Glucose, Galactose, Mannose

Now even Stereoisomers are divided into different Categories which are:

1. Enantiomers: Stereoisomers which are mirror images of each other. Like the human hand, if you put them face to face they're mirror images, but they're not super imposable (Don't fit over each other).

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If molecules are super imposable it means that they're identical molecules. Enantiomers are either: L or D, L stands for Levorotatory and D stands for

Dextrorotatory The doctor advises us to consider L as Left and D as Right.

But Left and Right in terms of what?!

- "The designation of L or D depends on the configuration at the highest numbered chiral carbon atom"

- in other words - In consideration of the farthest Chiral carbon from the functional group, and

this carbon has a Hydroxyl group. In case of determining Left or Right, we shall take 3 conditions into consideration: 1- Farthest Chiral carbon from the Functional group ( Aldehyde or Ketone ) 2- Does it have a Hydroxyl Group 3- The side where this hydroxyl group resides is it left or right.

Refer to slide 3 Page 16 and Answer the following questions:

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1- How many Stereoisomers do we have for this molecule? (Hint: Chiral Carbons)

o 4 2- Are these Molecules considered Enantiomers?

o Yes Let's go back to Stereoisomers types, We mentioned the first type which is Enantiomers… Now:

2** Diastereomers: They are Stereoisomers which are not mirror images of each

other.

Not mirror Images, Not Identical, Not super imposable, Same

molecular formula Diastereomers

Diastereomers can be Epimers or Non-Epimers: - Epimers: Diastereomers which differ in 1 Hydroxyl group position on a Chiral

Carbon. The molecule above is A diastereomer and an Epimer.

** If a Diastereomer differs in 2 Chiral carbons orientation, it is NOT an epimer.

Reactions of Monosaccharides: - Aldehyde + Alcohol Hemi-Acetal + Alcohol Acetal

- Ketone + Alcohol Hemi-Ketal + Alcohol Ketal

The difference Between (Hemi-acetal and Hemi-ketal), also (Acetal with Ketal) is that Acetal and Hemi-Acetal Ends with a Hydrogen group, but Ketals and Hemi-ketals end with another Carbon (single or chain).

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3- Formation of Cyclic Sugars: - The functional group of the sugar chain ( Aldehyde ) is active, It's nucleophilic

and it's looking for a hydroxyl group to react with, where can it find one?

- The hydroxyl group on carbon 5 bends and reacts with that functional group which creates a Hemi-acetal. This is also possible with Carbon number 4 Which also creates a Hemi-acetal.

- Hexoses easily form a Cyclic molecule by creation of a Hemi-acetal Group.

- The hexose sugar creating a hemi-acetal group can either create a 5 membered ring or a 6 membered ring.

- The ring with 5 members is called Furanose and the ring with 6 membersis called Pyranose.

R

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- Summary: Cyclization occurs as a result of interaction between functional groups at distant carbons to form derivatives called hemiacetals or hemiketals, in other words it’s a general reactions between Aldehydes OR ketones with Alcohols to form those derivatives .

*Extra Question: 1. What are these Molecules called?

2. Are They Trioses, Pentoses, Hexoses? 3. Are they Ketoses or Aldoses? 4. Are they Sterisomers? Specify if yes ( Enantiomers / Diasteromers ) 5. Which of Those 2 Molecules is Levorotatory and Which is Dextrorotatory?

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Answers: 1. Glucose 2. Hexoses 3. Aldoses 4. Enantiomers 5. Right Molecule is Levorotatory(L-Glucose), Left Molecule is

Dextrorotatory(D-Glucose)

For I am a beacon of knowledge blazing out

of a black sea of ignorance "Everyone is an idiot" – House M.D.