Isomers AP Biology > The Chemistry of Life > The Chemistry of Life

ORGANIC CHEMISTRY REVIEW FOR CARBOHYDRATE BIOCHEMISTRY: ISOMERS Here, we address:

• Structures and characteristics of aldose and ketose sugars

• Differences between various types of isomers.

• Carbohydrates being carbon-rich molecules with the empirical formula (CH2O)n, where "n" is a variable number.

• Carbohydrates being polymers that comprise simple sugars.

These sugars may have 3, 4, 5, or 6 carbon atoms in them, and are called trioses, tetroses, pentoses and hexoses, respectively. OVERVIEW CARBOHYDRATES

• Carbon-rich molecules

• Empirical formula: (CH2O)n

• Polymers = simple sugars (3, 4, 5, or 6 C-atoms)

SIMPLE SUGAR NOMENCLATURE

• 3-carbon: triose

• 4-carbon: tetrose

• 5-carbon: pentose

• 6-carbon: hexose

TWO TYPES OF SUGARS

• Aldose sugars: aldehydes; carbonyl group at terminal carbon

Example Glyceraldehyde (glycolysis): triose sugar C3H6O3

• Ketose sugars: ketones; carbonyl group bound to 2 C-atoms

Dihydroxyacetone (glycolysis): triose sugar C3H6O3 Dihydroxyacetone & glyceraldehyde are isomers: same general formula ISOMERS

• Same structural formula with different order/ spatial arrangement of atoms

1. Constitutional isomers: different order of attachments but same formula

• Gyceraldehyde and dihydroxyacetone are constitutional isomers

1 / 5 2. Stereoisomers: same order of attachments but different spatial arrangements. Non-superimposable isomers: have at least one chiral carbon Two types: enantiomers & diastereomers Enantiomers: mirror images Diastereomers: not mirror images STEREOISOMERS

• Enantiomers of glyceraldehyde:

C2 is chiral --> all stereoisomers have at least one chiral carbon atom D-glyceraldehyde and L-glyceraldehyde.

• Optical isomers: D- and L-isomers

Distinguishable by the direction they rotate plane-polarized light Most carbohydrates occur naturally as D form isomers Most amino acids are L form isomers

FULL-LENGTH TEXT

• Here we will begin to review some of the organic chemistry principles that are important in our understanding of carbohydrate biochemistry, especially as it relates to how the body metabolizes carbohydrate compounds.

- Specific topics we will address are the structures and characteristics of aldose and ketose sugars and the differences between various types of isomers.

First, let's make a table:

• Indicate that:

- Carbohydrates are carbon-rich molecules with the empirical formula (CH2O)n, where n is a variable number.

- Carbohydrates are polymers that comprise simple sugars. These sugars may have 3, 4, 5, or 6 carbon atoms in them, and are called trioses (tri-oses), tetroses, pentoses and hexoses, respectively.

• There are two types of sugars:

- aldose sugars, which are aldehydes; they have a carbonyl group at the terminal carbon of the molecule.

- ketose sugars, which are ketones; they have a carbonyl group attached to two other carbon atoms within the molecule.

Now, let's draw an example of an aldose sugar and a ketose sugar. For this example, we will draw glyceraldehyde and dihydroxyacetone, both of which are produced during glycolysis.

• To draw glyceradehyde, first draw the aldehyde group as follows: draw a carbon atom double-bonded to an oxygen atom and single-bonded to a hydrogen atom.

• Now, add another carbon atom on our carbon, as its fourth bond.

• To this carbon atom attach a hydroxyl group and a hydrogen atom.

2 / 5 • Then add a third carbon atom with two hydrogen atoms and a hydroxyl group attached to it.

• With our glyceraldehyde completed, next indicate the aldehyde group as follows: dash a box around the first carbon atom and the attached carbonyl and hydrogen.

Finally, let's see how glyceraldehyde relates to what we have just learned about carbohydrates.

• Write that glyceraldehyde is a triose sugar with the general formula C3H6O3.

Now let's draw dihydroxyacetone.

• Draw a carbon atom with a hydroxyl group and two hydrogen atoms.

• Now, add another carbon atom on our carbon, as its fourth bond.

• To this carbon attach a double-bonded oxygen atom (a carbonyl group).

• Then add a third carbon atom with a hydroxyl group and two hydrogen atoms attached to it.

• With our dihydroxyacetone molecule completed, next indicate the ketone group as follows: dash a box around the second carbon atom, the one with the double-bond to the oxygen atom.

Finally, let's see how dihydroxyacteone relates to what we know about carbohydrates.

• Write that dihydroxyacetone is also a triose sugar with the general formula C3H6O3.

• Notice that dihydroxyacetone and glyceraldehyde have the same general formula.

- In fact, glyceraldehyde and dihydroxyacetone are isomers of each other.

Let's define what isomers are and what kinds of isomers exist in biochemistry.

• Write that an isomer is a molecule that has the same structural formula as another molecule, but has a different order or a different spatial arrangement of atoms.

Isomers can be divided into two broad categories:

3 / 5 • Constitutional isomers, which have a different order of attachments but the same formula;

• Stereoisomers, which have the same order of attachments but different spatial arrangements.

• Write that there are two types of stereoisomers: enantiomers and diastereomers.

- Indicate that in BOTH types isomers are non-superimposable isomers. With both hands palms down, place one on top of the other and see that you can't place one hand perfectly on top of the other: your fingers go in different directions – this is what we mean by non-superimposable.

- Next, indicate that enantiomers are mirror images of each other whereas diastereomers are not. So now have your palms face one another, and see that they are mirror images of each other.

- Both enantiomers and diastereomers have at least one chiral carbon atom.

Next let's address examples of constitutional and stereoisomers. First, address constitutional isomers.

• Indicate that glyceraldehyde and dihydroxyacetone, which we have already drawn, are constitutional isomers.

Next, let's address stereo isomers. Although two forms exist: enantiomers and diastereomers, we will only cover enantiomers, here (we draw diastereomers elsewhere).

Let's draw the enantiomers of glyceraldehyde, since we are already familiar with this molecule.

• Redraw our glyceraldehyde molecule.

• Label the carbon atom with the aldehyde group 1, the middle carbon atom 2 and the carbon atom with the hydroxyl group attached 3.

- Note that carbon 2 is chiral, meaning that each of the four attachments to this carbon atom is different.

- It is this quality that allows for the formation of enantiomers.

- All stereoisomers have at least one chiral carbon atom.

Now let's draw this in a three dimensional representation.

• Redraw our glyceraldehyde molecule, specifically D-glyceraldehyde, with the following changes: on carbon 2, draw a wedge as the bond between the carbon atom and the hydroxyl group to represent that this group is coming towards us in space.

• For the hydrogen atom attached to carbon 2, dash short horizontal lines to show that this atom is going away from us in space.

4 / 5 • Now dash a line to the right of our glyceraldehyde molecule. This line represents our mirror.

Let's now draw the enantiomer of our first glyceraldehyde molecule, the L-glyceraldehyde as follows:

• Draw a second glyceraldehyde molecule with the attachments on carbon 2 mirroring those on the first glyceraldehyde molecule, meaning dash short horizontal lines that connect the hydroxyl group to carbon 2 and draw a wedge that connects the hydrogen atom to carbon 2.

- Note that D- and L- isomers are called optical isomers, because the way to distinguish them from each other is based on the direction that they rotate plane-polarized light.

- Most carbohydrates occur naturally as D form isomers (whereas most amino acids are L form isomers).

In the next tutorial we will continue learning about isomers with a review of diastereomers, as well as some other organic chemistry principles that are important in carbohydrate biochemistry

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