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Stereoisomers

Notes

Stereoisomers

Sections

organic chemistry review for carbohydrate biochemistry: Stereoisomers

Overview

STEREOISOMERS

  1. Enantiomers differ at all chiral carbons (aka stereo-centers)
  • Mirror images of each other
  • Identical physical properties except they rotate plane-polarized light in opposite directions
  1. Diastereomers: differ at one or more, but not all stereocenters
  • Epimers: diastereomers differ at only one stereocenter (and have multiple stereocenters)
  • Anomers: differ at new stereocenters created during ring closing
  • Some diastereomers are neither epimers nor anomers

ALDOHEXOSES:

  • Aldose sugars with six carbons.

D-mannose

  • Used in protein glycosylation
  • C2- 5: chiral = 2^4 = 16 different stereoisomers

D-glucose

  • Primary sugar used in metabolism to generate ATP
  • D-mannose & D-glucose are diastereomers: not mirror images, differ in spatial arrangement of atoms at C2
  • D-mannose & D-glucose are epimers: only differ at one stereocenter, C2

D-galactose

  • Diastereomer of D-mannose & D-glucose
  • Epimer of D-glucose: only differ at C4
  • Differs from D-mannose at C2 & 4 (diastereomer/not epimer)

Alpha-D-glucose and Beta-D-glucose

  • Anomers: diastereomer of ring structures
  • Chirality at C1 only in ring form of glucose
  • Differ in –OH position at C1
  • Straight chain form of glucose: C1 double-bond to oxygen atom achiral
  • Ring alpha- & beta-D-glucose: C1 = 5th chiral carbon

Full-Length Text

  • Here we will continue to review some of the organic chemistry principles that are important in our understanding of carbohydrate biochemistry
    • We will specifically address the isomers present in carbohydrate biochemistry.

Start a table.

  • Write that there are two types of stereoisomers
  1. Enantiomers, which differ at all chiral carbons (aka stereo-centers), making them mirror images of each other.
  • Enantiomers have identical physical properties to one another except that they rotate plane-polarized light in opposite directions, as discussed elsewhere.
  1. Diastereomers, which differ at one or more, but not all stereocenters.
  • Diastereomers are called epimers if they differ at only one stereocenter (in molecules with multiple stereocenters) and are called anomers if they differ at new stereocenters created during ring closing reactions
  • However, there are diastereomers that are neither epimers or anomers (and are simply unspecified).

We will now draw examples of epimers, anomers, and non-epimer, non-anomer diastereomers. To illustrate this concept, we will use aldohexoses; as their name implies, they are aldose sugars with six carbons.

  • First, draw a vertical chain of six carbons.
  • Label the carbon atoms 1-6 from top to bottom.
  • To carbon 1, add a carbonyl group and a hydrogen atom.
  • To carbons 2 and 3 add a hydroxyl group on the left and a hydrogen atom on the right.
  • For carbons 4 and 5 add a hydrogen atom on the left and a hydroxyl group on the right.
  • Finally, on carbon 6 add two hydrogen atoms and a hydroxyl group.
  • Indicate that this molecule is D-mannose, a sugar commonly used in the glycosylation of proteins.
    • Notice that there are 4 carbons (2 through 5) that are all chiral (shown in pink); thus, giving 2 to the power of 4 (which equals 16) different stereoisomers.
    • Only a few of these are generally considered relevant to human biochemistry.

Now let's draw one of the diastereomers of D-mannose to the right of it.

  • Once again draw a vertical chain of six carbons.
  • Label the carbon atoms 1-6 from top to bottom.
  • To carbon 1, add a carbonyl group and a hydrogen atom.
  • To carbon 2, 4 and 5, add a hydrogen atom on the left and a hydroxyl group on the right.
  • For carbon 3, add a hydroxyl group on the left and a hydrogen atom on the right.
  • Finally, on carbon 6, add two hydrogen atoms and a hydroxyl group.
  • Indicate that this molecule is D-glucose, the primary sugar used in metabolism to generate ATP.

Next, let's indicate how D-mannose and D-glucose are both diastereomers and epimers of one another.

  • First, dash a box around both C2s to highlight that D-mannose and D-glucose are diastereomers of each other, since they are not mirror images but they differ in the spatial arrangement of atoms at carbon 2.
  • Second, indicate that since they only differ at one stereocenter, carbon 2, they are also epimers of each other.

Now, let's draw D-galactose because it is a good example of a molecule that is a diastereomer of both D-mannose and D-glucose, but only an epimer of D-glucose.

  • Next to our drawing of D-mannose and D-glucose, draw a third vertical chain of six carbons and label the carbon atoms 1-6 from top to bottom.
  • Add the carbonyl group and hydrogen to carbon 1 and the two hydrogens and hydroxyl group to carbon 6.
  • To carbons 2 and 5, add a hydrogen atom on the left of the carbon and a hydroxyl group on its right.
  • To carbons 3 and 4 do the opposite, add the hydroxyl group on the left and the hydrogen atom on the right.
  • Indicate that this molecule is D-galactose, another simple sugar used in metabolism and in the formation of carbohydrates.
    • Notice that D-galactose differs from D-mannose at carbons 2 and 4, making it a diastereomer but not an epimer.
    • On the other hand, D-galactose only differs from D-glucose at carbon 4, making it a diastereomer and an epimer of D-glucose.

Finally, we will draw examples of anomers. Anomers are specifically diastereomers of ring structures, so for these next drawings we will be drawing ring form aldohexoses.

First, let's draw the sugar alpha-D-glucose.

  • Draw a hexagon with a pair of parallel sides going horizontally, and insert an oxygen atom at the apex of the two lines on the top right of the hexagon.
    • Recall from organic chemistry that in stick drawings of organic compounds, carbon atoms are at the end of the lines or at the points where two lines meet.

Let's label the carbon atoms in this molecule.

  • Moving clockwise from our oxygen atom, label the carbons 1-5.
    • Notice that our sixth carbon atom is missing. Let's add it as an attachment to carbon 5 as follows:
      draw a vertical line upwards from carbon 5 and add a carbon with two hydrogens and a hydroxyl group attached to it.

Now let's add the other hydroxyl groups.

  • To carbons 1, 2, and 4, add hydroxyl groups going vertically down.
  • To carbon 3, draw a hydroxyl group going vertically up.

Next, let's draw its anomer, beta-D-glucose.

  • Redraw the hexagon, insert the oxygen, label the carbon atoms, add the sixth carbon and its attachments.

Now let's add the hydroxyl groups, which determine the difference between these two molecules.

  • To carbons 1 and 3, add a hydroxyl group going vertically up.
  • To carbons 2 and 4, add the hydroxyl groups going vertically down.
  • Dash a box around carbon 1 (and its attachments) on each molecule to show that the chirality at carbon 1 only exists in the ring form of glucose, and that these molecules only differ in the position of the hydroxyl group at carbon 1.
    • In the straight chain form of glucose, carbon 1 has a double-bond to an oxygen atom, making it achiral.
    • However, with the formation of the ring in alpha- and beta- D-glucose, carbon 1 is now a fifth chiral carbon in the glucose molecule.