1. Lecture 2: Monosaccharides
Wood Chemistry PSE 406
Lecture 2:
Monosaccharides
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2. Why Study Monosaccharides?
As we discussed in lecture 1, wood carbohydrates (cellulose and hemicelluloses) are made of various sugars such as glucose, mannose, xylose, etc. In order to understand the properties and reactions of the carbohydrates, we need to understand monosaccharides.
D-glucose
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3. What is a Monosaccharide?
Simple sugars
Monosaccharides are polyhydroxy aldehydes or ketones (aldoses or ketoses) of 3-9 carbons.
Polyhydroxy means that there is more than 1 hydroxyl group (OH) on the carbon backbone.
Aldehydes and ketones both contain carbonyl groups.
Typically monosaccharides contain either 5 or 6 carbons.
Aldehyde
Ketone
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4. Glyceraldehyde This 3 carbon sugar is the simplest monosaccharide.
It is an aldehyde.
As is the case with all monosaccharides, it contains an asymmetric or chiral carbon. This means there are 4 different groups attached to this carbon
Because of this, there are 2 different stereoisomers of this structure known as enantiomers.
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5. Glyceraldehyde Representations
1. Wedge and Dash
2. Fischer Formula
In this representation,
the dotted lines indicate
bonds pointing back
the wedges indicate bonds
pointing forward (a bear wants
hug you).
In sugar chemistry, D and L are use to describe the absolute configuration around the lat asymmetric carbon in the molecule (the farthest away from the carbonyl groups). If the OH groups in the Fisher projection on this carbon is written on right hard site, the for is designated as D. If is on the left hand site it is L form, this is something different from (d and l and + or -) and describes how the compound polarizes light in the solution. For example, D glyceraldehydes is d or (+) but D-erythrose is levorotary (l or -).
This corresponds to how these compounds rotate polarized light in solution. If a compound rotates light in a clockwise direction it is said to be dextrorotatory and is designated as (+) or as d. When the sample rotates light in the counterclockwise direction it is said to be levorotatory and is designated as (-) or L.
There are significant differences between d and l compounds. Most sugars in wood are D although there is some L arabinose present.
In the Fischer representation,
vertical lines indicate bonds
pointing back while
horizontal lines indicate bonds
pointing forward..
Notes
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6. 3 D Views of the Enantiomers of Glyceraldehyde
= Carbon
= Oxygen
= Hydrogen
L-Glyceraldehyde
D-Glyceraldehyde
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7. Fisher projections
Using these rules the distinguishing features of the 3D structure of stereoisomers can easily and accurately represented with 2D drawings
Fischer projection named after Emil Fischer who established the molecular structures of many sugars
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8. Four Carbon Sugars (Not Found in Hemicelluloses or Cellulose)
With 2 chiral centers in the molecule, there are 4 stereoisomers; two pairs of enantiomers and two pairs of diastereomers.
Diastereomers
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9. Are these compounds the same?
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10. Nonsuperimposable mirror images
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11. Enantiomers are different compounds
Nonsuperimposable so different compounds
One carbon atom is chiral (not symmetric)
Nonsuperimposable mirror images are called enantiomers
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12. Enantiomers
Stereoisomers are molecules that have the same bonds connecting the same atoms but different relative orientations of the bonds
Enantiomer : one type of stereoisomer
Nonsuperimposable mirror images
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13. Stereochemistry
Enantiomers are identical in physical properties except that they rotate polarized light in opposite directions. From a biochemical standpoint, they are different (Nonsuperimposable mirror images).
Diastereoisomers possess different physical properties (melting point, solubility) and often undergo chemical reaction in the different fashion.
For n asymetric carbons there are:
2n maximum possible number of stereoisomers
2n-1 maximum possible number of enantiometric pairs
That means there are 2n-1 possible diastereoisomer pairs
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14. Enatiomers of Erythrose
D-Erythrose
L-Erythrose
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15. Are you a 3D type of Student?
We don’t care
about steriochemistry
How do chemical representations correlate with the actual spatial arrangement of the molecule?
The representations are a simplified (but tricky way) of showing the molecule.
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16. Simple Method for 3D
Remember, in the Fischer formula, the vertical lines always point back. You don’t want your molecule to be a circle so in 3D the carbon backbone alternates in and out of the page.
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17. Simple Method for 3D (II)
Check out the second carbon down, in the 3D version the carbons attached to this atom both are pointing back. The OH group and the H group attached to this carbon are pointing forward and pointing to the left and right respectively. Therefore, in the Fischer formula these are drawn to the left and right.
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18. Simple Method for 3D (III)
Now if you go to the 3rd carbon from the top, you will notice that the carbon above and below it are pointing forward; this is unacceptable. To fix this, we rotate the molecule 180°.
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19. Simple Method for 3D (IV)
If you look at the molecule now you will see that the OH and H groups on the 3rd carbon now stick out of the page and are on the left and right side respectively. Draw them into the Fischer formula and you get your final molecule.
It may seem a little tricky but it is a fun thing to do at parties.
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