CHEM 30 B  Dr. R. Rinehart  
Chapter 17 
 CARBOHYDRATES

 At last! The long-awaited transition from organic chemistry to biochemistry begins with the important class of compounds called carbohydrates - sugars, starches, and fibers. We’ll see that there is no clean-cut division between  organic chemistry and biochemistry - both will come into play.

I.  What is a carbohydrate?
            A.  Composition

   
                      1. Treat with heat or sulfuric acid --> carbon + water
                         2. General formula
[CH2O]n or Cn(H2O)m

B.  Structure and Functionality of monosaccharides
           
1.
Polyhydroxy aldehydes or ketones with 3 to 9 carbons
           
2
. Classification by:
                     
a. 
Functionality: Aldoses or Ketoses
                     
b. 
# of C: trioses, tetroses, pentoses, hexoses, etc.
                      c.       Prefixes can be combined: e.g.,
glucose is an aldohexose 

C. Larger molecules assembled from monosaccharides
           
1.
Disaccharides: 2 monosaccharides in “glycosidic  [acetal/ketal] linkage
           
2. Oligosaccharides: 3-10 monosaccharide units 
           
3.
Polysaccharides: may contain thousands of monosaccharide units 

II.  Through the looking glass: STEREOCHEMISTRY
           
A. 
Nonidentical [nonsuperimposible] mirror-image molecules = enantiomers
           
B. 
Chiral centers: C with 4 different substituents [may be >1 in a molecule]
           
C
.  Fischer projections: a way to show stereochemistry
                       
1. D- vs. L-  structures
           
D.  for a structure with
n chiral centers, there are 2n possible stereoisomers
           
E.  Detection by rotation of plane-polarized light
                       
1. Clockwise = dextrorotatory;  angle is +
                       
2. Counterclockwise = levorotatory;  angle is - 

III.  Monosaccharides or “simple sugars”
           
A.
Physical properties
                       
1. Highly soluble in water
                       
2. Most taste sweet
 

            B. Chemical properties
                       
1. Formation of
cyclic hemiacetals or hemiketals
                        2.
New chiral center => 2 “anomers”  designated  a and b

see the table of carbohydrates to learn more about structure of anomers 
and to see structures of the carbohydrates mentioned in this outline..
You might also want to check out the page on carbohydrates by Clarke Earley at the Stark campus of  Kent State University at http://www.personal.kent.edu/~cearley/PChem/sugar/makering.htm  -- it has a beautiful drawing showing the correspondence of Fisher and Haworth projections. {if the direct link doesn't work, go to the following page and select "Cyclization of carbohydrates" http://www.personal.kent.edu/~cearley/PChem/pchem.htm
PowerPoint-type presentation of basic carbohydrate chemistry
by Warren Gallagher at U Wisconsin Eau Claire is viewable [with Acrobat Reader] at:
http://web.archive.org/web/20060504011433/http://www.chem.uwec.edu/Chem150_S06/Pages/lecture-materials.html
http://www.chem.uwec.edu/Chem150_S06/Pages/lecture-materials.html  > Lectures 9 & 10
 
 

                        3. Oxidation: Benedict’s test; “reducing sugars”
                       
4. Formation of
phosphate esters
                       
5. Acetal/ketal formation with alcohols yields  nonreducing “glycosides” 

            C. Important monosaccharides [you should learn these structures]
                       
1.
D-glyceraldehyde [aldotriose] and dihydroxyacetone [ketotriose]
                       
2. Aldopentoses:
D-ribose and D-2-deoxyribose
                       
3. Aldohexoses
                                   
1.
D-glucose [glc]
                                   
2.
D-galactose [gal]
                                   
3.
D-mannose [man]
                       
3. Ketohexose:
D-fructose [fru] 

IV.   Disaccharides:  glycosides [acetals or ketals] where the alcohol comes from a second monosaccharide
           
A. Many possible combinations; we only care about 3
           
B.
Maltose : D-glc a(1à4) D-glc; formed by breakdown of starch
           
C.
Lactose: D-gal b(1à4) D-glc: found only in milk
            D.
Sucrose: D-glc(a1↔b2)D-fru: cane or beet sugar; nonreducing

You can see Dave Woodcock’s Chime structures of maltose, lactose, cellobiose, and sucrose at:
http://www.molecularmodels.ca/molecule/modelfiles/nat0011.html  maltose
http://www.molecularmodels.ca/molecule/modelfiles/nat0014.html  lactose
http://www.molecularmodels.ca/molecule/modelfiles/nat0012.html  cellobiose
http://www.molecularmodels.ca/molecule/modelfiles/nat0013.html  sucrose
[or select from http://www.molecularmodels.ca/molecule/Natural_Products.htm]

V.   Polysaccharides:  many possible; we’ll mainly look at 3
           
A.
Starch: 2 forms
                       
1.
Amylose: All D-glc with a(1à4) linkages
                       
2.
Amylopectin: amylose chains with a(1à6) branches every ~20 units
           
B.
Glycogen: “animal starch” like amylopectin, but branches more frequent
            C.
Cellulose: All D-glc with b(1à4) linkages; the most abundant organic substance

Check out Chime structures of polysaccharides by Paul Schueler of Raritan Valley College at:
http://www.raritanval.edu/departments/Science/molecules.html 

VI.  Carbohydrate derivatives [probably not in your text, but you will be hearing about them at some point]
  
                     A. Sugar alcohols:
                                    1.  sorbitol
                                    2.  mannitol
                                    3.   inositol

  
                     B.  Sugar acids:
  
                                 1.   -onic
  
                                  2.  -uronic
  
                                 3.  –aric

  
                     C.  Amino sugars
  
                                 1. D-glucosamine
  
                     D. Heteropolysaccharides

 

Other References:
  Carbohydrate page by Gordon Rule at CMU  http://web.archive.org/web/20031204144005/http://stingray.bio.cmu.edu/~web/bc/Lec/Lec24/lec24.html
and http://web.archive.org/web/20031006114441/http://stingray.bio.cmu.edu/~web/bc/Lec/Lec25/lec25.html

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© Ronald W. Rinehart, 2002-2007