Take Home Lessons from Lecture 6

 

1. The popular medical diagnostic technique of magnetic resonance imaging (MRI):

A. MRI is based on the same principles as NMR, namely the flipping (i.e. resonance) of nuclear spins of H atoms by radio frequency irradiation when a patient is placed in a strong magnetic field.

B. Magnetic field gradients are used to gain imaging information.

C. Rotation of the gradient around the center of the object gives imaging in an entire plane (i.e. slice inside patient).

D. In an MRI image, you are looking at individual slices that when stacked make up the three-dimensional image of relative amounts of protons, especially the protons from water and fat, in the different tissues. Click here for a handout on MRI.

2. Carbohydrates can be aldoses or ketoses, and the most common are pentoses or hexoses.

3. THE KEY issue with carbohydrates is stereochemistry.

A. The different carbohydrates are largely just stereoisomers of each other.

B. The stereochemistries of carbohydrates are based on (+) D and (-) L glyceradehyde. A D-sugar has the same configuration as (+)-D-glyceraldehyde at the chiral center farthest from the carbonyl group. An L-sugar has the same configuration as (-) -L-glyceraldehyde at the chiral center farthest from the carbonyl group.

C. The best way to analyze carbohydrate stereochemistry is with Fisher projections (think Teddy bear hugging you).

4. Carbohydrates exist in the cyclic, hemiacetal form in solution. Click here for a discussion of this. These are generally shown as Hayworth projections. The mechanism involves the same steps as the hemiacetal formation mechanism you learned.

5. A new chiral center is created (at the anomeric carbon) as the carbohydrate cyclizes, and the OH group can be axial (alpha equals axial for glucose) or equatorial (beta equals equatorial for glucose). For monosaccharides, these two forms are in equilibrium, and interconversion is catalyzed by acid (mutarotation).

6. Cyclic form carbohydrates form either five (furanose) or six (pyranose) membered rings.

7. Carbohydrate monomers can be linked together via acetal bonds, and this linkage can be alpha or beta. (For glucose, alpha is axial) This type of acetal bond is called a glycosidic bond and is named as alpha or beta referring to the stereochemistry of the anomeric carbon and also by the numbers of the ring carbons attached via the -C-O-C- linkage (i.e. alpha 1,4-). Click here for a discussion of this.

8. Carbohydrates can also exist as polymers (polysaccharides), linked via gylcosidic bonds. Poly beta-1,4-D-Glucose forms cellulose, while poly alpha-1,4-D-Glucose forms starch.

SUMMARY OF SKILLS YOU MUST MASTER FOR THE MCAT:

1) Understand the unique nomenclature of carbohydrates including terms such as aldose, ketose, pentose, hexose, pyranose, furanose, anomer, anomeric carbon.

2) Remember how to distinguish D vs. L sugars

3) Recognize glycosidic bonds and be able to name then (alpha 1,4 linkage, etc.)

4) Remember that for glucose, apha is axial.

I have enjoyed getting to know you this past week. My intent has been to cover organic chemistry "a mile wide and an inch deep." Just as important I have tried to create a one-stop and concise website that you can use as a resource to guide your very efficient study. I will leave this up on the server so that you may use it well into the future. Best of luck on the MCAT, DAT or whatever test you will be taking. Finally, this has been thefirst time through the material. I welcome any feedback concerning how I could improve what I am doing, especially after you have taken one of these tests for yourself!!