Sunday, December 12, 2010


In chemistry, up to 4 different groups can be affixed to a carbon atom. If there are 4 different groups, we call that carbon (actually this is true with  other atoms ) a chiral center. Molecules can have multiple chiral centers, which makes them very difficult to synthesize. In the first sample above, I have drawn the two enantiomers of phenylalanine, a building block of life. These two enantiomers are identical in every respect just like a left glove and a right glove would be but try to put a left glove on a right hand. It doesn't work. In medicinal chemistry, we use the lock and key motif to describe how a drug may interact with a receptor (only both lock and key are moving and changing shape). One enantiomer  may fit into a receptor and the other one won't. If  our compound is an equal mixture of the two enantiomers, we call it racemic. Since it used to be close to impossible to make a compound enantiomerically pure, drugs were frequently racemic. Now that one can make a compound enantiomerically pure (with great difficulty I might add) we have to make each enantiomer to see which one is active.
Who figured this out? Louis Pasteur,who must be the most productive scientist who ever lived. Early in his career, he was employed by the wine industry to figure out what went wrong with some of the batches. It was thought that grape juice turned into wine all by itself. He proved there was a microorganism(yeast) responsible  by killing them off with heat. No fermentation until he reintroduced the yeast. Frequently crystals formed inside the wine bottles. These turned out to be the potassium salt of tartaric acid ( in German: weinsaure, wine acid). He knew the structure of it. The leap of logic that I don't understand is why he tested it to see if it would rotate the plane of polarized light, which it did. I could understand why he thought that theoretically there would be enantiomers but to see what properties each have is another story. He made some tartaric acid in the lab, not every difficult to do, but it did not rotate the light. Somehow he crystallized the acid and using a microscope and tweezers, separated the right handed molecules from the left. A solution of each rotated the light in opposite directions.
My record for dealing with enatiomers is a molecule with 3 chiral centers giving you 8 possible molecules: 4 pairs of enantiomers. The isomers that aren't mirror images are called diastereomers, a word I used to stumble to pronounce.
His process for selectively killing pathogens by heating is pasteurization.  Pasteurized milk is not sterile milk. Leave it out a few hours and it will go sour.
Aside from saving the wine industry and silk industry, he developed the rabies vaccine.

So now we have real snow on the ground. Yesterday we had a break in which the temps were in the 40s and sunny. This might have been  my last long run for a while as running on a hot track for longer than an hour just is so difficult.

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