Sunday, August 3, 2014

198: We're All Pasteurized

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Before we start, I'd like to thank listener Dan Unger, who posted another nice review on iTunes. Thanks Dan!

Now, on to today's topic. Most of us know the name of Louis Pasteur, the 19th-century French biologist and chemist, from seeing his name embedded in the term 'pasteurized' on milk cartons. And he is rightly remembered for his discoveries in the area of the germ theory of disease, which led to the development of many vaccines as well as the famous process for heating milk to reduce the bacterial content, collectively saving human lives in the millions over the past two centuries. But did you know that before his famous medical contributions, he made an equally revolutionary discovery in chemistry: the fact that molecules could posses chirality, that is left-handedness or right-handedness, just like ordinary macroscopic 3-D objects?

We're all familiar with the concept of left/right asymmetry in the real world. Simple examples are a glove, which cannot be changed from left-handed to right-handed without turning it inside out, or an ordinary screw, where we have to remember the "lefty loosey righty tighty" rule to use it properly. It seems natural that molecules should be able to exhibit such asymmetry as well, but at the dawn of the 19th century, scientists hadn't really discussed that idea much. In 1848, a young Louis Pasteur was studying a chemical called tartaric acid, a byproduct of wine production. He was trying to understand a strange anomaly related to this substance and a very similar one called racemic acid. Scientists had determined the chemical compositions of both acids, and they were exactly the same. Yet they had some very different physical properties: in particular, tartaric acid in solution would rotate a beam of polarized light passing through it, while racemic acid had no such effect.

Pasteur decided that there must be some geometric difference at the molecular level, so generated crystals of both acids to examine under a microscope. Remember that crystals are essentially an endless repetition of a small molecular structure, so studying the shapes of pure crystals can grant some insight into the shapes of the molecules themselves. Other scientists thought he was wasting his time, since the experiment had already been done by others, and they found that tartaric and racemic acid crystals were exactly the same shape. But Pasteur noticed something that his colleagues had missed: while tartaric acid crystals were all the same shape, racemic acid was a mixture of two types: the tartaric acid crystals, plus another form that was a mirror image of the other. Since the form was somewhat asymmetric, this meant that the two forms were truly different, even though other scientists had dismissed these differences as insignificant: no rotation in our physical space could transform one into the other.

Pasteur then came up with a clever experiment. He crystallized some racemic acid, then with a microscope and needle, carefully separated the left-handed and right-handed crystals into two piles. Then he created solutions of the two types of crystals. As he suspected, one of the solutions was a solution of tartaric acid, and rotated polarized light in the same way that tartaric acid normally would. But the other was a new substance, which rotated light in the opposite direction. In other words, the reason racemic acid did not usually bend light was that it was a mix of right-handed and left-handed molecules, while tartartic acid consisted purely of the left-handed form. Pasteur had separated out the right-handed component of the racemic acid. The result was so shocking to the scientists of the day that the French Academy of Sciences made Pasteur repeat the experiment in front of witnesses before they would accept it.

Today awareness of the chirality, or handedness, of molecules plays a critical role in biochemical and medical research. This is mainly due to the fact that life is "homochiral", a fancy way of saying that our basic building blocks all have a single handedness. This differs from most naturally occurring nonliving materials, which tend to exhibit both kinds of handedness randomly in roughly even proportions. The biological origin of Pasteur's tartaric acid was responsible for its one-sided content. Almost all amino acids found in living creatures are left-handed, and we use them to interact with right-handed sugars to supply most of our energy. At first, scientists found this very surprising, since attempts to artificially synthesize most biological molecules result in roughly equal quantities of right- and left- handed forms. This can have tragic consequences: the infamous pregnancy drug thalidomide was a great treatment for morning sickness in its left-handed form, but the right-handed version caused serious birth defects.

The reason for life's left-handedness is one of science's great mysteries. You can see links to articles in the show notes with a few different theories. One is that it stems from a fundamental left-handedness in physics, since certain types of radioactive decay have a leftward electron spin. The effect is tiny, though, and you could argue that this is just begging the question, since you still have to explain the left-handedness of physics. Another theory is that life was seeded by left-handed molecules from space: meteors with both types of amino acids may have happened to pass through regions of space with polarized light that destroyed one kind more often than the other. Or it could just be luck-- maybe left-handed life randomly formed first in the primordial soup, and once it had a toehold it crowded out any other possibilities. Whatever the real answer is, this geometry is critically important to every cell in our bodies.

And this has been your math mutation for today.


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