31 January 2009

The Most Important Crystal Structure Ever Determined

Ask anyone what they think was the most important crystal structure ever determined and they'll say, DNA.

For me, that just isn't so.

As chemistry progressed it was found that some molecules had almost the same configuration; ie, the atoms were joined together in the same places and in the same sequence but they were mirror images of one another.

At the time it was difficult to get a handle on these mirror image molecules. In the lab, they were usually formed in 50:50 mixtures. Techniques were available to separate the two different forms and hence purify them. When pure, it was found that their physical properties, eg melting point, boiling point ,etc, were identical; except for their interaction with plane polarised light.

Get a source of light. Put a piece of polarising filter in front of it (remember polarising sunglasses?) then shine it through a solution containing the purified molecule and the plane of polarised light is rotated. Repeat the experiment with the other molecule and the plane of polarised light is rotated in the opposite direction.

This meant that the molecules could be labelled on the basis of this property. The problem was that it wasn't possible to correlate the two forms of the molecule to what was called their absolute configuration. In other words, after separating the two molecules and getting them pure it wasn't possible to tell what the molecules looked like in three dimensional space - yes they were mirror images of one another but which image represented which molecule?

At this point in the history of chemistry someone (Fisher) made a guess. Molecules were thus assigned on the basis of this guess from then on. This process went on until Bijvoet,

"In their seminal experiment on sodium rubidium tartrate, (I), the group of Bijvoet (Peerdeman et al., 1951; Bijvoet et al., 1951) performed the first determination of the absolute configuration of an organic compound."

Also,

"This experiment changed the history of organic chemistry, because the stereochemistry could now be determined by experiment. It also started a new era in X-ray crystallography, because molecular properties could now be related to the absolute configuration."

This 1951 paper has been looked at again by, Martin Lutz and Antoine M. M. Schreurs.

Have a read and the next time someone asks your opinion as to the most important crystal structure ever determined, you can say ...

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