Science with Richard Bleil
Compounds that have the same formulas, but different structures are called “isomers”. For example, C2H6O can be arranged to make ethanol or ether. Chemically, and physically, they are very different. They have different melting points, vapor pressures, boiling points, and their reactivities are very different as well. If you oxidize ethanol, you form an aldehyde, but ether cannot be oxidized into anything useful.
There is one class of very special isomers. They are called “chiral compounds”. These isomers are put together in more or less the same way, except that they are mirror images of one another. Unlike other isomers, chiral compounds are almost identical, with the same physical properties, and the same chemical reactivities with one exception. Chiral compounds only react differently with other chiral compounds.
To understand chirality, it’s a little like your hands. Typically, a person’s hands are the same. They have four fingers, and a thumb, except that they are mirror images of one another. They say that they are “non-superimposable”, meaning that there is no way to rotate or flip them to make them the same. When the palms face one another, the thumbs are on the same side, but when both palms are facing the same direction, the thumbs are always on opposite sides. Nothing you can do will make them on the same side without, again, putting one palm up and the other down.
Chiral compounds are actually fairly common, especially in organic compounds. For example, sugars are chiral. The sugar produced naturally, in beets or sugar cane, is called “right-handed” sugar. There is actually a procedure that chemists use to determine if chiral compounds are right or left-handed. As it turns out, our enzymes are right-handed as well.
Enzymes are a kind of organic catalyst. Catalysts speed up chemical reactions, but it’s important to note that they do not make reactions occur that would not otherwise occur. In other words, the catalyst in your exhaust speeds up the reaction of carbon monoxide with oxygen to form carbon dioxide. This reaction would occur without the catalyst, but very slowly. But no catalyst will cause the breakdown of carbon dioxide into carbon monoxide without a chemical reaction or the input of some for of energy to force it to happen.
Sugar has a lot of stored energy. That’s the point of sugar. However, the conversion of sugar to carbon dioxide, releasing its energy, occurs very slowly when left alone. This is why the shelf-life of sugar is so long. The reaction is very slow, which means the release of energy is very slow, far too slow, in fact, to be useful as a biochemical source of energy. So, we have enzymes that speed up (or catalyze) this reaction allowing the energy in the sugar to be released in a timelier fashion (and in such a way that the body can utilize the energy as well).
But this enzyme is right-handed. When reaction with right-handed sugar, the enzyme works. However, left-handed sugar, which has been made, does not interact with the enzyme. This means that left-handed sugar tastes the same and cooks the same, but in our bodies, it would pass straight through without reacting. This would be the perfect artificial sweetener except for one thing.
See, left-handed sugar can only be made in the lab. This means it takes tremendous amounts of energy and is exceptionally expensive since it can only be made in small batches. Plants make large amounts of sugar over a very long period of time using the energy of the sun in the process. We simply cannot artificially compete with this.
Not all chiral compounds are harmless. In the 1950’s, thalidomide was introduced to help expecting mothers deal with morning sickness. Thalidomide is a chiral compound and is more or less harmless if it is the right-handed compound. Unfortunately, the left-handed version caused birth defects, and eventually caused it to have its approval removed. Making just one chiral form of a compound is not easy, and clearly the pharmaceutical companies didn’t have it down. But, as you recall, chiral isomers have the same properties, including melting point, boiling point, and solubility. That means it’s also incredibly difficult to separate chiral isomers. All of the usual techniques that chemists use just don’t work on chiral isomers.
Back in Ohio, a local network television station put together its own television series, a crime drama wherein an innocent man is being hunted for crimes that his heretofore unknown identical twin brother committed. As a curious side note, while people believe DNA to be completely unique, it is not. The exception are identical twins who have identical DNA. The only way forensics can distinguish between them is through their fingerprints, which are still unique. Thalidomide is kind of like this. The innocent and useful right-handed version that helped pregnant women deal with morning sickness, and the evil left-handed version that attacked the babies.
Bleil's Banter 