Why Does the Same Molecule Come in Left and Right Hands? And Why It Matters

A carbon atom bonded to four different groups can arrange them in two ways that are mirror images of each other, and no amount of rotation will superimpose one on the other, exactly as a left hand cannot be turned into a right one. The two forms are identical in nearly every measurable property, having the same melting point, the same solubility and the same spectrum, and they differ in only two respects. They rotate the plane of polarised light in opposite directions, which is how they were first detected. And they interact differently with anything else that is itself handed.

Living systems are built almost entirely from handed molecules:

• •Proteins are made from amino acids of a single handedness
• •So enzyme binding sites are themselves handed
• •A handed site fits one form of a molecule and not its mirror image
• •One form may be active and the other inert
• •Or the other may do something entirely different
• •Smell and taste receptors show the same effect clearly

Thalidomide is the example every account reaches for and the real story is more complicated than the version usually told. The drug was sold as an equal mixture of both forms, and one form is an effective sedative while the other causes severe birth defects, which is the point usually made. What is frequently omitted is that the two forms convert into each other inside the body, so administering the safe form alone would not have prevented the harm. The episode nonetheless drove regulators to require that the two forms of any new handed drug be assessed separately, which is now standard and has changed how such medicines are developed.

Biology uses one handedness almost exclusively and the reason is an open question. Amino acids in proteins are essentially all of one form and the sugars in DNA all of the other, and there is no chemical reason why the opposite arrangement would not work equally well, since a mirror-image organism would function identically. Suggested explanations include a chance asymmetry in whatever chemistry started, amplified because a mixed system works badly, and small physical asymmetries including polarised light from certain astronomical sources that destroys one form faster. Meteorites carry a slight excess of one form, which supports the idea that the bias predates life.

Obtaining a single form is genuinely difficult, since the two are so alike, and several approaches are used. Reacting the mixture with something already handed produces two products that are no longer mirror images and can be separated by ordinary means, after which the added group is removed. Passing the mixture through a column packed with a handed material separates them by how strongly each sticks. Some mixtures crystallise into separate crystals of each form that can be picked apart, which is how Louis Pasteur first demonstrated the phenomenon in 1848 using tweezers and a lens. And modern synthesis increasingly builds only the wanted form from the start, using handed catalysts.

A carbon bonded to four different groups gives two mirror-image arrangements that cannot be superimposed and differ only in rotating polarised light and in how they meet other handed molecules. Since proteins are built from amino acids of one handedness, enzyme sites are handed and fit one form only. Separating the two is hard, and Pasteur first did it with tweezers in 1848.