Bioquimica

An Adventure in Stereochemistry: Alice in Mirror Image Land*
by Frank J. Dinan, Department of Chemistry and Biochemistry, Canisius College, Buffalo, NY Gordon T. Yee, Department of Chemistry, Virginia Polytechnic Institute and State University

The beautiful spring day made Alice reluctant to go into her dorm. She knew, though, that she had to study hard for her organic chemistry examProfessor Emil Fischer was giving the following day. As she entered her room, she found her cat, Kitty, eagerly awaiting her arrival. Alice sighed. “I’m sorry, Kitty. I’d love to play with you, but I have to study for my stereochemistry test tomorrow.” Kitty tilted her head as if to say, “What’s that?” Alice smiled at the cat’s reaction, then settled into her favorite soft chair, opened her organicchemistry text, and began to read. Alice’s stereochemistry reading went well for a while, but the hypnotic effect of the cat’s purring and the long day had their inevitable effect. The text began to blur, and Alice’s head nodded more and more frequently.

Alice found herself staring at her image in the full-length mirror hanging in her room. “We’re always talking about mirror images in my organiccourse, Kitty. I think that I’ll see what life is like on the other side of the mirror.” And with that, Alice stepped magically through the looking glass and entered Mirror Image Land. * * * * * * * * * * * * * *

“Who are you?” asked a startled old man wearing a rumpled tweed jacket.

“I’m Alice, from the other side of the mirror. Who are you?” asked Alice. “I am Professor Rehcsif Lime, anorganic chemist. May I ask why you came here?” “I came here because I want to see what life is like in Mirror Image Land.” “I see,” said the Professor. “I think that you will find that life is quite different here. Would you like a stick of sugar-free spearmint gum?” “Why yes, thank you,” replied Alice. She unwrapped the gum and started to chew. “Bleeeech!” exclaimed Alice as she spit the gum out. “Ittastes like sweet rye bread.”

“Yes, I thought that would be the case. You are not like us, you see,” said Professor Lime, grinning like the Cheshire cat. “What do you mean?” asked Alice, still trying to get rid of the terrible taste in her mouth. “Well, the spearmint plant in your world makes l-carvone, which is associated with the smell and taste of spearmint. But this is a mirror image world,and our spearmint plants make its mirror image, d-carvone. Because your taste buds and scent receptors are still the same, you perceive our spearmint gum to smell and taste like caraway, which is associated with d-carvone.” “I still don’t understand,” said Alice. “Is everything going to be different on this side?” “Well, no. Take baseballs, for instance. They’re the same in our world as in yours.But the gloves, now they’re different. The mirror image of a right-handed glove from your world doesn’t fit onto your right hand very comfortably. We say that balls are achiral, but gloves are chiral, that is, not superimposable on their mirror images. Of course, the mirror image of a right-handed glove fits your left hand perfectly.” The Professor looked sad as he added, “I’m afraid you can’tstay here for very long Alice or you’ll run into problems.” “Problems? But why? I’m able to walk and breathe just as if I were at home. Could I eat when I got hungry?”

“I’m afraid that would be the difficulty for you, Alice. All of your enzymes are mirror images of our enzymes, and yours aren’t adapted for digesting our food. Here, everything is the mirror image isomer, the enantiomer, of whatexists in your world. Your enzymes won’t allow you to use any of our proteins, because our amino acids are mirror images of yours. Enzymes and the molecules that they act on have to fit together in a lock-and-key fashion for the enzymes to catalyze reactions. Your enzymes just aren’t arranged to process some of our molecules.” “I study organic chemistry,” said Alice, “so I know that my enzymes...