Why does the same chemical smell different in oranges and lemons?

Cambridge University's Sarah Madden sniffed out the answer to this question...

Sarah -They have the same chemical in them - limonene. It is truly the same chemical; it’s made up of the same atoms; it has the same bonds. But the difference is, the two versions that you find in oranges and lemons are actually the mirror image of each other. The way to think about this is to compare it to our hands. If we look at our hands we have two hands and they are the same but not, they are mirror images of each other.

What happens is that these two same, but different, chemicals interact with our nose differently. Again, we have receptors in our nose and because these compounds aren’t exactly the same it interacts differently with our receptors and we can smell the difference.

Chris - Right, OK. So you’ve got the same molecule except that it’s almost like I’ve shone it in a mirror and I’m looking at the mirror image molecule. Oranges have one form and lemons have the other but, when they go up your nose, because they are a bit like a right hand and a left hand, when they try and go into the receptor in your nose because that’s like a right hand going into a right handed glove, you can’t put a left hand in a right handed glove, at least not easily. Therefore, you smell one as one kind of smell because it’s registering with one group of receptors and one is a different smell because it’s registering with a totally different set of receptors.

Sarah - Yeah, that’s exactly right. It’s strange how such a small difference can have such a big effect on the body.

Chris - And in the drugs industry, this must be important because if you can make the same molecule and a chemist will tell you “yeah, I’ve made chemical A,” but I could have chemical A or chemical B, which is its mirror image, only one of those two forms might work in the body to kill cancer or do a job?

Sarah - Yeah, that’s really true. So often we have them as a mixture because it’s just basically as a chemist I know it’s so much easier and, therefore, so much cheaper to make it as a mixture of the two mirror images. But this can have effects on that you have a lot more side effects sometimes when you have the mixture. So you often find the really expensive medicine is the pure one form of the mirror image.