Can chemicals tell right from left?

31 March 2014

Interview with

Stephen Driver, University of Cambridge

So humans have handedness, but is the same true for chemistry? Harriet Johnson was R and S Carvonejoined by Stephen Driver from the University of Cambridge.

Steven - there are certainly left-handed and right-handed versions of certain types of molecule.

Harriet -   And so, this affects the way that they work?

Steven -   Chemically, they're the same, but in biological systems, it can be very important to keep track of the differences between the left-handed versions and the right-handed versions.

Harriet -   I went to Hills Road Sixth Form College in Cambridge, armed with a smell challenge, using a chemical called carvone, kindly provided by (Dr. Stephan Hugh) from the University of Nottingham.  First, I wanted to tell how well the students and staff could identify the smell of carvone.  So, I gave a group of them a tube to sniff.

Female -   Peppermint.

Female -   Peppermint.

Female - Peppermint.

Male -   Spearmint.

Male -   Mint.

Harriet -   So, that was a pretty clean sweep with mint, but I gave another tube of mintscarvone to a different group and they had a little more trouble.

Male -   I think it smells a bit lemony.  It also reminds me of baby wipes.

Female -   Maybe something you'd have in like shower gels or something.

Female -   I thought it smell like aniseed or liquorish.

Female -   I wouldn't cook with it.  It sounds like musty clothing to me.  I don't like it.  It's something that makes me could've think of it (yuck!)

Chris -   And that was the same stuff?

Harriet -   Well, both groups were given a tube of carvone except one was this tube labelled s-carvone and the other one was labelled r-carvone.  So Steven, what's the difference?

Steven -   So, it's all about carbon atoms.  So, the carbon atom wants to form 4 chemical bonds and the four chemical bonds want to get as far away from each other in space as they possibly can.  So, they've taken a particular geometric arrangement called the tetrahedron.  So imagine if you like, three bonds pointing downwards and outwards forming a little tripod and fourth bond pointing straight up.  So, if you put different atoms or different chemical groups onto those four bonds, you form a molecule.  It turns out that you can do that as it were in a car with a left-handed way or an anti-clockwise way and the clockwise way.  What you have then are molecules that are the mirror images of each other.  In chemistry, we call those enantiomers and we say that molecules are chiral.  So, they're chemically the same but biochemically, they can be very different.

Harriet -   So, they have a different shape and this shape works in different ways.

Steven -   And one is the mirror image of the other.

Harriet -   Yeah.  So, there's different shapes in each of these tubes.  So, the S-form is what we'd call left-handed form.  And so, that form of carvone is found naturally in caraway which people seem to have a hard time identifying whereas the right-handed is found in mint which we saw people could recognise quite easily.  So, these are bonding with the receptors in their noses.  So, to test if they could see it the other way, we swapped the tubes around in the groups and this is what they thought...

Female -   It smells like a mixture of lemon and mint.

Female -   I think I viewed it like in Chinese cooking.

Female -   Caraway.

Male -   Pepper.

Male -   Vaguely, eucalyptusy.

Harriet -   And investigating the smelling abilities of the second group, here they are with the right-handed carvone...

Male -   I think it smells minty.

Female -   Yeah, just like a peppermint.

Female -   I think it's spearmint.

Female -   Toothpaste.

Female -   My aunt.

Harriet -   Apparently her aunt did always carry mints on her.  So, that was triggering her aunt receptor in her nose.  So Steven, is it just smells that this is working with or are there other effects everywhere else?

Steven -   There are other effects everywhere else as well.  So, in terms of applications, one very important area is pharmaceuticals for example.  Artificial senses is another one.  We've talked about the nose as a biological sensor, but if we want to make artificial senses, little gadgets that can detect molecules, it may be that they need to be sensitive to the left-handed and right-handed forms as well.

Harriet -   And so, there could be serious consequences if the chirality of the shape of the molecule is the wrong way around as we see in some drug examples.

Steven -   That's right.  So, there are examples we're one enantiomer, so one chirality of the drug molecule is effective and the chirality either is not effective where it can even be harmful.  So, it can be very important to get that right.

Joanne -   Yeah, the classic example was the thalidomide drug that was prescribed that was completely safe in one form and then they didn't know that the other form would cause these birth defects and that's how that accident happened, right?

Steven -   I believe it's actually slightly more complicated than that in that particular example.

Chris M. -   I think the problem is that if you have the pure one, then they spontaneously turn into the other one.  So, they haven't got a half-life which is very long, so you can't give the pure version.

Harriet -   So, it's really difficult to control the shape of these molecules.

Chris M. -   Of that molecule.

Harriet -   So Steven, this is what you research at the minute, trying to create molecules that only go one way around?

Steven -   That's right.  So, there are ways of doing that and there are ways of doing that that currently I used in the chemical industry.  But they involve a technique known as homogeneous catalysis.  So, I need to explain what a catalyst is.  If you're trying to do a chemical reaction, you're taking reactants and converting them into products.  Often, you need to control how that's done.  So either you need to speed the reaction up or you need to push the reaction towards a particular product.  In this case, we want to push it towards a particular chirality of product.  So, homogeneous catalysis, imagine that your reactants are liquid, your products are liquid, and your catalysts is also a liquid.  So, everything is in a liquid phase and you do your reaction, you get your product.  Then you have to somehow separate out any leftover reactants and the catalysts so that you can re-use it.  That's doable and that's done in industry, but it's difficult.  So, we're interested in a different way of doing that.  it's called heterogeneous catalysis.  That's widely used in the chemical industry and in car exhaust catalytic converters for example.  What's happening there is that the reactants of for example, gas phase, the products of also gas phase, but the catalyst is a solid.  Typically, it's a metal solid, so it's a surface of a metal.  So, we're looking at whether it's possible to use metal surfaces to do enantioselective heterogeneous catalysis.

Harriet -   And how is that working out for you?

Steven -   Well, we're getting promising results.  So, we're looking at how these kind of molecules interact with metal surface whether they're either symmetric surfaces that is or asymmetric.  I'm looking at the different ways in which chirality can manifest itself.

Harriet -   So very quickly, we saw in the carvone experiment that natural systems like the spearmint leaf are controlling chirality in the oils that they're making.  So, why is it so hard to replicate this in situ, in the lab.

Steven -   I don't quite understand the question.

Harriet -   Sorry.  So, these things are happening like in our bodies.  We're making chemicals that all go the same way around.  So, why is it so hard to do it artificially when you're synthesizing chemicals like drugs?

Steven -   I think it's not so difficult to do.  It's difficult to separate the products.  So, our interest is simply in moving from homogeneous to heterogeneous catalysis to make it an easier industrial process.

Harriet -   Okay, fantastic!  Good luck with that.  Thank you to Steven Driver from Cambridge University.

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