Oranges and lemons and quantum biology in Y12

The Rosy Periwinkle
from Madagascar

In yesterday's session, you set out to develop a series of plant extracts in order to test their anti-bacterial properties. This is a form of "Bioprospecting", a generic term sometimes used to describe the traditional approach to Drug Discovery. So for example, anti-cancer drugs such as vincristine and vinblastine have been found in the Rosy Periwinkle (left) found in Madagascar. Often these initial observations are based on local knowledge and cultural practices. Pharmaceutical companies have historically investigated such compounds and then developed methods for their synthesis in order to control the costs of production and to ensure the stability of supply. There are of course important issues relating to the impact on the communities who might be adversely affected by such discoveries. Indeed morphine, which can be found in poppy seeds is on the one hand invaluable in pain management, but on the other hand a major cause for social unrest in view of its illegal use as a narcotic, which in a similar way to cocaine impacts on certain developing countries such as Afghanistan.

Whilst I was homogenising the lemons, I was immediately transported to my

organic chemistry classes as an undergraduate at Sheffield and I remembered the story of oranges and lemons and the chirality of limonene. The characteristic difference between the smell of oranges and lemons couldn't be more subtle, from a chemical perspective. As you can see left the two molecules differ only with respect to their chirality. That is, both molecules are mirror images, like left and right hands. But that's clearly enough to change the way our brain interprets their smell, or odour. Which brings me nicely on to "quantum biology". 

It has been suggested that the translation of the information from a small molecule such as limonene into a sensory perception is partly a result of the shape of the molecule, but is possibly a result of the molecule's quantum level vibrational fingerprint! So as you know, all bonds have a characteristic set of vibration frequencies derived from bending, stretching etc. You may have come across this in Chemistry. These phenomena can be understood using quantum theory and unlike the lock and key concept (which we will explore soon) it has been suggested that neuronal responses to odour are triggered by quantum vibrations. So why would a good experiment be to compare the sell of limonene in both chiral forms with a deuterated version?