Your left hand looks much the same as your right but try putting your right into a left-handed glove to see just how different they are. The mismatch occurs because they are asymmetrical, mirror images of one another.
Something similar happens when organic chemists devise new drugs. Molecules have a distinct three-dimensional shape, described by the term stereo-chemistry. But they can occur in two mirror image forms. They may look very similar but as with the left and right hands, they are different in shape and often very different in their actions, Dr Anita Maguire, a lecturer in organic chemistry at University College Cork, explains.
There are many examples of how these enantiomers or mirror image forms could differ, she explains. The chemical limonene smells like an orange in one form and like a lemon in the other. Another molecule, carvone, smells like caraway in one form and like spearmint in the other.
Knowing the activities of the two forms becomes crucial in drug development, Dr Maguire said. The main treatment for Parkinson's Disease is based on the `lefthanded' version of Dopa (LDopa). L-Dopa is broken down in the brain and eases tremors, but its mirror image breaks down into a toxic substance that can accumulate in the brain.
Drugs work in the body because of their ingredients but also because they have a particular shape that can interact with cell surface molecules. The actions of a drug may change on the basis of its handedness, she explained. Regulatory authorities controlling release of new drugs demand full biological studies of both forms to ensure safety.
The problem for synthetic organic chemists however is that when these compounds are synthesised they readily form both mirror images even though only one might be wanted. "One of the most important areas in research in organic chemistry worldwide is the development of new methods for the construction of organic compounds as one mirror image form," Dr Maguire stated.
Drugs are often produced as a mix of left and right and then companies apply "resolution" separation technology to recover the required form. The unwanted form is often just thrown away, she says. "Obviously that is not very attractive from an environmental point of view." The alternative is to find ways to produce only the form that is wanted, a process known as asymmetric synthesis.
Dr Maguire heads a team of 12 postdoctoral and postgraduate researchers in the Department of Chemistry who are looking at this problem and in fact the team has had considerable success with asymmetric synthesis. Chemists can control the way that substances react by using catalysts which can alter both the speed and the way that chemicals combine. Researchers have spent many years looking at biological catalysts and catalytic metals such as rhodium to achieve asymmetric synthesis. Dr Maguire's group decided to combine the two catalysts, one of the first groups in the world to attempt this.
"By combining the rhodium catalyst and the biocatalyst, ordinary baker's yeast and sugar bought from the local supermarket, we were able to do some transformations that were amazingly novel," she said. They were able to create molecules of one handedness and could easily choose either form.
The metal catalyst gives control over the position where atoms joine and the yeast catalyst controls the three-dimensional form. "We are broadening out the applications. We are trying to expand this approach by using copper catalysts," Dr Maguire says.
The object would be to expand the degree of control that could be exerted over these asymmetric synthesis reactions.
They have already synthesised a range of compounds including a number of insect pheromones, the chemical attractants insects use to communicate over distances, producing both forms. "We are expanding the scope of the research by looking at the use of other biocatalysts and other transition metal catalysts and more importantly by careful design of the substrate structure to lead to novel transformations."