19 September 2005

Biochemists Turn To Quantum Physics

by Kate Melville

Charge is a property of electrons most people are familiar with while another property, spin, is lesser known and typically the preserve of physicists. Electron spin occurs in one of two opposing directions - up or down - and biochemists want to start factoring electron spin into their computer simulations of biochemical reactions to make them more accurate.

Biochemists have long used computers to model how large, complex biological molecules will react with each other but a U.S. researcher believes that to gain an even more fundamental understanding of the reactions taking place, electron spin must be considered. The man with spin on his mind, Jorge H. Rodriguez, from Purdue University, says that factoring spin into molecular models could save the pharmaceutical industry years of work and millions of dollars. "Whereas we have had to be satisfied with observing the chemistry in living things and describing it afterward without complete understanding, we are developing computational tools that can predict what will happen between molecules before they meet in the test tube," said Rodriguez. A paper describing Rodriguez' efforts appears in the Journal of Chemical Physics.

Rodriguez' special area of interest is a class of metal-based proteins that includes hemoglobin and chlorophyll, and their reactions in plants and animals. "Physicists have long known that, according to the laws of quantum mechanics, there are some chemical reactions in our bodies that are 'forbidden' - such as hemoglobin's binding oxygen in our lungs when we breathe. But they do happen nonetheless. So, because these reactions involve electron spin, we decided to take a closer look at them," explained Rodriguez.

"Nature loves balance, and you see evidence of it in both charge and spin," Rodriguez continued. "For example, electrons of opposite spin like to pair up with each other as they fly around the nucleus. This allows their spins to balance one another, just as positive and negative charges do between protons and electrons. Even when you have hundreds of electrons forming an immense cloud around a complex molecule, you still see balance in both charge and spin. But sometimes the electrons in metalloproteins seem to be playing a trick on us. As we see with hemoglobin, nature appears to be conserving electronic charge while sacrificing this conservation in spin."

At hemoglobin's center is the transition-metal iron, where several electrons can fly around the nucleus unpaired. When a red blood cell encounters oxygen in our lungs, its hemoglobin is able to grasp some of the oxygen with some of these unpaired electrons, carrying it to the rest of our body. But in the process, the cumulative spin of the system changes in a way that is not conserved, which seems impossible to a physicist. "This chemistry is vital for life, but physicists wonder how it can happen," Rodriguez explained. "The charge between the electrons in the bonded oxygen and hemoglobin is balanced in the end, which makes sense to chemists. But the electronic spin of the entire system is not conserved, making a physicist frown at what appears to be a formally forbidden process. Of course, we needed to learn more about nature at the microscopic level."

As many of these supposedly forbidden reactions involve biomolecules and transition metals, which can flip back and forth between different spin states under certain conditions, Rodriguez theorized that it was this variability in spin state that was influencing the rate of these reactions.

Rodriguez' team is using supercomputers which he says will soon run virtual models of molecules that can then 'react' with one another in simulations that accurately predict what will happen when they meet in the physical world. "We are at the point where we have developed computational tools to analyze the spin-dependent processes of biomolecules and have applied them to a few important test cases," he said. "But our methods are based on approaches that are valid for any molecular system. Therefore, hundreds more metalloproteins that are of great scientific and practical interest may be studied in the future with the methods we have developed. We are creating a new field that attempts to understand biochemical processes at the most fundamental level - that of quantum mechanics. It could be the most important step toward making biochemistry a predictive science rather than a descriptive one," he concluded.

Source: Media release - Purdue University