Volume 3, Issue 21 June 2006
Molecular Rules Of Engagement
Phil Geissler is a fellow of the David and Lucile Packard Foundation and of the Alfred P. Sloan Foundation.
A single molecule doesn't usually do much of anything. But combine a large number of molecules together and various sorts of amazing things can happen. Networks of polymers interact to propel cells, molecules in a plant's leaf work together to convert sunlight into energy, and so on. But what are the rules that give rise to these emergent behaviors and can we control them? UC Berkeley scientist Phil Geissler is using the tools of theoretical chemistry to find out.
"What intrigues us in a very generic sense is how molecules can act in concert to give rise to behavior that is unexpected on scales that are relevant for things like biology and materials science," says Geissler, a professor in the Department of Chemistry and a faculty affiliate in the California Institute for Quantitative Biomedical Research (QB3).
In each research effort, Geissler and his colleagues construct simple models to gain insight into how molecules may interact. For example, they might start with a group of molecules at equilibrium and then attempt to discern and predict what will happen when that system is disturbed by adding, say, light, or triggering a chemical reaction. The goal is not to create a highly-detailed model, but rather one that "has just enough meat on it" to be valuable, Geissler says. Simplified models, he explains, allow both thorough statistical analysis and a clear way to discern the effects of specific physical ingredients.
"The tools we use are pencil and paper and some computer simulation," says Geissler, who recently earned a prestigious Sloan Foundation Research Fellowship. "I haven't gotten my hands wet in a long time."
An illustration of the models of biological polymers that Geissler's group is studying in connection with Dan Fletcher's experiments on actin networks. Each line represents an actin filament. The color and width of a segment indicate its strain during a typical equilibrium fluctuation. (Red and thick means more highly distorted, thin and blue means more weakly distorted.) This configuration demonstrates the emergence of force patterns that result from connections between different molecules. (courtesy the researchers)
Instead, Geissler is collaborating with UC Berkeley experimental nanoscientists Dan Fletcher, Matthew Francis, and Paul Alivisatos, to apply his models to the nanoscale realm of self-assembly. In recent years, synthetic chemists and biologists have identified or created a wide array of organic and inorganic materials with fascinating electronic and optical properties. Some of these materials could perhaps lead to the likes of nanoscale computers that are extraordinarily faster than today's processors, or solar cells far more efficient than state-of-the-art photovoltaics. The challenge though is arranging these tiny components into precise patterns that can be wired together into a useful system.
"It sounds like a pipe dream," Geissler says, "but we're exploring ways to alter the conditions in which these simple components live so that they self-assemble into interesting structures."
In recent work, Geissler and Fletcher have investigated how rather stiff biological polymers can induce motion within a cell. Consisting of proteins such as actin, these filaments provide the cell's structure and mechanical integrity but also work together to power a cell's forward motion.
"You'd think that if you could figure out how one filament works, you could jump to understanding the entire network," Geissler says. "But that's not the case. So we're involved in developing models that describe how these filaments talk to one another in a network and the consequences of that communication and cooperation."
Results of a computer simulation for a system of "light harvesting complexes," in this case the protein shells of viruses, whose interactions can be turned on and off. From these simulations, Geissler and Matt Francis hope to learn how varying the strength, variety, and time-dependence of interactions leads to controllable capsid arrangements. Eventually, they aim to mimic this behavior in a bio-artificial solar cell.
Based on such models, Geissler and Fletcher are now considering whether they can take inspiration from the motion of chloroplasts, responsible for the energy conversion within plant cells, to build bio-artificial solar cells. One reason plant cells are so efficient at converting sunlight into energy, Geissler explains, is because they can modulate how much light is absorbed. That's possible because the chloroplasts can move to and away from the light as necessary to achieve maximum efficiency and avoid damage from too much light. The researchers hope to eventually incorporate that into a synthetic solar cell inspired by nature's emergent behavior.
"It's all a matter of figuring out how to program complex, rational behavior into systems that live by simple rules."