Penn Researchers Devise a Theory That Describes Mysterious Rafts in Membranes

Ali Sundermier | alisun@upenn.edu | 215-898-8562
Friday, January 20, 2017
Viral Membrane

An illustration of a viral membrane with rafts surrounded by depletant particles.

If you held a ball up to a mirror, it would produce an image that, if pulled out, would sit perfectly on top of the ball. Yet if you held your right hand up to a mirror, the image produced would be a left hand, which is not identical to the original; a glove for one hand cannot comfortably fit the other. 

Objects different from their mirror images, such as your hand, are known as chiral objects. Most of the molecules in the human body, including DNA, are chiral.

In a new paper published in the Proceedings of the National Academy of Sciences, researchers at the University of Pennsylvania produced a theory that describes how the chirality of microscopic rod-like particles can stabilize small domains called rafts in membranes composed of these particles.


Louis Kang

Louis Kang


The study was led by Louis Kang, a graduate and medical student in Penn’s M.D.-Ph.D. program in the Perelman School of Medicine, and ​​​​​​​Tom Lubensky, Christopher H. Browne Distinguished Professor of Physics in the School of Arts & Sciences.

The Penn researchers considered membranes formed from filamentous viruses, which can be conceptualized as microscopic rods, in the presence of tiny polymer particles called depletants, which can be conceptualized as spheres. Experiments carried out by Zvonimir Dogic and his group at Brandeis University have shown that, when short and long viruses are mixed together in a single membrane, the shorter viruses segregate into mutually repelling rafts of a fixed size.

Kang and Lubensky's theory demonstrates that the composition, size and interactions of these rafts can be explained by the natural tendency for chiral rods to adopt twisted structures and the thermal forces that the depletant particles exert on the viruses, which is related to the concept of entropy.

"It's a simple system,” Kang said. “There's just water, viruses and depletants. So you should be able to explain it by using simple principles.”


Tom Lubensky

Tom Lubensky


​​​​​​​​​​​​​​According to Lubensky, though biological cells are significantly thinner and more complicated than the system used in this study, there is a potential connection with this theory and rafts in cell membranes. In mathematical descriptions for how molecules are configured in cell membranes and how particles are aligned in viral membranes, the two equations would share many common features.

While the existence of cell membrane rafts is still controversial, they have received a large amount of attention in the biological and biomedical literature, and their disruption has been hypothesized to play a role in certain diseases and anesthesia.

The research was supported by a grant from the National Science Foundation and Simons Foundation.