Three-dimensional formation of blurred microscopic membranes

Image: Image (false color) of a sponge-like phase of liquid colloidal films, self-assembled from a binary mixture of short and long rods
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Credit: Ayantika Khanra

Cell membranes seamlessly transition between distinct three-dimensional configurations. It is a remarkable feature that is essential for many biological phenomena such as cell division, cell movement, transport of nutrients into cells, and viral infections. Researchers at the Indian Institute of Science (IISc) and their collaborators recently devised an experiment that sheds light on the mechanism by which such processes might occur in real time.

The researchers looked at colloidal membranes, which are micrometer-thick layers of compact, stick-like particles. Colloidal membranes provide a more amenable system to study because they exhibit many of the same properties as cell membranes. Unlike a plastic sheet, where all the molecules are immobile, cell membranes are liquid sheets in which each component is free to diffuse. “This is an essential property of the cell membranes available in our region [colloidal membrane] system too,” explains Prerna Sharma, Associate Professor in the Department of Physics, The Institute of Ismaili Studies and corresponding author of the study published in the journal Proceedings of the National Academy of Sciences.

Colloidal membranes were formed by preparing a solution of rod-shaped viruses of two different lengths: 1.2 μm and 0.88 μm. The researchers studied how the shape of the colloidal membranes changes as one increases the fraction of short sticks in the solution. “I made several samples by mixing different sizes of the two viruses and then observing them under a microscope,” explains Ayantika Khanra, a doctoral student in the Department of Physics and first author of the research paper.

When the proportion of short rods increased from 15% to between 20-35%, the membranes transitioned from a flat disc-like shape to a saddle-like shape. Over time, the membranes began to fuse together and grow in size. Saddles are categorized by their order, which is the number of ups and downs encountered as one moves along the edge of the saddle. The researchers note that when saddles fused laterally, they form a larger saddle of the same order or higher. However, when they merged at almost a right angle, away from their edges, the final formation had an object-like shape. The catenoids then fused with other saddles, giving rise to increasingly complex structures, such as trienoids and quadrupoles.

To explain the observed behavior of membranes, the researchers also proposed a theoretical model. According to the laws of thermodynamics, all physical systems tend to move towards low-energy configurations. For example, a drop of water takes on a spherical shape because it has less energy. For diaphragms, this means shapes with shorter edges, such as flat disc, are more preferred. Another property that plays a role in determining the composition of the membrane is the Gaussian curvature modulus. The main idea of ​​the study was to show that the Gaussian bending modulus of the films increases when the fraction of the short rods is increased. This explains why adding more short rods pushes the membranes toward saddle-like shapes, which are lower in energy. He also explains another observation from their experiment where low-order membranes were small in size, while high-order membranes were large.

We have proposed a new mechanism for generating curvature of liquid films. This mechanism for tuning the curvature by changing the Gaussian modulus could play a role in biological membranes as well,” says Sharma. They want to further study how other microscopic changes in membrane components affect the large-scale properties of the membranes, she says.

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