By Gail Gallessich

An interdisciplinary team of UCSB researchers in physics and biology has made a discovery at the nanoscale level that could be instrumental in the production of miniaturized materials with many applications. Dubbed a “living necklace,” the finding was completely unexpected.

The scientists studied microtubules from the brain tissue of a cow to understand the mechanisms leading to their assembly and shape. Microtubules are nanometer-scale hollow cylinders inside cells that are fundamental to cells’ internal structure and how they work.

In an organism, microtubules and their assembled structures are critical components in a broad range of cell functions—from providing tracks for the transport of nutrient cargo to forming the spindle structure for cell replication. Their functions include the transport of neurotransmitters in neurons. However, the assembly mechanism of microtubules within an organism has been poorly understood.

This was the puzzle facing the collaborating labs of Cyrus Safinya, professor of materials and physics and faculty member of the Biomolecular Science & Engineering Program, and Leslie Wilson, professor of biochemistry in the Department of Molecular, Cellular, and Developmental Biology.

The researchers discovered a new type of higher order assembly of microtubules. Positively charged, large, linear molecules (tri-, tetra- and penta-valent cations) resulted in a tightly bundled hexagonal grouping of microtubules—a result that was predicted. Unexpectedly, the scientists found that small, spherical divalent cations caused the microtubules to assemble into a “necklace.”

They discovered distinct linear, branched, and loop-shaped necklaces. They also found that the living necklace bundle is highly dynamic and that thermal fluctuations cause it to change shape.

This latter discovery could influence the development of vehicles for chemical, drug, and gene delivery; enzyme encapsulation systems and biosensors; circuitry components and templates for nanosized wires and optical materials. The findings were reported in the Proceedings of the National Academy of Sciences.

The first author of the paper is Safinya’s graduate student, Daniel Needleman. Postdoctoral researchers Uri Raviv and Miguel Ojeda-Lopez from Safinya’s group and Herbert Miller, a researcher in Wilson’s group, completed the team.

Safinya and Needleman commented that from a formal theoretical physics perspective, the living necklace phase is the first experimental realization of a new type of membrane where long microtubule molecules are oriented in the same direction but can diffuse within the living membrane.

The scientists envision applications based on both the tight bundle and living necklace phases. For example, metallization of necklace bundles with different sizes and shapes would yield nanomaterials with controlled optical properties.

A more original application is in the area of using the assemblies—encased by a lipid bilayer—as drug or gene carriers where each nanotube may contain a distinct chemical, as noted by the team. In delivery applications the shape of the bundle determines its property. For example, the linear necklace phase with its higher surface to volume ratio would have a larger contact area and a faster delivery rate compared to the tight bundle phase.