First developed in the early 2000s, molecular cages or nanocages, are hollow nanostructures with porous walls.

Scientists have used self-assembling protein molecules to build nanocages of different shapes and sizes for biomedical applications, like controlled drug delivery.

The tunable structure of nanocages allows them to trap therapeutic or diagnostic molecules inside their walls to deliver them on-demand.

Gold-Bonded Spherical Molecular Cages

International researchers report their “impossible” achievement in molecular cages: they created a nanocage with an incomparable spherical shape.

The team claimed their breakthrough to be “unparalleled in nature or even in mathematics.”

The researchers who built this “impossible sphere” was headed by the Heddle Initiative Research Unit at the Japanese RIKEN Institute in Wako.

Technically speaking, a nanocage is a polyhedron, a geometric figure with six or more flat faces. From the five Platonic polyhedra, geometry limits scientists to build nanocages with only three shapes. Those are the tetrahedron (4 plane faces), the octahedron (8 faces), and the icosahedron (20 faces).

Theoretically, they shouldn’t end up with this shape using the outline of an equilateral triangle as building bricks.

“Fortunately, Platonic idealism is not a dogma of the physical world. If you accept certain inaccuracies in the solid figure being constructed, you can create structures with shapes that are not found in nature, what’s more, with very interesting properties,” says Dr. Tomasz Wrobel from the Cracow Institute of Nuclear Physics of the Polish Academy of Sciences.

The team built a nanocage shaped like a sphere out of 24 protein rings each with eleven walls. The structure of this molecular cage is ultrastable, and scientists can control its assembly and disassembly.

It’s the addition of a gold glue, or gold (i)-triphenylphosphine, that triggers the self-assembly of protein rings. However, researchers weren’t sure gold atoms would form a permanent chemical bond between the rings of the nanocage.

Using spectroscopic imaging and X-ray photoelectron spectroscopy, they showed that the gold “really did form bonds with sulphur atoms in cysteines.” They noted:

“In other words, in a difficult, direct measurement, we proved that gold ‘glue’ for bonding protein rings in cages really does exist.”

The 24 walls of this “impossible” nanocage were held together using 120 gold atoms to form a structure with an outer diameter of 22 nanometres and an inner diameter of 16 nm.

“This work establishes an approach for linking protein components into robust, higher-order structures and expands the design space available for supramolecular assemblies to include previously unexplored geometries,” write authors of the paper.

Read More: Researchers Toughen Glass Using Silicon Carbide Nanoparticles

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