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Tuning organic–inorganic interactions

14 August 2019

A modular lattice of programmable proteins is the first step toward hybrid materials.

When the human body grows bone, it produces a protein that self-assembles in three dimensions into a scaffold that directs inorganic crystals to form around it. The ability to mimic that process of organic materials forming in tandem with inorganic ones could have applications in semiconductor and medical research. Harley Pyles, Shuai Zhang, James De Yoreo, and David Baker, of the University of Washington and Pacific Northwest National Laboratory, recently took the first step toward re-creating the miracle of bone by designing a protein that they used to tailor organic–inorganic interactions.

The team’s designer protein, DHR10-micaX, adsorbs to the crystalline surface of mica. Pyles, who used Baker’s computer program Rosetta to design the protein, made each of DHR10’s subunits 10.4 Å long, double the separation between potassium cations in the mica. When placed in a potassium chloride solution, the protein nanorods matched up perfectly with the K+ sublattice on the mica plane.

DHR10-micaX lattice.
An atomic force microscopy image of a DHR10-micaX lattice. The field of view is an 800 nm square (200 nm square in the lower left inset). At upper right is a Fourier transform of the main image.

Later versions of DHR10 formed higher-order structures. The team introduced new features into the protein that allowed three nanorods to assemble into a trimer, which then connected with other trimers to form a hexagonal lattice, like the one in the figure. The honeycomb structure proved to be modular: Researchers could alter the size of the hexagons by varying the number of repeating subunits in each protein nanorod.

The team tuned the protein–inorganic interaction to account for a bevy of forces. If the protein nanorods were too short or too long, they wouldn’t assemble into the honeycomb lattice. In addition, the proteins bound to the mica and assembled only if the salt solution had a narrow range of concentrations. Each modification to the salt solution was made on the fly as Zhang took measurements with atomic force microscopy.

Even though the study highlights the delicate nature of programmable, composite systems, it shows that such systems are possible. The researchers will next attempt to reproduce their findings on semiconductor surfaces that are less hospitable but more technologically useful than mica. They’re also hoping to find a way to use mica as a scaffold to grow proteins that can later be removed from the surface entirely. (H. Pyles et al., Nature 571, 251, 2019.)

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