In the not-too-distant future, scientists may be able to use
DNA to grow their own specialized materials, thanks to the concept of directed
evolution. University of California, Santa
Barbara scientists have, for the first time, used
genetic engineering and molecular evolution to develop the enzymatic synthesis
of a semiconductor.
“In the realm of human technologies it would be a new
method, but it’s an ancient approach in nature,” said Lukmaan Bawazer,
first author of the paper, “Evolutionary selection of enzymatically
synthesized semiconductors from biomimetic mineralization vesicles,”
published in the Proceedings of the
National Academy of Sciences. Bawazer, who was a PhD student at the time,
wrote the paper with co-authors at UCSB’s Interdepartmental Graduate Program in
Biomolecular Science and Engineering; Institute for Collaborative
Biotechnologies; California NanoSystems Institute and Materials Research
Laboratory; and Department of Molecular, Cellular and Developmental Biology.
Daniel Morse, UCSB professor emeritus of biochemistry of molecular genetics,
directed the research.
Using silicateins, proteins responsible for the formation of
silica skeletons in marine sponges, the researchers were able to generate new
mineral architectures by directing the evolution of these enzymes. Silicateins,
which are genetically encoded, serve as templates for the silica skeletons and
control their mineralization, thus participating in similar types of processes
by which animal and human bones are formed. Silica, also known as silicon, is
the primary material in most commercially manufactured semiconductors.
In this study, polystyrene microbeads coated with specific
silicateins were put through a mineralization reaction by incubating the beads
in a water-in-oil emulsion that contained chemical precursors for
mineralization: metals of either silicon or titanium dissolved in the oil or
water phase of the emulsion. As the silicateins reacted with the dissolved
metals, they precipitated them, integrating the metals into the resulting
structure and forming nanoparticles of silicon dioxide or titanium dioxide.
With the creation of a silicatein gene pool, through what
Bawazer only somewhat euphemistically calls “molecular sex”––the
combination and recombination of various silicatein genetic materials––the
scientists were able to create a multitude of silicateins, and then select for
the ones with desired properties.
“This genetic population was exposed to two
environmental pressures that shaped the selected minerals: The silicateins
needed to make (that is, mineralize) materials directly on the surface of the
beads, and then the mineral structures needed to be amenable to physical
disruption to expose the encoding genes,” said Bawazer. The beads that
exhibited mineralization were sorted from the ones that didn’t, and then
fractured to release the genetic information they contained, which could either
be studied, or evolved further.
The process yielded forms of silicatein not available in
nature, that behaved differently in the formation of mineral structures. For
example, some silicateins self-assembled into sheets and made dispersed mineral
nanoparticles, as opposed to more typical agglomerated particles formed by
natural silicateins. In some cases, crystalline materials were also formed,
demonstrating a crystal-forming ability that was acquired through directed
evolution, said Bawazer.
Because silicateins are enzymes, said Bawazer, with
relatively long amino acid chains that can fold into precise shapes, there is
the potential for more functionality than would be possible using shorter
biopolymers or more traditional synthetic approaches. In addition, the process
could potentially work with a variety of metals, to evolve different types of
materials. By changing the laboratory-controlled environments in which directed
evolution occurs, it will be possible to evolve materials with specific
capacities, like high performance in an evolved solar cell, for example.
we’ve demonstrated the evolution of material structure; I’d like to take it a
step further and evolve material performance in a functional device,” said