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Research on the Ability of Protein to Conduct Electricity Like Metals

Even the most experienced of scientists can get a surprise jolt at times from a totally unpredictable result when pushing the boundaries of discovery.

Lindsay explored the interactions between a protein, called an integrin (alphaVbeta3), and its target, called a ligand (RGDfC). Lindsay's team was able to manufacture a nanodevice to more finely control a series of experiments with a carefully sized gap to control the protein, an electrode holding the ligand in position, and control the amount of voltage that can be applied to it. CREDIT Weisi Song, Biodesign Institute, Arizona State University

This indeed was the case for ASU Regents' Professor and biophysicist Stuart Lindsay, who spent his career developing new microscopes that are considered to be the eyes of nanotechnology and next-generation, low-cost, rapid DNA and amino acid readers as they enable making precision medicine more of a reality.

In the process, Lindsay's research team succeeded in learning a thing or two about how single molecules act when tethered between a pair of electrodes, which is the groundwork for how his DNA readers work.

The technology, known as recognition tunneling, is capable of threading single molecules down a nanopore just like a thread through the eye of a needle.

Electrodes, as they go down the nano-rabbit hole, have the potential to measure the electrical properties of these single DNA or amino acid molecules in order to determine their sequence identity.

After spending a substantial amount of time developing DNA and amino readers, the idea was to give whole proteins a try.

"The technological goal here was, can we use our technology to electronically detect whole proteins.

Stuart Lindsay, ASU Regents' Professor and biophysicist

However, about four years ago, Lindsay's research team obtained a lab result that even he could not quite believe.

It goes against all current perceptions as with most scientific surprises.

"What we've done here is to use our recognition tunneling to measure the electrical conductance of intact proteins. The thought was, that if you can specifically trap a whole protein between a pair of electrodes, you would have a label-free electronic reader."

The possibility of having a nanotechnology device sensitive enough to identify a single protein molecule may become a powerful new diagnostic tool in medicine.

However, proteins, the building blocks in every cell, were assumed to behave electrically as inert organic blobs. Electronically, they were believed to behave as insulators, just like placing a piece of plastic over a metal wire.

There is just a large amount of swept under the rug data on the electrical properties of proteins, there is one camp who dismiss these claims. There is another camp that says proteins are incredible electrical conductors. And never the twain shall meet, just like American politics."

Stuart Lindsay, ASU Regents' Professor and biophysicist

Four years ago, Yanan Zhao, one of his graduate students at the time, gave the protein challenge. He tethered a protein between two electrodes, followed by turning up the voltage, and voila! The protein began acting like a metal, with a wild and "remarkably high electronic conductance."

"If it's true, it's amazing," said Lindsay.

After years of attempting to disprove the results himself and also trying to account for every single possible wrong avenue or detour, his research group has now published their latest findings in the advanced online edition of the Institute of Physics journal Nano Futures.

"What this paper is mainly testing out are all the alternative explanations of our data, and ruling out all of the artifacts," said Lindsay.

The first extraordinary results were carried out with the help of a technology Lindsay helped spearhead, known as Scanning Tunnel Microscopy (STM). An integrin, a glue-like protein that helps cells stick together and assemble into organs and tissue, was employed in the experiment.

An electrode attached to a small molecule, known as a ligand, and extending from the tip of the STM, and this electrode specifically binds to the integrin protein. After being set in place, the STM is available with a lever arm and probe similar to a stylus and needle on a turntable in order to make the ligand come in contact with its integrin target.

This is where the weirdness started.

"I just didn't believe it, because what he saw were giant pulses of current when the probe was known to be a great distance from the surface," said Lindsay.

That gap would have been extremely great for the electricity to run through by electron hopping, or tunneling, as what happens with Lindsay's recognition tunneling sequencing technology.

In vain, Lindsay scratched his head trying to match a theory in order to explain the phenomena.

"That data simply cannot be explained by electron tunneling," said Lindsay.

A major turning point referred to Lindsay uncovering the work of theoretical biophysicist Gabor Vattay from the Department of Physics of Complex Systems, Eötvös Loránd University, Budapest, Hungary.

We had this data for a number of years, then I read this paper by Gabor Vattay that involved some absolutely amazing quantum mechanics, it turns out that energy level spacings in a quantum system signal whether the system is a conductor or insulator. There is a special signature of a state poised between conducting and insulating, and Gabor Vattay looked at a bunch of proteins, finding them poised at this critical (and highly improbable) point. An exception was spider silk which is a pure structural protein.

Stuart Lindsay, ASU Regents' Professor and biophysicist

The theory suggests that an electrical fluctuation is capable of kick-starting a protein into being a great insulator. "It's just poised to do this fluctuating thing," said Lindsay.

"In our experiments, we were seeing this weird behavior in this huge protein conducting electricity, but it is not static. It's a dynamic thing."

The electronic spikes were observed with increasing frequency as the voltage was upped across the protein. There is indeed a threshold to cross. "Below a certain bias, it's just an insulator, but when the fluctuations start kicking in, they are huge," said Lindsay.

"Because of this, I contacted Gabor, and he had to use some of the best supercomputers in Europe to analyze our large protein. Basically, there are 3 curves for the distribution of energy level spacings, one corresponding to a metallic state, another to an insulator state, and middle third, corresponding to the quantum critical state."

"Low and behold our protein is in the quantum critical state if you believe the theory."

Lindsay's team next succeeded in manufacturing a nanodevice in order to control another series of experiments ver carefully, with a cautiously sized gap to control the protein and voltage amount that can be applied to it.

"And the nice thing about having our chips is that we know we can make them small enough to where we just have a single protein molecule there in the gap."

That was a big change from earlier experiments as they did not know precisely what was happening at the tip of the STM.

"In the device, you get this beautiful switching on and off of the electrical conductance of the protein," said Lindsay.

His results confirmed that basic quantum forces are work to explain the manner in which the integrin protein was performing in the experiments.

Basically, we've eliminated all of those sources of "I don't believe this data" and we are still seeing this weird behavior of this huge protein conducting electricity. It's still there and it's beautiful.

Stuart Lindsay, ASU Regents' Professor and biophysicist

It is also overturning the way scientists are observing the electrical properties of proteins.

"There are people who are beginning to think of proteins as quantum mechanical objects," said Lindsay.

Lindsay next aims at exploring other medically important proteins and measuring their behavior with the help of the solid-state nanodevices.

Could proteins very important to health and disease turn out to act like metals? Or insulators?

One thing that is certain refers to a totally new way of analyzing how protein behavior has made room for new scientific vistas that earlier, Lindsay and many others did not think was possible.

"I believe the data now, but it's only one protein so far," cautions Lindsay.

As a serial entrepreneur with successful ASU spin-out companies, Lindsay may have one more trick up his sleeve in order to translate a fundamental discovery into the marketplace.

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