Specifically, they are trying to understand the properties of artificial proteins. Proteins are chain-like molecules that fold into particular shapes to act as enzymes, structural components, hormones, and much of everything else in living things. In order to be able to do all those things, they have the capacity to be fantastically complex. And it is this complexity that attracted Johnson to study proteins. In fact, he wanted to get started studying the electro-optical properties of proteins when he first arrived at Penn in 1994. But the theoretical understanding and physical tools needed had not yet been developed. And another molecule came calling: the carbon nanotube. These hollow tubes of crystalline carbon measure just over a nanometer in diameter but can be several thousand times that size in length. They have become the darling of the nanotechnology world because of their unheard-of mechanical and electronic properties: they are stronger than steel but lighter than aluminum, they can act as either conductive wires or semiconductors like those that make up computer chips, they can transport current or heat along their length with little resistance, they can even be made to emit light.

As fascinating a material as carbon nanotubes may be, they are quite simple when compared to proteins, says Johnson. And the NBIC researchers are able to take advantage of their understanding of nanotubes. “We’re building on our knowledge of electronics at the molecular scale, except we want to work with some molecules that are more complicated than carbon nanotubes,” says Johnson. “It’s an idea whose time has come from a physics standpoint.”

The proteins Johnson is working with are chains consisting of any of 20 biochemicals called amino acids. The sequence in which the amino acids occur determines what shape the chain folds into, and that shape determines many of its properties. In particular it determines what other chemicals, called cofactors, can be attached to it and where they fit. An example of a protein and its cofactors is hemoglobin, where the protein carries the cofactor heme, which does the work of carrying oxygen in blood. The sequence of amino acids that make up proteins in the NBIC experiments do not correspond to anything found in nature. Instead they are designed to hold cofactors—not all of them are found in nature, either—in particular spots and at particular orientations in the protein. While the proteins are important too, the cofactors are the heart of the molecule’s electro-optical properties.

Johnson’s optoelectronics experiments are a perfect example of interdisciplinary science. Biophysics Professor William DeGrado designs the proteins based in part on computational theory by Associate Professor of Chemistry Jeffery G. Saven. The cofactors are designed by Chemistry Professor Michael J. Therein. Using his own techniques and some tools developed in Dawn Bonell’s lab, Johnson puts the molecule on a chip between two electrodes that are separated by just a few nanometers and measures how current flows through it both in the presence and absence of light.

Being able to measure a single molecule at a time lets scientists observe the effects of subtle things in the molecule’s environment. You can see, for instance, the effect of the kind of metal the electrodes are made of, the influence of a nearby molecule, or the importance of the material on which the protein is lying.

Bonnell points out, too, that unless you look at the individual molecule, you can’t resolve the problems posed by the environment or discover some unexpected but useful physics that can be exploited. A circuit being influenced by something in its environment in a predictable way is the very definition of a sensor, and Johnson thinks sensor design is one discipline that could benefit from the work his group at NBIC is doing.

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