Foresight Update 5 Page 4

Recent Progress: Steps Toward Nanotechnology

by Russell Mills

Electronics

Researchers at Caltech, JPL, and Univ. São Paulo, Brazil have designed (but not built) a molecular-sized shift register--a memory storage device with 1000 times the density and a ten-thousandth the energy consumption of its VLSI equivalent. Bits are stored by bumping individual electrons into the energy levels of a polymer, where they are moved along the polymer as more bits are written. The design has been worked out in some detail, and specifies the orbital energy levels of the molecules, the rates of competing (error-producing) electron transitions, spacing of the polymers, and the timing of the read/write cycle. The authors state that the register and associated read/write devices could be implemented with current technology; they provide chemical formulas of candidate molecules. [Science 241:817-820 (12Aug88)]

Molecular-sized conducting wires of lengths down to 3 nanometers have been made at the Univ. of Minnesota from polyacenequinone and imide subunits. The researchers hope to generate 3-D networks under 10 nm in size using similar chemical techniques. [New Scientist (19May88)]

Chemistry

Herschel Rabitz at Princeton Univ. proposes using femtosecond laser pulses to excite molecules in solution, measuring their response, and using the data to craft another pulse--thus homing in on the pulse structure needed to produce a desired chemical reaction. Once the correct pulse structure is known, it could be used routinely to carry out the reaction while dispensing with the elaborate techniques now required to protect one part of the molecule while another part is being modified. If Rabitz's method works, it may shorten many of the paths to nanotechnology by drastically simplifying the assembly of complicated molecules. [Science News 134:6 (2Jul88)]

Webmaster's Note: A recent bibliography of information on ultrafast lasers is available on the WWW at:

http://xro.physik.uni-jena.de/gibbon/bibhead.html

Micromanipulation

A technique has been developed at Bell Labs for trapping and manipulating microorganisms without damaging them. A lens is used to focus a laser on the organism; light refraction results in a force that pushes it toward the focal point of the beam. Viruses and bacteria can be trapped and immobilized by the technique; larger cells, such as yeast or protozoa, can be dragged around by moving the beam. The investigators even found that they could reach inside a cell with the laser beam, grasp internal organelles and move them around. One wonders whether a similar technique could be used to assemble components of micromachines like those discussed elsewhere in this article. [Science 241:1042 (26Aug88)]

Physicists at the National Bureau of Standards are now able to confine groups of sodium atoms between a set of laser beams and then slow down their motions to under 20 cm/sec. Under these conditions the properties of atoms can be studied with very high precision; such information will someday be needed for the design of nanomachines and zero-tolerance materials. [Science 241:1041-1042 (26Aug88)]

A step forward in our ability to handle individual molecules has been made by Japanese researchers at Osaka Univ. who have directly measured the tensile strength of an intermolecular bond--by pulling on it until it broke. The bond is that between protein subunits in a skeletal muscle filament. The filaments are chains of "actin" molecules held together by non-covalent bonds; two such chains wind around each another to form an actin filament. Another protein, "myosin", contains the motor apparatus of the muscle. The researchers obtained a value of 108 piconewtons for the tensile strength of actin filaments. They proceeded to measure the force exerted by each myosin "motor" as it pulls on an actin filament--about 1 pN. Since each actin filament is pulled on by roughly 50 myosin molecules, there would seem to be a safety factor of 2 built into our muscles. [Nature 334:74-76 (Jul88)]

Webmaster's Note: For more current information on:
using optical techniques to manipulate cells, sub-cellular components, and other micrometer-scale objects:

using lasers to manipulate atoms and single molecules:

Viewing

Biochemists at Cornell Univ. are now able to take 120 picosecond x-ray diffraction exposures of organic molecules and enzymes. This breakthrough is made possible by a magnetic "undulator" that produces an intense x-ray beam. Until now, x-ray diffraction analysis has required long exposures, especially for large molecules. Molecular motion would cause the images to blur, thus limiting the resolution obtained. With exposure times now reduced by a million-fold, it should be possible to watch enzymes change shape as they catalyze reactions and to troubleshoot nanomachines by observing them in action. [Science 241:295 (15Jul88)]

For more recent results, see article in Update 28.

Micromechanics

An electric motor less than half a millimeter across, miniature air-driven turbines, and gear trains--these are among the various micromachines recently fabricated at the Univ. of Calif. at Berkeley, Cornell Univ., and Bell Labs using the techniques of integrated circuit manufacture. Intended to provide measurements of friction, wear, viscosity, lubrication, stress, deformation, fatigue and other factors at the scale of microtechnology, they may be forerunners of practical devices: tiny fans for cooling integrated circuits, drug-dispensing mechanisms for smart pills, cutting tools for unblocking blood vessels, cell sorters for diagnostic tests. Similar methods might be used to make even smaller machines, but true nanomachines are probably beyond the range of these techniques. [Science 242:379-380 (21Oct88)]

Public attitudes

Victim of numerous court-ordered delays inspired by unfounded fears, the U.S. biotechnology industry has finally realized that it can no longer take public awareness for granted. Some companies have dealt with the problem by hiring public relations firms to promote positive attitudes toward them; often this approach has led to company-sponsored public meetings in communities where the testing of genetically modified organisms is being planned. The effectiveness of the effort is already evident--more than a dozen field tests have been conducted recently without controversy. [Science 242:503-505 (28Oct88)] Nanotechnology proponents: take note! Technophobia is an easy nut to crack when moderate resources are devoted to the effort.

Protein engineering

Wm. DeGrado's group at the duPont Co. has continued to make remarkable progress in protein design and production. Having designed a four-helix protein that self-assembles into a stable bundle, they proceeded to synthesize the gene for this protein, insert the gene into a bacterium, and show that the bacterium produces the desired protein. Although this effort aimed at studying the relationship between amino-acid sequence and 3-dimensional structure of proteins, the designed protein will probably be used as a "platform" for adding functional features. [Science 241:976-978 (19Aug88)]

A comment on the above paper by Eric Drexler

The molecules responsible for photon-capture in photosynthesis were mapped in detail several years ago. To find out how they work, scientists at MIT and Washington Univ. (St. Louis) are making amino-acid substitutions in the reaction center of photosynthetic bacteria. When they altered an important amino acid linking a chlorophyll molecule with its protein support, one of the chlorophyll subunits lost its magnesium atom--yet the system still functioned at about 50% efficiency. This suggests that photosynthesis does not depend critically on the molecular structures arrived at through traditional evolution, and that better and simpler molecules may be developed for powering some kinds of nanomachinery. [Science News 134:292]

Biological membranes are equipped with a variety of channels connecting the inside and outside of cells or organelles. These channels, made of protein, can be opened and closed; when open they allow certain ions to pass through the cell membrane. Wm. DeGrado's group at duPont has designed and synthesized a number of simple ion channel proteins and tested their ability to form functional ion channels in a phospholipid membrane. The proteins were chains of 14 to 21 serine and leucine residues, arranged into helical structures with the polar serines running down one side and the apolar leucines along the opposite side. A number of these helices would then aggregate in parallel to form a cylindrical bundle around a central channel. The researchers determined that 21-residue proteins spanned the membrane and created a conductive path for ions. The amino-acid sequence of the proteins determined the number of helices in a bundle, and this in turn determined the size of ions that could pass through the channel. [Science 240:1177-1181 (27May88)]

Protein engineering advances swiftly. In each of the following three summaries, researchers have programmed Escherichia coli bacteria to produce and secrete redesigned antibody molecules. Bacteria are far easier to program and grow than eukaryotic (nucleated) cells, but in earlier experiments bacteria would not output functional proteins. In the latest work the bacteria have been persuaded to produce "antigen-binding fragments" (Fabs) with the same specificity and affinity for their substrates as the original antibodies.

Researchers at Max Planck Institute developed a bacterial expression system mimicking the one eukaryotes use. In eukaryotic cells, an antibody's protein chains are synthesized in the cell's cytoplasm, then transported into an organelle called the "endoplasmic reticulum," where they are trimmed, folded, bonded, and paired into a functioning configuration. The researchers first examined the 3-dimensional structure of the antibody MCPC603 and decided which portions of it to keep. They next constructed a custom plasmid (mini-chromosome) consisting of: the DNA sequences coding for the antigen-binding portions of the antibody's protein chains, two bacterial "signal sequences" coding for protein appendages that tell the bacterial cell membrane to secrete the proteins, and several other sequences required for replication and translation of the DNA via RNA into protein. When this plasmid was introduced into Escherichia coli, the bacteria used the new DNA to make and secrete the Fab protein chains. The chains then folded and bonded themselves correctly. [Science 240:1038-1041 (20May88)]

A group at International Genetic Engineering, Inc. used essentially the same technique to produce a chimeric Fab consisting of antigen recognition domains taken from a mouse antibody, and the remainder taken from human antibody (presumably to forestall an immune attack on the Fab if it should be used therapeutically in humans). This particular Fab was chosen because it attacks human colon cancer cells. [Science 240:1041-1043 (20May88)]

Genex Corp. researchers have gone a step further in simplifying antibody molecules. Traditional antibodies are composed of four polypeptide chains. In the Genex design, two of these chains are eliminated and the other two are joined by a short chain of amino acids. The result is called a "single-chain antigen-binding protein." Genes to encode several such proteins were constructed and expressed in E. coli. The proteins produced by the bacterium proved to have the same specificity and affinity for the substrates as the original antibodies. Single-chain antigen-binding proteins are expected to replace monoclonal antibodies in such areas as cancer and cardiovascular therapy, assays, separations, and biosensors. [Science 242:423-426 (21Oct88)]

Amidases are enzymes that catalyze the hydrolysis of amide bonds. Of particular interest to biotechnologists are amidases specific for the amide bonds connecting amino acids together in proteins; what is needed are tools for cutting a protein at any desired place along its amino acid sequence. Researchers at Scripps Clinic and Penn State Univ. have overcome a major hurdle by developing a Fab that catalyzes the hydrolysis of a somewhat different amide bond joining two aryl components. Mice were immunized with a compound resembling the transition state of amide hydrolysis; whole antibodies collected from the mice were then enzymatically trimmed. The resulting Fabs sped up the hydrolysis reaction by a factor of 250,000. [Science 241:1188-1191 (2Sep88)]

Dr. Mills has a degree in Biophysics and assists in the production of Update.

From Foresight Update 5, originally published 1 March 1989.

Foresight thanks Dave Kilbridge for converting Update 5 to html for this web page.

Foresight Update 5

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