Linus Pauling and the Structure of Proteins: A Documentary History Narrative  
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A Question of Size
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In part because of the sticky, messy, hard-to-work-with nature of many proteins, in part because they did not have adequately sensitive equipment, in part because they did not know how proteins were built, researchers in the early part of the twentieth century had come to wildly different conclusions about the molecular size of the substances. Some scientists believed that native, active proteins were aggregates of relatively small molecules. Some thought they were single giant molecules, many times larger than any inorganic molecules found in nature, many times larger, in fact, than any molecule ever discovered. At the time Pauling was starting college, estimates of protein sizes varied by more than an order of magnitude.

It was not until the 1920s that a new invention helped decide the question of protein size. Theodor (The) Svedberg, a Swedish chemist, was a specialist in the study of colloids (a colloid is a dispersion of smaller molecules in another material, like globules of fat in milk, or bits of ash in smoke; many protein chemists believed that proteins formed gel-like colloids). To study colloids better, Svedberg invented a machine called an ultracentrifuge. It basically consisted of test tubes whirling around a central spindle at high speed, creating in the tubes an effect similar to increased gravitational pull. Spin a complex mixture like human blood fast enough, and molecules would settle out, moving toward the bottom of the tubes at a rate determined by their molecular size, shape, and the nature of the liquid in which they were suspended. Ultracentrifugation proved a powerful tool in protein studies, allowing at least the partial separation and purification of substances in complex mixtures.

Svedberg's careful studies indicated that many proteins were indeed giants of the molecular world, thousands of times larger than most inorganic molecules. It also hinted that many of these enormous, biologically active proteins might be composed of smaller subunits of roughly the same size. Perhaps, he thought, amino acids preferred linking into groups of a certain size, with these groups somehow aggregating to form active proteins. The idea of a basic protein subunit caught on. Estimates indicated that these common units might be made of 288 amino acids.

For his work with the ultracentrifuge, Svedberg was awarded the Nobel Prize in 1926 - just seven years before Pauling started his own research into proteins.

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Audio Clip  Audio: The Evolution of Svedberg and the Ultracentrifuge. November 1, 1991. (3:47) Transcript and More Information

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The Svedberg, 1940s.

Page 3
"Linus Pauling Notebook. Solvay Congress 1953" April 1953.

"The machine was a simple idea that required superb engineering. Rotor and bearings allowed great rotational speeds to be built up, monitored, and maintained for hours and days. Cooling systems kept the experiment at constant low temperature. The individual cells for solutions were made of glass or quartz, and a high-speed camera was set up so that one cell was photographed repeatedly as it passed by."

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