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.