Using the promise of interesting research, steady money, and California sunshine,
Pauling convinced Mirsky to come to Pasadena for two years starting in the summer
of 1935. It was the beginning of a fruitful collaboration. Mirsky, too, believed in
Fischer's long-chain model of protein structure. Pauling and Mirsky began putting
together a picture of how a long chain might conform to a two-step model of denaturation.
The conclusion they came up with, after some months of brainstorming and experimental
work, was that proteins did indeed consist of long chains of amino acids, each connected
to the next with a covalent peptide bond. That much Fischer had already postulated.
But Pauling and Mirsky went farther. Once formed, the chains, they thought, might
then be looped or twisted into specific shapes, with weaker hydrogen bonds connecting
one part of the chain with another, pinning the final structure into place. Breaking
the hydrogen bonds with gentle heating would leave the chain intact but without its
native shape, strung out, denatured; careful cooling of the denatured protein would
allow the hydrogen bonds to reform and the chain would return to its original shape
and activity. That explained reversible denaturation. More severe treatment, however,
higher heat or stronger acids, would not only destroy the hydrogen bonds but also
would sever some of the covalent links holding the chain together, breaking it into
pieces. Once the covalent bonds were broken, cooling would not rejoin them. The denaturation
would be irreversible.
They published a paper on the subject in 1936. In a few clear sentences, they summarized
what they believed to be a basic structural feature of all proteins: "Our conception
of a native protein molecule (showing specific properties) is the following. The molecule
consists of one polypeptide chain which continues without interruption throughout
the molecule," they wrote. "This chain is folded into a uniquely defined configuration,
in which it is held by hydrogen bonds." The work cleverly explained two-step denaturation,
and it did more. Pauling and Mirsky were proposing that all proteins were basically
the same at one level – they were all long chains of amino acids – but differed in
their properties because of their final shape. In their view it was this specific
shape, delicately held in place by weak bonds, that gave each protein its personality.
Shape was everything.
Their paper, published in the Proceedings of the National Academy of Sciences, was quickly recognized as an important advance in protein studies. It appeared that
Warren Weaver's belief in Pauling was paying off.