isotopes of carbon and nitrogen into proteins. These isotopes made NMR more powerful because they are structural atoms in protein, which made it easy to walk the length of the chain. In addition, since the isotopes transferred their magnetic spin to other atoms more efficiently, it became possible to magnetize triplets, even quadruplets of atoms, resulting in obtaining bigger chunks of information at once. A final advantage is that the spectra of the isotopes are about 20 times wider than the spectrum for hydrogen.

THE STRUCTURE OF PROTEIN IS RESILIENT

Today, researchers are using both techniques for structural analysis of several dozen proteins. T4 lysozyme, a 164 amino acid protein that viruses use to break open bacterial cell walls, is one of the most thoroughly studied. It was the subject of Alber's postdoctoral research under Brian Matthews of the University of Oregon. (Lysozyme is a powerful antibacterial and is present in tears to protect the eyes.)

Matthews first turned his attention to lysozyme in the early 1970s. His colleague, the late George Streisinger, had created about 100 versions of the protein, using a traditional genetic technique that introduces random mutations. "We thought it would be a wonderful opportunity to take advantage of that genetic information," says Matthews, to find out how the mutations affected three-dimensional structure and function.

There was an intrinsic advantage to the use of lysozyme for the study of structure. An important characteristic of proteins is their stability. Heat a protein and eventually the structure falls apart as the weaker noncovalent bonds are breached. The measure of stability is the temperature at which the structure melts. Some proteins are hard to work with because they coagulate in the melted state, like an egg white, losing the ability to reform their proper structure—but not T4 lysozyme.

From 1974 to 1979, Matthews spent most of his time identifying interesting mutants, purifying the proteins, and analyzing their three-dimensional structure with the help of his colleague Rick Dahlquist, an expert on NMR. At that time, "There was a perception that protein structures were very delicately poised between folding and unfolding … and that a single mutation might tip the balance," says Matthews. But he doubted this. He had noted that, while the hemoglobins of horses and humans are structurally similar, they differ in amino acid sequence by nearly 50 percent. Still, he says, "what you don't know in looking at these naturally occurring variants is whether the 50 percent that are



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