Fortunately for the scientists, barstar, unlike most proteins, is unfolded at zero degrees Celsius and begins to fold as it warms.
The folding takes place in microseconds (thousandths of a second).
How the method works
The scientists begin by injecting a cold solution of barstar and a tiny amount of hydrogen peroxide into an optical fiber so thin it is difficult to believe it is actually hollow.
"Plugs" of sample in the fiber are then hit with two laser pulses in quick succession.
The first pulse, called a T jump, heats the solution just enough to make a different protein conformation energetically favorable.
The second pulse then breaks some of the hydrogen peroxide (H2O2) molecules into two haves, each of which is a very reactive hydroxyl (-OH) radical.
The radicals react with those parts of the protein that are exposed to the solution, "painting" them with oxygen atoms.
"Imagine," says Gross, "that you suspended a styrofoam model of a partially folded protein and spray-painted it blue. The outside parts would be painted blue; those buried within would remain white."
The radical reactions must be terminated rapidly; otherwise some "painting" may occur within the structure. Within a microsecond, a scavenger amino acid clears away any remaining hydroxyl radicals to prevent them from breaking bonds and altering the protein's configuration.
The same process is repeated 500 times, taking rapid-fire "snapshots" of the protein's quickly changing confirmation.
"The hydroxyl radicals don't mark everything," says Gross. "But they mark about half the amino acids, which is really pretty good. Most other chemical reagents are too specific and too slow for this experiment. Compared to hydroxyl radicals they're just plain ponderous."
Weighing the painted proteins
"We collect each drop of marked protein as it emerg
|Contact: Diana Lutz|
Washington University in St. Louis