The use of pulses of X-rays from a synchrotron to generate hydroxyl radicals, which will quickly react with, and thus label exposed residues of, a macromolecule as time intervals as short as 10 msec. A large unfolded tRNA of Tetrahymena, for instance, may be induced to begin its folding by the addition of Mg²⁺, while at timed intervals, bursts of hydroxyl radicals label the exposed regions of the RNA.