Determination of macromolecular folding and structure by synchrotron X-ray radiolysis techniques

Radiolysis of water by synchrotron X-rays generates oxygen-containing radicals that undergo reactions with solvent accessible sites of macromolecules inducing stable covalent modifications or cleavage on millisecond time scales. The extent and site of these reactions are determined by gel electrophoresis and mass spectrometry analysis. These data are used to construct a high-resolution map of solvent accessibility at individual reactive sites. The experiments can be performed in a time-resolved manner to provide kinetic rate constants for dynamic events occurring at individual sites within macromolecules or can provide equilibrium parameters of binding and thermodynamics of folding processes. The application of this synchrotron radiolysis technique to the study of lysozyme protein structure and the equilibrium urea induced unfolding of apomyoglobin are described. The Mg²⁺-induced folding of Tetrahymena thermophila group I ribozyme shows the capability of the method to study kinetics of folding.