Unfolding dynamics of a beta-sheet protein studied by mass spectrometry

The unfolding dynamics of cellular retinoic acid-binding protein I (CRABP I), an 18 kDa predominantly beta-sheet protein, were studied by monitoring the hydrogen-deuterium (H-D) exchange reaction under various solution conditions. A bimodal charge state distribution was observed when a denaturing agent was added to the protein aqueous solution. These two populations exhibit different kinetics of H-D exchange, with the high charge state ions undergoing very rapid isotope exchange, while the low charge state protein ions exchange cooperatively but at much slower rates. Transiently populated intermediate states were detected indirectly using hydrogen exchange measurement in aqueous solution at various pHs. At pH 2.5 and room temperature, three distinct populations of CRABP I ions exist over an extended period of time, each corresponding to a specific degree of backbone amide hydrogen atom protection. Mass spectral data are complementary to hydrogen exchange measurements by NMR, since the former samples a much faster time-scale of dynamic events in solution.