Electrostatics of hydrogen exchange for analyzing protein flexibility.

Electrostatic interactions at the protein-aqueous interface modulate the reactivity of solvent-exposed backbone amides by a factor of at least a billion fold. The brief (∼10 ps) lifetime of the peptide anion formed during the hydroxide-catalyzed exchange reaction helps enable the experimental rates to be robustly predictable by continuum dielectric methods. Since this ability to predict the structural dependence of exchange reactivity also applies to the protein amide hydrogens that are only rarely exposed to the bulk solvent phase, electrostatic analysis of the experimental exchange rates provides an effective assessment of whether a given model ensemble is consistent with the properly weighted Boltzmann conformational distribution of the protein native state.