GUANIDINIUM CHLORIDE INDUCTION OF PARTIAL UNFOLDING IN AMIDE PROTON-EXCHANGE IN RNASE-A

Amide (NH) proton exchange rates were measured in 0.0 to 0.7 M guanidi nium chloride (GdmCl) for 23 slowly exchanging peptide NH protons of r ibonuclease A (RNase A) at pH 5.5 (uncorrected pH measured in D2O), 3 4-degrees-C. The purpose was to find out whether GdmCl induces exchang e through binding to exchange intermediates that are partly or wholly unfolded. It was predicted that, when the logarithm of the exchange ra te is plotted as a function of the molarity of GdmCl, the slope should be a measure of the amount of buried surface area exposed to GdmCl in the exchange intermediate. The results indicate that these concentrat ions of GdmCl do induce exchange by means of a partial unfolding mecha nism for all 23 protons; this implies that exchange reactions can be u sed to study the unfolding and stability of local regions. Of the 23 p rotons, nine also show a second mechanism of exchange at lower concent rations of GdmCl, a mechanism that is nearly independent of GdmCl conc entration and is termed ''limited structural fluctuation.''