JBC, Vol. 250, Issue 2, 440-444, Jan, 1975
The solvent dependence of hydrogen exchange kinetics of folded proteins
C. K. Woodward, L. M. Ellis and A. Rosenberg
The effects of ethanol, ethylene glycol, dioxane, and other organic
co-solvents upon the hydrogen exchange rates of randomly coiled oxidized
RNase, native RNase, and native trypsin have been measured. The exchange
rate of oxidized RNase, the model compound for the proton transfer step in
hydrogen exchange, is decreased by all of the co-solvents studied at
temperatures in the range 3-20 degrees. This has been ascribed to the
combined effects of the disruption of peptide bond solvation due to a
reduction in the concentration of water, and of changes in [OH-] ion
concentration due to changes in the acid dissociation constant of water,
Kw. The solvent dependence for both native RNase and native trypsin is
similar in all of the solvents studied. At a low temperature (3-20
degrees), the exchange rates go through a minimum as the solvent
concentration is increased. At higher temperatures (20-35 degrees) the
exchange rates are increased at all concentrations of the co-solvent. The
apparent rate minimum at lower temperatures is due to two opposing effects.
Co-solvents decrease the rate of exchange that occurs directly from the
folded molecule. At higher concentrations and higer temperature. The
decrease in rates for exchange directly from folded protein is primarily
due to the effects on the proton transfer step, and not to binding or the
solvent effects on protein structure. The solvents used in this study have
no apparent effect on conformational processes contributing to the hydrogen
exchange process in folded proteins.
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