Hydration amide hydrogen exchange

Fig. 22. Hydration dependence of amide hydrogen exchange in lysozyme powder at pH 5. Individual samples of pH 5 fully labeled (with H O) lysozyme were equilibrated at 25°C for 24 hr at various water contents obtained by isopiestic equilibration ( ) or by the addition and mixing of solvent (A). The samples were then dissolved to a concentration of 20 mg/ml and 100-/U.1 aliquots were analyzed by gel filtration. The arrow indicates the 24-hr solution H . end point. H, represents the number of hydrogens remaining unexchanged. From Schinkel el al. (1985).

Amide hydrogen exchange in protein powders depends weakly on water activity, and its hydration dependence is complete within the low-hydration region (0.15 A). Apparently, the rate-determining step for the exchange of buried hydrogens is not much influenced by the protein surface. This is unexplained. 

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