Denaturation behavior of alpha-chymotrypsinogen A in urea and alkylurea solutions: fluorescence studies

The solvent denaturation of alpha-chymotrypsinogen (alpha-ctg A) in aqueous solution of urea, methyl-, N,N'-dimethyl-, ethyl-, propyl- and butylurea was studied by fluorescence measurements. Data were analyzed on the assumption of a two-state approximation to obtain the apparent equilibrium constant, Ku and the apparent Gibbs free energy of transition delta G0u. It has been observed that alkyl-substitution of urea significantly lowers the denaturant concentration needed to denature alpha-ctg A at 25 degrees C. Denaturation was accompanied by the red shift of emission maxima, the increase of the half-width of the fluorescence spectra, the increase of the fluorescence intensity, and the decrease of the fluorescence polarization. The differences of these fluorescence parameters observed for alpha-ctg A in alkylureas and urea can be ascribed to different unfolded states of the protein in different denaturant solutions. Minor differences in the extent of unfolding were confirmed by size-exclusion chromatography.