Estimation of the free energy of stabilization of ribonuclease A, lysozyme, alpha-lactalbumin, and myoglobin.

The denaturation of ribonuclease A, lysozyme alpha-lactalbumin, and myoglobin by urea, guanidine hydrochloride, and guanidine thiocyanate has been followed with the use of difference spectral measurements. The free energy of stabilization (delta GH2OD) has been determined by the linear extrapolation of the free energy of denaturation to zero denaturant concentration. The values of delta GH2OD are 7.3 +/- 0.2, 8.9 +/- 0.1, 4.3 +/- 0.1;, and 7.9 +/- 0.2 kcal/mol at 25 degrees C for ribonuclease A (pH 7.0), lysozyme (pH 7.0), alpha-lactalbumin (pH 7.0), and myoglobin (pH 6.6), respectively. The dependence of the free energy of denaturation on concentration ranges from 0.88 to 2.08 kcal/mol.M for urea and from 1.27 to 4.22 kcal/mol.M for guanidine hydrochloride for four proteins. The ratio of this dependence in guanidine hydrochloride to that in urea may depend on the polarity of the amino acid residues in the unfolding unit.