GLOBAL ANALYSIS OF THE THERMAL AND CHEMICAL DENATURATION OF THE N-TERMINAL DOMAIN OF THE RIBOSOMAL-PROTEIN L9 IN H2O AND D2O - DETERMINATION OF THE THERMODYNAMIC PARAMETERS, DELTA-H-DEGREES, DELTA-S-DEGREES, AND DELTA-C-DEGREES(P), AND EVALUATION OF SOLVENT ISOTOPE EFFECTS

The stability of the N-terminal domain of the ribosomal protein L9, NT L9, from Baccilus stearothermophilus has been monitored by circular di chroism at various temperatures and chemical denaturant concentrations in H2O and D2O. The basic thermodynamic parameters for the unfolding reaction, Delta H degrees, Delta S degrees, and Delta C(p)degrees, wer e determined by global analysis of temperature and denaturant effects on stability. The data were well fit by a model that assumes stability varies linearly with denaturant concentration and that uses the Gibbs -Helmholtz equation to model changes in stability with temperature. Th e results obtained from the global analysis are consistent with inform ation obtained from individual thermal and chemical denaturations. NTL 9 has a maximum stability of 3.78 +/- 0.25 kcal mol(-1) at 14 degrees C. Delta H degrees(25 degrees C) for protein unfolding equals 9.9 +/-. 0.7 kcal mol(-1) and T Delta S degrees(25 degrees C) equals 6.2 +/- 0 .6 kcal mol(-1). Delta C(p)degrees equals 0.53 +/- 0.06 kcal mol(-1) d eg(-1). There is a small increase in stability when D2O is substituted for H2O. Based on the results from global analysis, NTL9 is 1.06 +/- 0.60 kcal mol(-1) more stable in D2O at 25 degrees C and T-m is increa sed by 5.8 +/- 3.6 degrees C in D2O. Based on the results from individ ual denaturation experiments, NTL9 is 0.68 +/- 0.68 kcal mol(-1) more stable in D2O at 25 degrees C and T-m is increased by 3.5 +/- 2.1 degr ees C in D2O. Within experimental error there are no changes in Delta H degrees (25 degrees C) when D2O is substituted for H2O.