pK(a) values and the pH dependent stability of the N-terminal domain of L9as probes of electrostatic interactions in the denatured state. Differentiation between local and nonlocal interactions

pK(a) values were measured for the 6 carboxylates in the N-terminal domain of L9 (NTL9) by following NMR chemical shifts as a function of pH. The cont ribution of each carboxylate to the pH dependent stability of NTL9 was esti mated by comparing the pK(a) values for the native and denatured state of t he protein. A set of peptides with sequences derived from NTL9 were used to model the denatured state. In the protein fragments, the pK(a) values meas ured for the aspartates varied between 3.8 and 4.1 and the pK(a) values mea sured for the glutamates varied between 4.1 and 4.6. These results indicate that the local sequence can significantly influence pK(a) values in the de natured state and highlight the difficulties in using standard pK(a) values derived from small compounds. Calculations based on the measured pK(a) val ues suggest that the free energy of unfolding of NTL9 should decrease by 4. 4 kcal mol(-1) when the pH is lowered from 6 to 2. In contrast, urea and th ermal denaturation experiments indicate that the stability of the protein d ecreases by only 2.6 kcal mol(-1) when the carboxylates are protonated. Thi s discrepancy indicates that the protein fragments are not a complete repre sentation of the denatured state and that nonlocal sequence effects perturb the pK(a)'s in the denatured state. Increasing the salt concentration from 100 to 750 mM NaCl removes the discrepancy between the stabilities derived from denaturation experiments and the stability changes calculated from th e pK(a) values. At high concentrations of salt there is also less variation of the pK(a) values measured in the protein fragments. Our results argue t hat in the denatured state of NTL9 there are electrostatic interactions bet ween groups both local and nonlocal in primary sequence.