Proteins can now be constructed with any amino acid sequence. The importance of applications of this technology in health and other areas is now clear and limited mostly by our current knowledge and imagination. Thus, it is essential that we learn to predict how changes in the amino acid sequence will affect the chemical and physical properties, the function, the folding, and the stability of a protein. The research proposed here will lead to a better understanding of 1) the forces contributing to protein stability; 2) the interactions that determine the pK values of the ionizable groups of proteins; and 3) the contribution of individual side chains to protein solubility. We will study the thermodynamics of folding of Thr to Val and Val to Thr mutants of ribonuclease Sa. These results should allow us to assess whether proteins gain more stability from burying polar -OH groups or nonpolar -CH 3 groups and will provide us with fundamental information on the forces stabilizing proteins. These results should help theoretical chemists improve the methods used to predict and enhance protein structure. In a related project, we will study the thermodynamics of dissolution of the very polar but water insoluble peptide, GNNQQNY, which is extensively hydrogen bonded in a parallel beta-sheet in the solid state. This process will serve as a model for the exposure of polar groups to solvent when a protein unfolds, and will improve our understanding of the amyloid plaques that form in several diseases. The pK values of individual ionizable residues in a protein depend mainly on the Born self energy, hydrogen bonding, and charge-charge interactions. To assess the contribution of the Born self energy to the pK values, we will measure the pK values of buried ionizable groups introduced into RNase Sa. To assess the contribution of hydrogen bonding to pKs, we will measure the changes in the pK values of Asp 76 (pK = 0.5) in RNase T1 and.Asp 33 (pK = 2.3) in RNase Sa as their hydrogen bonding partners are removed. To gain a better understanding of protein solubility, we will study the effect of single changes in the amino acid sequence of RNase Sa on the solubility of the folded and unfolded states of the protein.