A central problem in biochemistry is to understand the relationship between the amino acid sequence of a protein and its folded structure. The first step requires accurate thermodynamic characterization of the specific interactions in proteins such as hydrogen bonding, van der Waals contacts, ion pairing and hydrophobic effects. The comparison of such interactions in the folded state to the unfolded hydrated state determines the global stability of native folded structure. The folded structure of a protein exhibits by its packing density, compressibility and its precise structural nature the basic properties of a solid; therefore the study of small molecules (amino acid compounds) in the form of crystalline solids is an appropriate model for quantitative evaluation of the interactions in proteins. The relevant thermodynamic process is the dissolving of a small molecule crystalline solid into water. We seek to evaluate the thermodynamic properties of the dissolution process, namely the free energy change, the enthalpy change, the entropy change, and the heat capacity change for simple compounds composed of amino acids. A direct calorimetric procedure has been developed to determine these basic quantities. Equilibrium between the crystalline solid phase and the saturated solution is perturbed within a titration microcalorimeter by injection of a small volume of pure water, and the heat of dissolution in returning to equilibrium is measured. Determination over a range of temperatures permits full evaluation of the desired thermodynamic quantities. Deuterium isotope effects can also be evaluated by use of D2O as the solvent, and lattice interactions between proteins in a crystalline state can be studied as well. The choice of compounds to be studied is dictated by their structural and chemical features. Van der Waals interactions of polar and nonpolar side chains can be determined by use of amino acid compounds with suitable side chains provided the crystal structures are similar. The characterization of hydrogen bonding requires systematic investigation of crystalline compounds in which hydrogen bonding can be blocked. The role of buried charges can be investigated by exploration of zwitterion compounds. Examination of the effect of oligomer size or chain length provides a means to determine hydrogen bonding contributions between chains, ion pair contributions between ends, and the dependence of configurational entropy on chain length. The long range goal is to correlate these structural features with the thermodynamic properties and, if possible, to determine group-additivity relationships. The ultimate test of these group values is to predict thermodynamic properties of proteins. In particular one wishes to describe in quantitative thermodynamic terms the role of 1) amino acid composition upon protein folding 2) specific interfacial interactions on the formation of multisubunit proteins, and 3) lattice interactions between proteins held in a crystal.