Evaluating protein structures requires reliable free energies with good estimates of both potential energies and entropies. Although there are many demonstrated successes from using knowledge-based potential energies, computing entropies of proteins has lagged far behind. Here we take an entirely different approach and evaluate knowledge-based conformational entropies of proteins based on the observed frequencies of contact changes between amino acids in a set of 167 diverse proteins, each of which has two alternative structures. The results show that charged and polar interactions break more often than hydrophobic pairs. This pattern correlates strongly with the average solvent exposure of amino acids in globular proteins, as well as with polarity indices and the sizes of the amino acids. Knowledgebased entropies are derived by using the inverse Boltzmann relationship, in a manner analogous to the way that knowledge-based potentials have been extracted. Including these new knowledge-based entropies almost doubles the performance of knowledge-based potentials in selecting the native protein structures from decoy sets. Beyond the overall energy-entropy compensation, a similar compensation is seen for individual pairs of interacting amino acids. The entropies in this report have immediate applications for 3D structure prediction, protein model assessment, and protein engineering and design.knowledge-based | entropies | free energy | native structure | contact changes K nowledge of a protein's structure is required to understand its dynamics and function; so, improvements in protein structure prediction, especially template-free methods, are essential if the whole protein universe it to be fully comprehended. Computational methods of structure prediction typically yield large numbers of possible structure models (decoys), then require challenging discrimination in determining which of these models is most likely to be the native structure. This well-known bottleneck in protein structure prediction suffers from presentday limitations in structure evaluation. Because the folding of a protein into its native structure is dictated by its free-energy landscape (1), the development of accurate free-energy functions for native structure evaluation is an area of active research. The free energy ΔG of a protein structure can be represented as ΔG = ΔV -TΔS, where ΔV and ΔS represent the energetic (enthalpic) and entropic components, respectively, and T the temperature. In the conventional folding funnel hypothesis of protein folding, the energies and entropies are captured by the depth and by the width of the well (1). Both the energetic and entropic components are combinations of large numbers of contributions, and hence the accurate prediction of free energies is limited by the reliable ability to assess all of these contributions.The energetic contribution to free energy of proteins is usually captured by potential functions, either physics-based or knowledgebased. Physics-based force fields, such as CHARMM, AMBER, GROMOS...