Origin of the native driving force for protein folding

We derive an expression with four adjustable parameters that reproduces well the 20 x 20 Miyazawa-Jernigan potential matrix extracted from known protein structures. The numerical values of the parameters can be approximately computed from the surface tension of water, water-screened dipole interactions between residues and water and among residues, and average exposures of residues in folded proteins. PACS numbers: 87.15.By, 02.10.Sp, 64.75.+g.