A computer program is described for calculating the environment and the exposure to solvent of atoms of a protein. The computation is based on the atomic co-ordinates of the protein and on assumptions like those of Lee & Richards (1971). Results for lysozyme and insulin are presented. Changes in exposure to solvent and in the nature of contacts that develop through folding, association reactions and crystallization are described numerically. The computations suggest several generalizations. (a) Lattice contacts within the protein crystal are characterized by a significantly smaller involvement of non-polar side chains and a proportionately greater involvement of ionizable side chains than is found for protein folding or for protein association reactions important for biological function, (b) In helical regions the carbonyl oxygen of the first residue in the helix has high probability of being shielded from solvent, (c) Glycine is among the residues having exposure least affected by folding; this accords with the expectation that it lies at bends of the peptide chain on the surface of the molecule. © 1973.
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