A three-dimensional Voronoi tessellation of folded proteins is used in order to analyse geometrical and topological properties of a set of proteins. To each amino acid is associated a central point surrounded by a Voronoi cell. Voronoi cells describe the packing of the amino acids. Special attention is given to reproduction of the protein surface. Once the Voronoi cells are built, a lot of tools from geometrical analysis can be applied to investigate the protein structure; volume of cells, number of faces per cell, and number of sides per face are the usual signatures of the protein structure. A distinct difference between faces related to primary, secondary, and tertiary structures has been observed. Faces threaded by the main chain have on average more than 6 edges, while those related to helical packing of the amino acid chain have less than 5 edges. The faces on the protein surface have on average 5 edges within 1% error. The average number of faces on the protein surface for a given type of amino acid brings a new point of view in the characterisation of the exposition to the solvent and the classification of amino acid as hydrophilic or hydrophobic. It may be a convenient tool for model validation.

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