Polio virus pentamer. Molecular graphic of a pentamer from the outer coat of a polio virus. The protein coat of the polio virus is a tiny dodecahedron, 25-30 nanometres in diameter, consisting of 12 pentagonal units or pentamers". Each pentamer contains five copies of each of four different proteins, shown here by four colours (green only just visible). This view shows the outer, convex surface of the pentamer. Spheres represent amino acids. The coat of the polio virus is highly resistant to heat and acid, making the virus stable in such harsh environments as sewage systems and the human gut."