The background plots are the contours of the general density distributions of α-τ angle pairs coloured with different levels of density to indicate high 50% (most favoured), 75% (favoured), and 90% (allowed) regions. The dots correspond to the α τ angle pairs in the given protein structures. The plot for 1PHY has only 51.22%, 28.46%, and 10.57% angle pairs in allowed, favoured, and most favoured regions, respectively, while 2PHY has 93.44%, 76.23%, and 45.9% angle apirs in those regions.