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 1PTE has only 45.87%, 28.75%, and 14.68% angle pairs in allowed, favoured, and most favoured regions, respectively, while 3PTE has 89.24%, 73.84%, and 49.13% angle apirs in those regions.