Fig. 2

Generic φ,ψ map showing commonly forbidden areas. For this plot, “forbidden areas” is defined as those representing structures in which there is a close contact (“collision”) between atoms. The contact distance was set at 0.93 Å. These forbidden areas are generally less interesting than the “valleys” of the energy surface but they explain why certain residue types behave the way they do (in particular V, I and T). Residue types are shown in lower-case single letter code. This figure also explains the black regions in some of the members (especially V and D) of Figs. 3, 4, 5 and 6—the energy gradients are too steep to be properly rendered by the graphics