Figure 1.

Small-probe dots showing the internal contacts of an ideal-geometry Gln residue in the t + t rotamer (ref. 1; tp180° in our nomenclature). There are two bad clashes of Hɛ with Hα and/or Hβ at the mean angle (as shown) and for some distance on either side, but this is relieved if the side-chain amide is flipped by 180°.