Previous rotamer libraries showed little sig- nificant clustering for asparagine X_2 or glutamine X_3 values, but none of those studies corrected amide orientations or omitted disordered side chains. The current survey used 240 proteins at <=1.7 A resolution with <50 homology and <30 clashes per thousand atoms (atomic overlap >=0.4 A). All H atoms were added and optimized, and amide orientation was flipped hy 180deg if required by H bonding or atomic clashes. A side chain was included only if its amide orientation was clearly determined and if no atom had a B factor >=40 alternate conformation, or severe clash; that selection process yielded 1,490 Asn and 863 Gln side chains. Clear clustering was observed for Asn X_2 and Gln X_3 (except when Gln X_2 is trans). For Gln, five major and four minor rotamers cover 87 of examples. For Asn, there are seven backbone-independent rotamers covering 94 of examples plus rotamers specified for strictly #alpha#-helical, #beta#, and left-handed (+#phi#) Asn. Although the strongest influence on X angles is avoidance of atomic clashes (especially with the NH_2 hydrogens), some Asn or Gln rotamers are influenced by favorable van der Waals contacts and others by specific local H-bond patterns.
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