Schellman loop

Schellman loops incorporate a three amino acid residue RL nest (protein structural motif),[12][13] in which three mainchain NH groups (from Schellman loop residues i+3 to i+5) form a concavity for hydrogen bonding to carbonyl oxygens.

[15] The majority of Schellman loops (82%) occur at the C-terminus of an alpha-helix such that residues i, i+1, i+2 and i+3 are part of the helix.

[16] Residue i+4 is the one most-highly conserved; it has positive φ values; 70% of amino acids are glycine and none are proline.

Consideration of the hydrogen bonding in the nests of Schellman loops bound to mainchain oxygens reveals two main types of arrangement: 1,3-bridged or not.

In one (lower figure, ii) the first and third nest NH groups are bridged by an oxygen atom.

Schellman loop. Nitrogen atoms, blue; oxygens, red; carbons, grey. The purple and yellow lines are hydrogen bonds. Side chain and hydrogen atoms omitted. Hydrogen bond arrangement as in ii of the lower figure.
Hydrogen bond arrangements of Schellman loops. Residues are represented by black filled circles. Mainchain-mainchain hydrogen bonds are shown as dashed lines. The grey shading indicates the three nest residues.