Helix Packing of Leucine-Rich Peptides: A Parallel Leucine Ladder in the Structure of Boc- Aib-Leu- Aib- Aib-Leu-Leu-Leu- Aib-Leu- Aib-OMe

The packing of peptide helices in crystals of the leucine-rich decapeptide Boc-AibLeu- Aib AibLeu-Leu-Leu- AibLeu-Aib-OMe provides an example of ladder-like leucyl-leucyl interactions between neighboring molecules. The peptide molecule forms a helix with five 5-l hydrogen bonds and two 4-1 hydrogen bonds near the C terminus. Three head-to-tail NH - - - 0 = C hydrogen bonds between helices form continuous columns of helices in the crystal. The helicial columns associate in an antiparallel fashion, except for the association of Leu........Leu side chains, which occurs along the diagonal of the cell where the peptide helices are parallel. The peptide, with formula C58H102N10O13, crystallizes in space group P212121 with Z = 4 and cell parameters a = 16.774(3) A, b = 20.032(3) A and c = 20.117(3) A; overall agreement factor R = 10.7% for 2014 data with [Fobs] > 3\sigma(F); resolution 1.0 A