PARALLEL ZIPPERS FORMED BY ALPHA-HELICAL PEPTIDE COLUMNS IN CRYSTALS OF BOC-AIB-GLU(OBZL)-LEU-AIB-ALA-LEU-AIB-ALA-LYS(Z)-AIB-OME

The crystal structure of the decapeptide Boc-Aib-Glu(OBzl)-Leu-Aib-Ala-Leu-Aib-Ala-Lys(Z)-Aib-OMe (where Aib is α-aminoisobutyryl, Boc is t-butoxycarbonyl, OBzl is benzyl ester, and Z is benzyloxycarbonyl) illustrates a parallel zipper arrangement of interacting helical peptide columns. Head-to-tail NH ⋯ OC hydrogen bonding extends the α-helices formed by the decapeptide into long columns in the crystal. An additional NH ⋯ OC hydrogen bond in the head-to-tail region, between the extended side chains of Glu(OBzl), residue 2 in one molecule, and Lys(Z), residue 9 in another molecule, forms a "double tooth" on the side of the column. These double teeth are repeated regularly on the helical columns with spaces of six residues between them (≈10 Å). The double teeth on a pair of parallel columns (all carbonyl groups pointed in the same direction) interdigitate in a zipper motif. All contacts in the zipper portion are of the van der Waals type. The peptide, with formula C66H103)N11O17·H 2O, crystallizes in space group P212121 with a = 10.677(4) Å, b = 16.452(6) Å, and c = 43.779(13) Å; overall agreement R = 10.2% for 3527 observed reflections (|F0| > 30); resolution 0.9 Å.